378 lines
14 KiB
C++
378 lines
14 KiB
C++
//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass transforms loops by placing phi nodes at the end of the loops for
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// all values that are live across the loop boundary. For example, it turns
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// the left into the right code:
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//
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// for (...) for (...)
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// if (c) if (c)
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// X1 = ... X1 = ...
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// else else
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// X2 = ... X2 = ...
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// X3 = phi(X1, X2) X3 = phi(X1, X2)
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// ... = X3 + 4 X4 = phi(X3)
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// ... = X4 + 4
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//
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// This is still valid LLVM; the extra phi nodes are purely redundant, and will
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// be trivially eliminated by InstCombine. The major benefit of this
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// transformation is that it makes many other loop optimizations, such as
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// LoopUnswitching, simpler.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/LCSSA.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/BasicAliasAnalysis.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/PredIteratorCache.h"
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#include "llvm/Pass.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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using namespace llvm;
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#define DEBUG_TYPE "lcssa"
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STATISTIC(NumLCSSA, "Number of live out of a loop variables");
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/// Return true if the specified block is in the list.
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static bool isExitBlock(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
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return is_contained(ExitBlocks, BB);
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}
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/// For every instruction from the worklist, check to see if it has any uses
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/// that are outside the current loop. If so, insert LCSSA PHI nodes and
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/// rewrite the uses.
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bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
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DominatorTree &DT, LoopInfo &LI) {
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SmallVector<Use *, 16> UsesToRewrite;
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SmallVector<BasicBlock *, 8> ExitBlocks;
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SmallSetVector<PHINode *, 16> PHIsToRemove;
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PredIteratorCache PredCache;
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bool Changed = false;
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while (!Worklist.empty()) {
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UsesToRewrite.clear();
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ExitBlocks.clear();
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Instruction *I = Worklist.pop_back_val();
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BasicBlock *InstBB = I->getParent();
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Loop *L = LI.getLoopFor(InstBB);
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L->getExitBlocks(ExitBlocks);
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if (ExitBlocks.empty())
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continue;
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// Tokens cannot be used in PHI nodes, so we skip over them.
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// We can run into tokens which are live out of a loop with catchswitch
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// instructions in Windows EH if the catchswitch has one catchpad which
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// is inside the loop and another which is not.
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if (I->getType()->isTokenTy())
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continue;
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for (Use &U : I->uses()) {
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Instruction *User = cast<Instruction>(U.getUser());
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BasicBlock *UserBB = User->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(U);
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if (InstBB != UserBB && !L->contains(UserBB))
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UsesToRewrite.push_back(&U);
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}
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// If there are no uses outside the loop, exit with no change.
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if (UsesToRewrite.empty())
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continue;
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++NumLCSSA; // We are applying the transformation
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// Invoke instructions are special in that their result value is not
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// available along their unwind edge. The code below tests to see whether
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// DomBB dominates the value, so adjust DomBB to the normal destination
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// block, which is effectively where the value is first usable.
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BasicBlock *DomBB = InstBB;
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if (InvokeInst *Inv = dyn_cast<InvokeInst>(I))
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DomBB = Inv->getNormalDest();
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DomTreeNode *DomNode = DT.getNode(DomBB);
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SmallVector<PHINode *, 16> AddedPHIs;
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SmallVector<PHINode *, 8> PostProcessPHIs;
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SmallVector<PHINode *, 4> InsertedPHIs;
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SSAUpdater SSAUpdate(&InsertedPHIs);
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SSAUpdate.Initialize(I->getType(), I->getName());
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// Insert the LCSSA phi's into all of the exit blocks dominated by the
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// value, and add them to the Phi's map.
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for (BasicBlock *ExitBB : ExitBlocks) {
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if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
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continue;
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// If we already inserted something for this BB, don't reprocess it.
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if (SSAUpdate.HasValueForBlock(ExitBB))
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continue;
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PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB),
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I->getName() + ".lcssa", &ExitBB->