1000 lines
42 KiB
C++
1000 lines
42 KiB
C++
//===-- UnrollLoopRuntime.cpp - Runtime Loop unrolling utilities ----------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements some loop unrolling utilities for loops with run-time
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// trip counts. See LoopUnroll.cpp for unrolling loops with compile-time
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// trip counts.
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//
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// The functions in this file are used to generate extra code when the
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// run-time trip count modulo the unroll factor is not 0. When this is the
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// case, we need to generate code to execute these 'left over' iterations.
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//
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// The current strategy generates an if-then-else sequence prior to the
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// unrolled loop to execute the 'left over' iterations before or after the
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// unrolled loop.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/LoopIterator.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/MDBuilder.h"
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#include "llvm/IR/Metadata.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
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#include "llvm/Transforms/Utils/UnrollLoop.h"
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#include <algorithm>
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using namespace llvm;
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#define DEBUG_TYPE "loop-unroll"
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STATISTIC(NumRuntimeUnrolled,
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"Number of loops unrolled with run-time trip counts");
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static cl::opt<bool> UnrollRuntimeMultiExit(
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"unroll-runtime-multi-exit", cl::init(false), cl::Hidden,
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cl::desc("Allow runtime unrolling for loops with multiple exits, when "
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"epilog is generated"));
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static cl::opt<bool> UnrollRuntimeOtherExitPredictable(
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"unroll-runtime-other-exit-predictable", cl::init(false), cl::Hidden,
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cl::desc("Assume the non latch exit block to be predictable"));
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/// Connect the unrolling prolog code to the original loop.
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/// The unrolling prolog code contains code to execute the
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/// 'extra' iterations if the run-time trip count modulo the
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/// unroll count is non-zero.
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///
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/// This function performs the following:
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/// - Create PHI nodes at prolog end block to combine values
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/// that exit the prolog code and jump around the prolog.
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/// - Add a PHI operand to a PHI node at the loop exit block
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/// for values that exit the prolog and go around the loop.
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/// - Branch around the original loop if the trip count is less
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/// than the unroll factor.
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///
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static void ConnectProlog(Loop *L, Value *BECount, unsigned Count,
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BasicBlock *PrologExit,
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BasicBlock *OriginalLoopLatchExit,
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BasicBlock *PreHeader, BasicBlock *NewPreHeader,
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ValueToValueMapTy &VMap, DominatorTree *DT,
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LoopInfo *LI, bool PreserveLCSSA) {
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// Loop structure should be the following:
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// Preheader
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// PrologHeader
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// ...
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// PrologLatch
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// PrologExit
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// NewPreheader
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// Header
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// ...
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// Latch
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// LatchExit
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BasicBlock *Latch = L->getLoopLatch();
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assert(Latch && "Loop must have a latch");
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BasicBlock *PrologLatch = cast<BasicBlock>(VMap[Latch]);
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// Create a PHI node for each outgoing value from the original loop
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// (which means it is an outgoing value from the prolog code too).
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// The new PHI node is inserted in the prolog end basic block.
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// The new PHI node value is added as an operand of a PHI node in either
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// the loop header or the loop exit block.
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for (BasicBlock *Succ : successors(Latch)) {
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for (PHINode &PN : Succ->phis()) {
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// Add a new PHI node to the prolog end block and add the
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// appropriate incoming values.
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// TODO: This code assumes that the PrologExit (or the LatchExit block for
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// prolog loop) contains only one predecessor from the loop, i.e. the
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// PrologLatch. When supporting multiple-exiting block loops, we can have
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// two or more blocks that have the LatchExit as the target in the
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// original loop.
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PHINode *NewPN = PHINode::Create(PN.getType(), 2, PN.getName() + ".unr",
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PrologExit->getFirstNonPHI());
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// Adding a value to the new PHI node from the original loop preheader.
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// This is the value that skips all the prolog code.
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if (L->contains(&PN)) {
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// Succ is loop header.
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NewPN->addIncoming(PN.getIncomingValueForBlock(NewPreHeader),
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PreHeader);
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} else {
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// Succ is LatchExit.
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NewPN->addIncoming(UndefValue::get(PN.getType()), PreHeader);
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}
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Value *V = PN.getIncomingValueForBlock(Latch);
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if (Instruction *I = dyn_cast<Instruction>(V)) {
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if (L->contains(I)) {
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V = VMap.lookup(I);
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}
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}
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// Adding a value to the new PHI node from the last prolog block
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// that was created.
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NewPN->addIncoming(V, PrologLatch);
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// Update the existing PHI node operand with the value from the
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// new PHI node. How this is done depends on if the existing
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// PHI node is in the original loop block, or the exit block.
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if (L->contains(&PN))
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PN.setIncomingValueForBlock(NewPreHeader, NewPN);
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else
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PN.addIncoming(NewPN, PrologExit);
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}
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}
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// Make sure that created prolog loop is in simplified form
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SmallVector<BasicBlock *, 4> PrologExitPreds;
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Loop *PrologLoop = LI->getLoopFor(PrologLatch);
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if (PrologLoop) {
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for (BasicBlock *PredBB : predecessors(PrologExit))
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if (PrologLoop->contains(PredBB))
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PrologExitPreds.push_back(PredBB);
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SplitBlockPredecessors(PrologExit, PrologExitPreds, ".unr-lcssa", DT, LI,
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nullptr, PreserveLCSSA);
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}
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// Create a branch around the original loop, which is taken if there are no
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// iterations remaining to be executed after running the prologue.
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Instruction *InsertPt = PrologExit->getTerminator();
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IRBuilder<> B(InsertPt);
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assert(Count != 0 && "nonsensical Count!");
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// If BECount <u (Count - 1) then (BECount + 1) % Count == (BECount + 1)
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// This means %xtraiter is (BECount + 1) and all of the iterations of this
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// loop were executed by the prologue. Note that if BECount <u (Count - 1)
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// then (BECount + 1) cannot unsigned-overflow.
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Value *BrLoopExit =
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B.CreateICmpULT(BECount, ConstantInt::get(BECount->getType(), Count - 1));
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// Split the exit to maintain loop canonicalization guarantees
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SmallVector<BasicBlock *, 4> Preds(predecessors(OriginalLoopLatchExit));
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SplitBlockPredecessors(OriginalLoopLatchExit, Preds, ".unr-lcssa", DT, LI,
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nullptr, PreserveLCSSA);
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// Add the branch to the exit block (around the unrolled loop)
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B.CreateCondBr(BrLoopExit, OriginalLoopLatchExit, NewPreHeader);
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InsertPt->eraseFromParent();
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if (DT)
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DT->changeImmediateDominator(OriginalLoopLatchExit, PrologExit);
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}
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/// Connect the unrolling epilog code to the original loop.
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/// The unrolling epilog code contains code to execute the
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/// 'extra' iterations if the run-time trip count modulo the
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/// unroll count is non-zero.
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///
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/// This function performs the following:
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/// - Update PHI nodes at the unrolling loop exit and epilog loop exit
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/// - Create PHI nodes at the unrolling loop exit to combine
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/// values that exit the unrolling loop code and jump around it.
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/// - Update PHI operands in the epilog loop by the new PHI nodes
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/// - Branch around the epilog loop if extra iters (ModVal) is zero.
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///
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static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
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BasicBlock *Exit, BasicBlock *PreHeader,
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BasicBlock *EpilogPreHeader, BasicBlock *NewPreHeader,
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ValueToValueMapTy &VMap, DominatorTree *DT,
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LoopInfo *LI, bool PreserveLCSSA) {
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BasicBlock *Latch = L->getLoopLatch();
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assert(Latch && "Loop must have a latch");
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BasicBlock *EpilogLatch = cast<BasicBlock>(VMap[Latch]);
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// Loop structure should be the following:
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//
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// PreHeader
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// NewPreHeader
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// Header
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// ...
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// Latch
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// NewExit (PN)
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// EpilogPreHeader
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// EpilogHeader
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// ...
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// EpilogLatch
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// Exit (EpilogPN)
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// Update PHI nodes at NewExit and Exit.
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for (PHINode &PN : NewExit->phis()) {
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// PN should be used in another PHI located in Exit block as
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// Exit was split by SplitBlockPredecessors into Exit and NewExit
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// Basicaly it should look like:
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// NewExit:
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// PN = PHI [I, Latch]
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// ...
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// Exit:
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// EpilogPN = PHI [PN, EpilogPreHeader]
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//
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// There is EpilogPreHeader incoming block instead of NewExit as
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// NewExit was spilt 1 more time to get EpilogPreHeader.
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assert(PN.hasOneUse() && "The phi should have 1 use");
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PHINode *EpilogPN = cast<PHINode>(PN.use_begin()->getUser());
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assert(EpilogPN->getParent() == Exit && "EpilogPN should be in Exit block");
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// Add incoming PreHeader from branch around the Loop
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PN.addIncoming(UndefValue::get(PN.getType()), PreHeader);
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Value *V = PN.getIncomingValueForBlock(Latch);
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Instruction *I = dyn_cast<Instruction>(V);
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if (I && L->contains(I))
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// If value comes from an instruction in the loop add VMap value.
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V = VMap.lookup(I);
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// For the instruction out of the loop, constant or undefined value
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// insert value itself.
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EpilogPN->addIncoming(V, EpilogLatch);
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assert(EpilogPN->getBasicBlockIndex(EpilogPreHeader) >= 0 &&
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"EpilogPN should have EpilogPreHeader incoming block");
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// Change EpilogPreHeader incoming block to NewExit.
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EpilogPN->setIncomingBlock(EpilogPN->getBasicBlockIndex(EpilogPreHeader),
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NewExit);
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// Now PHIs should look like:
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// NewExit:
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// PN = PHI [I, Latch], [undef, PreHeader]
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// ...
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// Exit:
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// EpilogPN = PHI [PN, NewExit], [VMap[I], EpilogLatch]
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}
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// Create PHI nodes at NewExit (from the unrolling loop Latch and PreHeader).
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// Update corresponding PHI nodes in epilog loop.
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for (BasicBlock *Succ : successors(Latch)) {
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// Skip this as we already updated phis in exit blocks.
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if (!L->contains(Succ))
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continue;
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for (PHINode &PN : Succ->phis()) {
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// Add new PHI nodes to the loop exit block and update epilog
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// PHIs with the new PHI values.
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PHINode *NewPN = PHINode::Create(PN.getType(), 2, PN.getName() + ".unr",
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NewExit->getFirstNonPHI());
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// Adding a value to the new PHI node from the unrolling loop preheader.
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NewPN->addIncoming(PN.getIncomingValueForBlock(NewPreHeader), PreHeader);
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// Adding a value to the new PHI node from the unrolling loop latch.
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NewPN->addIncoming(PN.getIncomingValueForBlock(Latch), Latch);
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// Update the existing PHI node operand with the value from the new PHI
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// node. Corresponding instruction in epilog loop should be PHI.
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PHINode *VPN = cast<PHINode>(VMap[&PN]);
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VPN->setIncomingValueForBlock(EpilogPreHeader, NewPN);
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}
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}
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Instruction *InsertPt = NewExit->getTerminator();
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IRBuilder<> B(InsertPt);
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Value *BrLoopExit = B.CreateIsNotNull(ModVal, "lcmp.mod");
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assert(Exit && "Loop must have a single exit block only");
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// Split the epilogue exit to maintain loop canonicalization guarantees
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SmallVector<BasicBlock*, 4> Preds(predecessors(Exit));
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SplitBlockPredecessors(Exit, Preds, ".epilog-lcssa", DT, LI, nullptr,
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PreserveLCSSA);
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// Add the branch to the exit block (around the unrolling loop)
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B.CreateCondBr(BrLoopExit, EpilogPreHeader, Exit);
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InsertPt->eraseFromParent();
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if (DT)
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DT->changeImmediateDominator(Exit, NewExit);
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// Split the main loop exit to maintain canonicalization guarantees.
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SmallVector<BasicBlock*, 4> NewExitPreds{Latch};
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SplitBlockPredecessors(NewExit, NewExitPreds, ".loopexit", DT, LI, nullptr,
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PreserveLCSSA);
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}
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/// Create a clone of the blocks in a loop and connect them together.
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/// If CreateRemainderLoop is false, loop structure will not be cloned,
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/// otherwise a new loop will be created including all cloned blocks, and the
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/// iterator of it switches to count NewIter down to 0.
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/// The cloned blocks should be inserted between InsertTop and InsertBot.
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/// If loop structure is cloned InsertTop should be new preheader, InsertBot
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/// new loop exit.
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/// Return the new cloned loop that is created when CreateRemainderLoop is true.
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static Loop *
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CloneLoopBlocks(Loop *L, Value *NewIter, const bool CreateRemainderLoop,
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const bool UseEpilogRemainder, const bool UnrollRemainder,
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BasicBlock *InsertTop,
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BasicBlock *InsertBot, BasicBlock *Preheader,
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std::vector<BasicBlock *> &NewBlocks, LoopBlocksDFS &LoopBlocks,
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ValueToValueMapTy &VMap, DominatorTree *DT, LoopInfo *LI) {
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StringRef suffix = UseEpilogRemainder ? "epil" : "prol";
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BasicBlock *Header = L->getHeader();
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BasicBlock *Latch = L->getLoopLatch();
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Function *F = Header->getParent();
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LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
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LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
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Loop *ParentLoop = L->getParentLoop();
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NewLoopsMap NewLoops;
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NewLoops[ParentLoop] = ParentLoop;
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if (!CreateRemainderLoop)
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NewLoops[L] = ParentLoop;
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// For each block in the original loop, create a new copy,
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// and update the value map with the newly created values.
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for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
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BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, "." + suffix, F);
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NewBlocks.push_back(NewBB);
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// If we're unrolling the outermost loop, there's no remainder loop,
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// and this block isn't in a nested loop, then the new block is not
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// in any loop. Otherwise, add it to loopinfo.
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if (CreateRemainderLoop || LI->getLoopFor(*BB) != L || ParentLoop)
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addClonedBlockToLoopInfo(*BB, NewBB, LI, NewLoops);
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VMap[*BB] = NewBB;
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if (Header == *BB) {
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// For the first block, add a CFG connection to this newly
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// created block.
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InsertTop->getTerminator()->setSuccessor(0, NewBB);
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}
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if (DT) {
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if (Header == *BB) {
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// The header is dominated by the preheader.
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DT->addNewBlock(NewBB, InsertTop);
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} else {
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// Copy information from original loop to unrolled loop.
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BasicBlock *IDomBB = DT->getNode(*BB)->getIDom()->getBlock();
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DT->addNewBlock(NewBB, cast<BasicBlock>(VMap[IDomBB]));
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}
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}
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if (Latch == *BB) {
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// For the last block, if CreateRemainderLoop is false, create a direct
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// jump to InsertBot. If not, create a loop back to cloned head.
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VMap.erase((*BB)->getTerminator());
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BasicBlock *FirstLoopBB = cast<BasicBlock>(VMap[Header]);
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BranchInst *LatchBR = cast<BranchInst>(NewBB->getTerminator());
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IRBuilder<> Builder(LatchBR);
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if (!CreateRemainderLoop) {
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Builder.CreateBr(InsertBot);
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} else {
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PHINode *NewIdx = PHINode::Create(NewIter->getType(), 2,
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suffix + ".iter",
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FirstLoopBB->getFirstNonPHI());
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Value *IdxSub =
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Builder.CreateSub(NewIdx, ConstantInt::get(NewIdx->getType(), 1),
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NewIdx->getName() + ".sub");
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Value *IdxCmp =
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Builder.CreateIsNotNull(IdxSub, NewIdx->getName() + ".cmp");
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Builder.CreateCondBr(IdxCmp, FirstLoopBB, InsertBot);
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NewIdx->addIncoming(NewIter, InsertTop);
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NewIdx->addIncoming(IdxSub, NewBB);
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}
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LatchBR->eraseFromParent();
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}
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}
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// Change the incoming values to the ones defined in the preheader or
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// cloned loop.
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for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
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PHINode *NewPHI = cast<PHINode>(VMap[&*I]);
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if (!CreateRemainderLoop) {
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if (UseEpilogRemainder) {
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unsigned idx = NewPHI->getBasicBlockIndex(Preheader);
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NewPHI->setIncomingBlock(idx, InsertTop);
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NewPHI->removeIncomingValue(Latch, false);
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} else {
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VMap[&*I] = NewPHI->getIncomingValueForBlock(Preheader);
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cast<BasicBlock>(VMap[Header])->getInstList().erase(NewPHI);
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}
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} else {
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unsigned idx = NewPHI->getBasicBlockIndex(Preheader);
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NewPHI->setIncomingBlock(idx, InsertTop);
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BasicBlock *NewLatch = cast<BasicBlock>(VMap[Latch]);
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idx = NewPHI->getBasicBlockIndex(Latch);
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Value *InVal = NewPHI->getIncomingValue(idx);
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NewPHI->setIncomingBlock(idx, NewLatch);
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if (Value *V = VMap.lookup(InVal))
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NewPHI->setIncomingValue(idx, V);
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}
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}
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if (CreateRemainderLoop) {
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Loop *NewLoop = NewLoops[L];
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assert(NewLoop && "L should have been cloned");
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MDNode *LoopID = NewLoop->getLoopID();
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// Only add loop metadata if the loop is not going to be completely
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// unrolled.
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if (UnrollRemainder)
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return NewLoop;
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Optional<MDNode *> NewLoopID = makeFollowupLoopID(
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LoopID, {LLVMLoopUnrollFollowupAll, LLVMLoopUnrollFollowupRemainder});
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if (NewLoopID.hasValue()) {
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NewLoop->setLoopID(NewLoopID.getValue());
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// Do not setLoopAlreadyUnrolled if loop attributes have been defined
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// explicitly.
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return NewLoop;
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}
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// Add unroll disable metadata to disable future unrolling for this loop.
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NewLoop->setLoopAlreadyUnrolled();
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return NewLoop;
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}
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else
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return nullptr;
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}
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/// Returns true if we can safely unroll a multi-exit/exiting loop. OtherExits
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/// is populated with all the loop exit blocks other than the LatchExit block.
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static bool canSafelyUnrollMultiExitLoop(Loop *L, BasicBlock *LatchExit,
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bool PreserveLCSSA,
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bool UseEpilogRemainder) {
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// We currently have some correctness constrains in unrolling a multi-exit
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// loop. Check for these below.
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|
|
// We rely on LCSSA form being preserved when the exit blocks are transformed.
|
|
if (!PreserveLCSSA)
|
|
return false;
|
|
|
|
// TODO: Support multiple exiting blocks jumping to the `LatchExit` when
|
|
// UnrollRuntimeMultiExit is true. This will need updating the logic in
|
|
// connectEpilog/connectProlog.
|
|
if (!LatchExit->getSinglePredecessor()) {
|
|
LLVM_DEBUG(
|
|
dbgs() << "Bailout for multi-exit handling when latch exit has >1 "
|
|
"predecessor.\n");
|
|
return false;
|
|
}
|
|
// FIXME: We bail out of multi-exit unrolling when epilog loop is generated
|
|
// and L is an inner loop. This is because in presence of multiple exits, the
|
|
// outer loop is incorrect: we do not add the EpilogPreheader and exit to the
|
|
// outer loop. This is automatically handled in the prolog case, so we do not
|
|
// have that bug in prolog generation.
|
|
if (UseEpilogRemainder && L->getParentLoop())
|
|
return false;
|
|
|
|
// All constraints have been satisfied.
|
|
return true;
|
|
}
|
|
|
|
/// Returns true if we can profitably unroll the multi-exit loop L. Currently,
|
|
/// we return true only if UnrollRuntimeMultiExit is set to true.
|
|
static bool canProfitablyUnrollMultiExitLoop(
|
|
Loop *L, SmallVectorImpl<BasicBlock *> &OtherExits, BasicBlock *LatchExit,
|
|
bool PreserveLCSSA, bool UseEpilogRemainder) {
|
|
|
|
#if !defined(NDEBUG)
|
|
assert(canSafelyUnrollMultiExitLoop(L, LatchExit, PreserveLCSSA,
|
|
UseEpilogRemainder) &&
|
|
"Should be safe to unroll before checking profitability!");
|
|
#endif
|
|
|
|
// Priority goes to UnrollRuntimeMultiExit if it's supplied.
|
|
if (UnrollRuntimeMultiExit.getNumOccurrences())
|
|
return UnrollRuntimeMultiExit;
|
|
|
|
// The main pain point with multi-exit loop unrolling is that once unrolled,
|
|
// we will not be able to merge all blocks into a straight line code.
|
|
// There are branches within the unrolled loop that go to the OtherExits.
|
|
// The second point is the increase in code size, but this is true
|
|
// irrespective of multiple exits.
|
|
|
|
// Note: Both the heuristics below are coarse grained. We are essentially
|
|
// enabling unrolling of loops that have a single side exit other than the
|
|
// normal LatchExit (i.e. exiting into a deoptimize block).
|
|
// The heuristics considered are:
|
|
// 1. low number of branches in the unrolled version.
|
|
// 2. high predictability of these extra branches.
|
|
// We avoid unrolling loops that have more than two exiting blocks. This
|
|
// limits the total number of branches in the unrolled loop to be atmost
|
|
// the unroll factor (since one of the exiting blocks is the latch block).
|
|
SmallVector<BasicBlock*, 4> ExitingBlocks;
|
|
L->getExitingBlocks(ExitingBlocks);
|
|
if (ExitingBlocks.size() > 2)
|
|
return false;
|
|
|
|
// Allow unrolling of loops with no non latch exit blocks.
|
|
if (OtherExits.size() == 0)
|
|
return true;
|
|
|
|
// The second heuristic is that L has one exit other than the latchexit and
|
|
// that exit is a deoptimize block. We know that deoptimize blocks are rarely
|
|
// taken, which also implies the branch leading to the deoptimize block is
|
|
// highly predictable. When UnrollRuntimeOtherExitPredictable is specified, we
|
|
// assume the other exit branch is predictable even if it has no deoptimize
|
|
// call.
|
|
return (OtherExits.size() == 1 &&
|
|
(UnrollRuntimeOtherExitPredictable ||
|
|
OtherExits[0]->getTerminatingDeoptimizeCall()));
|
|
// TODO: These can be fine-tuned further to consider code size or deopt states
|
|
// that are captured by the deoptimize exit block.
|
|
// Also, we can extend this to support more cases, if we actually
|
|
// know of kinds of multiexit loops that would benefit from unrolling.
|
|
}
|
|
|
|
// Assign the maximum possible trip count as the back edge weight for the
|
|
// remainder loop if the original loop comes with a branch weight.
|
|
static void updateLatchBranchWeightsForRemainderLoop(Loop *OrigLoop,
|
|
Loop *RemainderLoop,
|
|
uint64_t UnrollFactor) {
|
|
uint64_t TrueWeight, FalseWeight;
|
|
BranchInst *LatchBR =
|
|
cast<BranchInst>(OrigLoop->getLoopLatch()->getTerminator());
|
|
if (LatchBR->extractProfMetadata(TrueWeight, FalseWeight)) {
|
|
uint64_t ExitWeight = LatchBR->getSuccessor(0) == OrigLoop->getHeader()
|
|
? FalseWeight
|
|
: TrueWeight;
|
|
assert(UnrollFactor > 1);
|
|
uint64_t BackEdgeWeight = (UnrollFactor - 1) * ExitWeight;
|
|
BasicBlock *Header = RemainderLoop->getHeader();
|
|
BasicBlock *Latch = RemainderLoop->getLoopLatch();
|
|
auto *RemainderLatchBR = cast<BranchInst>(Latch->getTerminator());
|
|
unsigned HeaderIdx = (RemainderLatchBR->getSuccessor(0) == Header ? 0 : 1);
|
|
MDBuilder MDB(RemainderLatchBR->getContext());
|
|
MDNode *WeightNode =
|
|
HeaderIdx ? MDB.createBranchWeights(ExitWeight, BackEdgeWeight)
|
|
: MDB.createBranchWeights(BackEdgeWeight, ExitWeight);
|
|
RemainderLatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
|
|
}
|
|
}
|
|
|
|
/// Insert code in the prolog/epilog code when unrolling a loop with a
|
|
/// run-time trip-count.
|
|
///
|
|
/// This method assumes that the loop unroll factor is total number
|
|
/// of loop bodies in the loop after unrolling. (Some folks refer
|
|
/// to the unroll factor as the number of *extra* copies added).
|
|
/// We assume also that the loop unroll factor is a power-of-two. So, after
|
|
/// unrolling the loop, the number of loop bodies executed is 2,
|
|
/// 4, 8, etc. Note - LLVM converts the if-then-sequence to a switch
|
|
/// instruction in SimplifyCFG.cpp. Then, the backend decides how code for
|
|
/// the switch instruction is generated.
|
|
///
|
|
/// ***Prolog case***
|
|
/// extraiters = tripcount % loopfactor
|
|
/// if (extraiters == 0) jump Loop:
|
|
/// else jump Prol:
|
|
/// Prol: LoopBody;
|
|
/// extraiters -= 1 // Omitted if unroll factor is 2.
|
|
/// if (extraiters != 0) jump Prol: // Omitted if unroll factor is 2.
|
|
/// if (tripcount < loopfactor) jump End:
|
|
/// Loop:
|
|
/// ...
|
|
/// End:
|
|
///
|
|
/// ***Epilog case***
|
|
/// extraiters = tripcount % loopfactor
|
|
/// if (tripcount < loopfactor) jump LoopExit:
|
|
/// unroll_iters = tripcount - extraiters
|
|
/// Loop: LoopBody; (executes unroll_iter times);
|
|
/// unroll_iter -= 1
|
|
/// if (unroll_iter != 0) jump Loop:
|
|
/// LoopExit:
|
|
/// if (extraiters == 0) jump EpilExit:
|
|
/// Epil: LoopBody; (executes extraiters times)
|
|
/// extraiters -= 1 // Omitted if unroll factor is 2.
|
|
/// if (extraiters != 0) jump Epil: // Omitted if unroll factor is 2.
|
|
/// EpilExit:
|
|
|
|
bool llvm::UnrollRuntimeLoopRemainder(
|
|
Loop *L, unsigned Count, bool AllowExpensiveTripCount,
|
|
bool UseEpilogRemainder, bool UnrollRemainder, bool ForgetAllSCEV,
|
|
LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
|
|
const TargetTransformInfo *TTI, bool PreserveLCSSA, Loop **ResultLoop) {
|
|
LLVM_DEBUG(dbgs() << "Trying runtime unrolling on Loop: \n");
|
|
LLVM_DEBUG(L->dump());
|
|
LLVM_DEBUG(UseEpilogRemainder ? dbgs() << "Using epilog remainder.\n"
|
|
: dbgs() << "Using prolog remainder.\n");
|
|
|
|
// Make sure the loop is in canonical form.
|
|
if (!L->isLoopSimplifyForm()) {
|
|
LLVM_DEBUG(dbgs() << "Not in simplify form!\n");
|
|
return false;
|
|
}
|
|
|
|
// Guaranteed by LoopSimplifyForm.
|
|
BasicBlock *Latch = L->getLoopLatch();
|
|
BasicBlock *Header = L->getHeader();
|
|
|
|
BranchInst *LatchBR = cast<BranchInst>(Latch->getTerminator());
|
|
|
|
if (!LatchBR || LatchBR->isUnconditional()) {
|
|
// The loop-rotate pass can be helpful to avoid this in many cases.
|
|
LLVM_DEBUG(
|
|
dbgs()
|
|
<< "Loop latch not terminated by a conditional branch.\n");
|
|
return false;
|
|
}
|
|
|
|
unsigned ExitIndex = LatchBR->getSuccessor(0) == Header ? 1 : 0;
|
|
BasicBlock *LatchExit = LatchBR->getSuccessor(ExitIndex);
|
|
|
|
if (L->contains(LatchExit)) {
|
|
// Cloning the loop basic blocks (`CloneLoopBlocks`) requires that one of the
|
|
// targets of the Latch be an exit block out of the loop.
|
|
LLVM_DEBUG(
|
|
dbgs()
|
|
<< "One of the loop latch successors must be the exit block.\n");
|
|
return false;
|
|
}
|
|
|
|
// These are exit blocks other than the target of the latch exiting block.
|
|
SmallVector<BasicBlock *, 4> OtherExits;
|
|
L->getUniqueNonLatchExitBlocks(OtherExits);
|
|
bool isMultiExitUnrollingEnabled =
|
|
canSafelyUnrollMultiExitLoop(L, LatchExit, PreserveLCSSA,
|
|
UseEpilogRemainder) &&
|
|
canProfitablyUnrollMultiExitLoop(L, OtherExits, LatchExit, PreserveLCSSA,
|
|
UseEpilogRemainder);
|
|
// Support only single exit and exiting block unless multi-exit loop unrolling is enabled.
|
|
if (!isMultiExitUnrollingEnabled &&
|
|
(!L->getExitingBlock() || OtherExits.size())) {
|
|
LLVM_DEBUG(
|
|
dbgs()
|
|
<< "Multiple exit/exiting blocks in loop and multi-exit unrolling not "
|
|
"enabled!\n");
|
|
return false;
|
|
}
|
|
// Use Scalar Evolution to compute the trip count. This allows more loops to
|
|
// be unrolled than relying on induction var simplification.
|
|
if (!SE)
|
|
return false;
|
|
|
|
// Only unroll loops with a computable trip count, and the trip count needs
|
|
// to be an int value (allowing a pointer type is a TODO item).
|
|
// We calculate the backedge count by using getExitCount on the Latch block,
|
|
// which is proven to be the only exiting block in this loop. This is same as
|
|
// calculating getBackedgeTakenCount on the loop (which computes SCEV for all
|
|
// exiting blocks).
|
|
const SCEV *BECountSC = SE->getExitCount(L, Latch);
|
|
if (isa<SCEVCouldNotCompute>(BECountSC) ||
|
|
!BECountSC->getType()->isIntegerTy()) {
|
|
LLVM_DEBUG(dbgs() << "Could not compute exit block SCEV\n");
|
|
return false;
|
|
}
|
|
|
|
unsigned BEWidth = cast<IntegerType>(BECountSC->getType())->getBitWidth();
|
|
|
|
// Add 1 since the backedge count doesn't include the first loop iteration.
|
|
const SCEV *TripCountSC =
|
|
SE->getAddExpr(BECountSC, SE->getConstant(BECountSC->getType(), 1));
|
|
if (isa<SCEVCouldNotCompute>(TripCountSC)) {
|
|
LLVM_DEBUG(dbgs() << "Could not compute trip count SCEV.\n");
|
|
return false;
|
|
}
|
|
|
|
BasicBlock *PreHeader = L->getLoopPreheader();
|
|
BranchInst *PreHeaderBR = cast<BranchInst>(PreHeader->getTerminator());
|
|
const DataLayout &DL = Header->getModule()->getDataLayout();
|
|
SCEVExpander Expander(*SE, DL, "loop-unroll");
|
|
if (!AllowExpensiveTripCount &&
|
|
Expander.isHighCostExpansion(TripCountSC, L, SCEVCheapExpansionBudget,
|
|
TTI, PreHeaderBR)) {
|
|
LLVM_DEBUG(dbgs() << "High cost for expanding trip count scev!\n");
|
|
return false;
|
|
}
|
|
|
|
// This constraint lets us deal with an overflowing trip count easily; see the
|
|
// comment on ModVal below.
|
|
if (Log2_32(Count) > BEWidth) {
|
|
LLVM_DEBUG(
|
|
dbgs()
|
|
<< "Count failed constraint on overflow trip count calculation.\n");
|
|
return false;
|
|
}
|
|
|
|
// Loop structure is the following:
|
|
//
|
|
// PreHeader
|
|
// Header
|
|
// ...
|
|
// Latch
|
|
// LatchExit
|
|
|
|
BasicBlock *NewPreHeader;
|
|
BasicBlock *NewExit = nullptr;
|
|
BasicBlock *PrologExit = nullptr;
|
|
BasicBlock *EpilogPreHeader = nullptr;
|
|
BasicBlock *PrologPreHeader = nullptr;
|
|
|
|
if (UseEpilogRemainder) {
|
|
// If epilog remainder
|
|
// Split PreHeader to insert a branch around loop for unrolling.
|
|
NewPreHeader = SplitBlock(PreHeader, PreHeader->getTerminator(), DT, LI);
|
|
NewPreHeader->setName(PreHeader->getName() + ".new");
|
|
// Split LatchExit to create phi nodes from branch above.
|
|
SmallVector<BasicBlock*, 4> Preds(predecessors(LatchExit));
|
|
NewExit = SplitBlockPredecessors(LatchExit, Preds, ".unr-lcssa", DT, LI,
|
|
nullptr, PreserveLCSSA);
|
|
// NewExit gets its DebugLoc from LatchExit, which is not part of the
|
|
// original Loop.
|
|
// Fix this by setting Loop's DebugLoc to NewExit.
|
|
auto *NewExitTerminator = NewExit->getTerminator();
|
|
NewExitTerminator->setDebugLoc(Header->getTerminator()->getDebugLoc());
|
|
// Split NewExit to insert epilog remainder loop.
|
|
EpilogPreHeader = SplitBlock(NewExit, NewExitTerminator, DT, LI);
|
|
EpilogPreHeader->setName(Header->getName() + ".epil.preheader");
|
|
} else {
|
|
// If prolog remainder
|
|
// Split the original preheader twice to insert prolog remainder loop
|
|
PrologPreHeader = SplitEdge(PreHeader, Header, DT, LI);
|
|
PrologPreHeader->setName(Header->getName() + ".prol.preheader");
|
|
PrologExit = SplitBlock(PrologPreHeader, PrologPreHeader->getTerminator(),
|
|
DT, LI);
|
|
PrologExit->setName(Header->getName() + ".prol.loopexit");
|
|
// Split PrologExit to get NewPreHeader.
|
|
NewPreHeader = SplitBlock(PrologExit, PrologExit->getTerminator(), DT, LI);
|
|
NewPreHeader->setName(PreHeader->getName() + ".new");
|
|
}
|
|
// Loop structure should be the following:
|
|
// Epilog Prolog
|
|
//
|
|
// PreHeader PreHeader
|
|
// *NewPreHeader *PrologPreHeader
|
|
// Header *PrologExit
|
|
// ... *NewPreHeader
|
|
// Latch Header
|
|
// *NewExit ...
|
|
// *EpilogPreHeader Latch
|
|
// LatchExit LatchExit
|
|
|
|
// Calculate conditions for branch around loop for unrolling
|
|
// in epilog case and around prolog remainder loop in prolog case.
|
|
// Compute the number of extra iterations required, which is:
|
|
// extra iterations = run-time trip count % loop unroll factor
|
|
PreHeaderBR = cast<BranchInst>(PreHeader->getTerminator());
|
|
Value *TripCount = Expander.expandCodeFor(TripCountSC, TripCountSC->getType(),
|
|
PreHeaderBR);
|
|
Value *BECount = Expander.expandCodeFor(BECountSC, BECountSC->getType(),
|
|
PreHeaderBR);
|
|
IRBuilder<> B(PreHeaderBR);
|
|
Value *ModVal;
|
|
// Calculate ModVal = (BECount + 1) % Count.
|
|
// Note that TripCount is BECount + 1.
|
|
if (isPowerOf2_32(Count)) {
|
|
// When Count is power of 2 we don't BECount for epilog case, however we'll
|
|
// need it for a branch around unrolling loop for prolog case.
|
|
ModVal = B.CreateAnd(TripCount, Count - 1, "xtraiter");
|
|
// 1. There are no iterations to be run in the prolog/epilog loop.
|
|
// OR
|
|
// 2. The addition computing TripCount overflowed.
|
|
//
|
|
// If (2) is true, we know that TripCount really is (1 << BEWidth) and so
|
|
// the number of iterations that remain to be run in the original loop is a
|
|
// multiple Count == (1 << Log2(Count)) because Log2(Count) <= BEWidth (we
|
|
// explicitly check this above).
|
|
} else {
|
|
// As (BECount + 1) can potentially unsigned overflow we count
|
|
// (BECount % Count) + 1 which is overflow safe as BECount % Count < Count.
|
|
Value *ModValTmp = B.CreateURem(BECount,
|
|
ConstantInt::get(BECount->getType(),
|
|
Count));
|
|
Value *ModValAdd = B.CreateAdd(ModValTmp,
|
|
ConstantInt::get(ModValTmp->getType(), 1));
|
|
// At that point (BECount % Count) + 1 could be equal to Count.
|
|
// To handle this case we need to take mod by Count one more time.
|
|
ModVal = B.CreateURem(ModValAdd,
|
|
ConstantInt::get(BECount->getType(), Count),
|
|
"xtraiter");
|
|
}
|
|
Value *BranchVal =
|
|
UseEpilogRemainder ? B.CreateICmpULT(BECount,
|
|
ConstantInt::get(BECount->getType(),
|
|
Count - 1)) :
|
|
B.CreateIsNotNull(ModVal, "lcmp.mod");
|
|
BasicBlock *RemainderLoop = UseEpilogRemainder ? NewExit : PrologPreHeader;
|
|
BasicBlock *UnrollingLoop = UseEpilogRemainder ? NewPreHeader : PrologExit;
|
|
// Branch to either remainder (extra iterations) loop or unrolling loop.
|
|
B.CreateCondBr(BranchVal, RemainderLoop, UnrollingLoop);
|
|
PreHeaderBR->eraseFromParent();
|
|
if (DT) {
|
|
if (UseEpilogRemainder)
|
|
DT->changeImmediateDominator(NewExit, PreHeader);
|
|
else
|
|
DT->changeImmediateDominator(PrologExit, PreHeader);
|
|
}
|
|
Function *F = Header->getParent();
|
|
// Get an ordered list of blocks in the loop to help with the ordering of the
|
|
// cloned blocks in the prolog/epilog code
|
|
LoopBlocksDFS LoopBlocks(L);
|
|
LoopBlocks.perform(LI);
|
|
|
|
//
|
|
// For each extra loop iteration, create a copy of the loop's basic blocks
|
|
// and generate a condition that branches to the copy depending on the
|
|
// number of 'left over' iterations.
|
|
//
|
|
std::vector<BasicBlock *> NewBlocks;
|
|
ValueToValueMapTy VMap;
|
|
|
|
// For unroll factor 2 remainder loop will have 1 iterations.
|
|
// Do not create 1 iteration loop.
|
|
bool CreateRemainderLoop = (Count != 2);
|
|
|
|
// Clone all the basic blocks in the loop. If Count is 2, we don't clone
|
|
// the loop, otherwise we create a cloned loop to execute the extra
|
|
// iterations. This function adds the appropriate CFG connections.
|
|
BasicBlock *InsertBot = UseEpilogRemainder ? LatchExit : PrologExit;
|
|
BasicBlock *InsertTop = UseEpilogRemainder ? EpilogPreHeader : PrologPreHeader;
|
|
Loop *remainderLoop = CloneLoopBlocks(
|
|
L, ModVal, CreateRemainderLoop, UseEpilogRemainder, UnrollRemainder,
|
|
InsertTop, InsertBot,
|
|
NewPreHeader, NewBlocks, LoopBlocks, VMap, DT, LI);
|
|
|
|
// Assign the maximum possible trip count as the back edge weight for the
|
|
// remainder loop if the original loop comes with a branch weight.
|
|
if (remainderLoop && !UnrollRemainder)
|
|
updateLatchBranchWeightsForRemainderLoop(L, remainderLoop, Count);
|
|
|
|
// Insert the cloned blocks into the function.
|
|
F->getBasicBlockList().splice(InsertBot->getIterator(),
|
|
F->getBasicBlockList(),
|
|
NewBlocks[0]->getIterator(),
|
|
F->end());
|
|
|
|
// Now the loop blocks are cloned and the other exiting blocks from the
|
|
// remainder are connected to the original Loop's exit blocks. The remaining
|
|
// work is to update the phi nodes in the original loop, and take in the
|
|
// values from the cloned region.
|
|
for (auto *BB : OtherExits) {
|
|
for (auto &II : *BB) {
|
|
|
|
// Given we preserve LCSSA form, we know that the values used outside the
|
|
// loop will be used through these phi nodes at the exit blocks that are
|
|
// transformed below.
|
|
if (!isa<PHINode>(II))
|
|
break;
|
|
PHINode *Phi = cast<PHINode>(&II);
|
|
unsigned oldNumOperands = Phi->getNumIncomingValues();
|
|
// Add the incoming values from the remainder code to the end of the phi
|
|
// node.
|
|
for (unsigned i =0; i < oldNumOperands; i++){
|
|
Value *newVal = VMap.lookup(Phi->getIncomingValue(i));
|
|
// newVal can be a constant or derived from values outside the loop, and
|
|
// hence need not have a VMap value. Also, since lookup already generated
|
|
// a default "null" VMap entry for this value, we need to populate that
|
|
// VMap entry correctly, with the mapped entry being itself.
|
|
if (!newVal) {
|
|
newVal = Phi->getIncomingValue(i);
|
|
VMap[Phi->getIncomingValue(i)] = Phi->getIncomingValue(i);
|
|
}
|
|
Phi->addIncoming(newVal,
|
|
cast<BasicBlock>(VMap[Phi->getIncomingBlock(i)]));
|
|
}
|
|
}
|
|
#if defined(EXPENSIVE_CHECKS) && !defined(NDEBUG)
|
|
for (BasicBlock *SuccBB : successors(BB)) {
|
|
assert(!(any_of(OtherExits,
|
|
[SuccBB](BasicBlock *EB) { return EB == SuccBB; }) ||
|
|
SuccBB == LatchExit) &&
|
|
"Breaks the definition of dedicated exits!");
|
|
}
|
|
#endif
|
|
}
|
|
|
|
// Update the immediate dominator of the exit blocks and blocks that are
|
|
// reachable from the exit blocks. This is needed because we now have paths
|
|
// from both the original loop and the remainder code reaching the exit
|
|
// blocks. While the IDom of these exit blocks were from the original loop,
|
|
// now the IDom is the preheader (which decides whether the original loop or
|
|
// remainder code should run).
|
|
if (DT && !L->getExitingBlock()) {
|
|
SmallVector<BasicBlock *, 16> ChildrenToUpdate;
|
|
// NB! We have to examine the dom children of all loop blocks, not just
|
|
// those which are the IDom of the exit blocks. This is because blocks
|
|
// reachable from the exit blocks can have their IDom as the nearest common
|
|
// dominator of the exit blocks.
|
|
for (auto *BB : L->blocks()) {
|
|
auto *DomNodeBB = DT->getNode(BB);
|
|
for (auto *DomChild : DomNodeBB->children()) {
|
|
auto *DomChildBB = DomChild->getBlock();
|
|
if (!L->contains(LI->getLoopFor(DomChildBB)))
|
|
ChildrenToUpdate.push_back(DomChildBB);
|
|
}
|
|
}
|
|
for (auto *BB : ChildrenToUpdate)
|
|
DT->changeImmediateDominator(BB, PreHeader);
|
|
}
|
|
|
|
// Loop structure should be the following:
|
|
// Epilog Prolog
|
|
//
|
|
// PreHeader PreHeader
|
|
// NewPreHeader PrologPreHeader
|
|
// Header PrologHeader
|
|
// ... ...
|
|
// Latch PrologLatch
|
|
// NewExit PrologExit
|
|
// EpilogPreHeader NewPreHeader
|
|
// EpilogHeader Header
|
|
// ... ...
|
|
// EpilogLatch Latch
|
|
// LatchExit LatchExit
|
|
|
|
// Rewrite the cloned instruction operands to use the values created when the
|
|
// clone is created.
|
|
for (BasicBlock *BB : NewBlocks) {
|
|
for (Instruction &I : *BB) {
|
|
RemapInstruction(&I, VMap,
|
|
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
|
|
}
|
|
}
|
|
|
|
if (UseEpilogRemainder) {
|
|
// Connect the epilog code to the original loop and update the
|
|
// PHI functions.
|
|
ConnectEpilog(L, ModVal, NewExit, LatchExit, PreHeader,
|
|
EpilogPreHeader, NewPreHeader, VMap, DT, LI,
|
|
PreserveLCSSA);
|
|
|
|
// Update counter in loop for unrolling.
|
|
// I should be multiply of Count.
|
|
IRBuilder<> B2(NewPreHeader->getTerminator());
|
|
Value *TestVal = B2.CreateSub(TripCount, ModVal, "unroll_iter");
|
|
BranchInst *LatchBR = cast<BranchInst>(Latch->getTerminator());
|
|
B2.SetInsertPoint(LatchBR);
|
|
PHINode *NewIdx = PHINode::Create(TestVal->getType(), 2, "niter",
|
|
Header->getFirstNonPHI());
|
|
Value *IdxSub =
|
|
B2.CreateSub(NewIdx, ConstantInt::get(NewIdx->getType(), 1),
|
|
NewIdx->getName() + ".nsub");
|
|
Value *IdxCmp;
|
|
if (LatchBR->getSuccessor(0) == Header)
|
|
IdxCmp = B2.CreateIsNotNull(IdxSub, NewIdx->getName() + ".ncmp");
|
|
else
|
|
IdxCmp = B2.CreateIsNull(IdxSub, NewIdx->getName() + ".ncmp");
|
|
NewIdx->addIncoming(TestVal, NewPreHeader);
|
|
NewIdx->addIncoming(IdxSub, Latch);
|
|
LatchBR->setCondition(IdxCmp);
|
|
} else {
|
|
// Connect the prolog code to the original loop and update the
|
|
// PHI functions.
|
|
ConnectProlog(L, BECount, Count, PrologExit, LatchExit, PreHeader,
|
|
NewPreHeader, VMap, DT, LI, PreserveLCSSA);
|
|
}
|
|
|
|
// If this loop is nested, then the loop unroller changes the code in the any
|
|
// of its parent loops, so the Scalar Evolution pass needs to be run again.
|
|
SE->forgetTopmostLoop(L);
|
|
|
|
// Verify that the Dom Tree is correct.
|
|
#if defined(EXPENSIVE_CHECKS) && !defined(NDEBUG)
|
|
if (DT)
|
|
assert(DT->verify(DominatorTree::VerificationLevel::Full));
|
|
#endif
|
|
|
|
// Canonicalize to LoopSimplifyForm both original and remainder loops. We
|
|
// cannot rely on the LoopUnrollPass to do this because it only does
|
|
// canonicalization for parent/subloops and not the sibling loops.
|
|
if (OtherExits.size() > 0) {
|
|
// Generate dedicated exit blocks for the original loop, to preserve
|
|
// LoopSimplifyForm.
|
|
formDedicatedExitBlocks(L, DT, LI, nullptr, PreserveLCSSA);
|
|
// Generate dedicated exit blocks for the remainder loop if one exists, to
|
|
// preserve LoopSimplifyForm.
|
|
if (remainderLoop)
|
|
formDedicatedExitBlocks(remainderLoop, DT, LI, nullptr, PreserveLCSSA);
|
|
}
|
|
|
|
auto UnrollResult = LoopUnrollResult::Unmodified;
|
|
if (remainderLoop && UnrollRemainder) {
|
|
LLVM_DEBUG(dbgs() << "Unrolling remainder loop\n");
|
|
UnrollResult =
|
|
UnrollLoop(remainderLoop,
|
|
{/*Count*/ Count - 1, /*TripCount*/ Count - 1,
|
|
/*Force*/ false, /*AllowRuntime*/ false,
|
|
/*AllowExpensiveTripCount*/ false, /*PreserveCondBr*/ true,
|
|
/*PreserveOnlyFirst*/ false, /*TripMultiple*/ 1,
|
|
/*PeelCount*/ 0, /*UnrollRemainder*/ false, ForgetAllSCEV},
|
|
LI, SE, DT, AC, TTI, /*ORE*/ nullptr, PreserveLCSSA);
|
|
}
|
|
|
|
if (ResultLoop && UnrollResult != LoopUnrollResult::FullyUnrolled)
|
|
*ResultLoop = remainderLoop;
|
|
NumRuntimeUnrolled++;
|
|
return true;
|
|
}
|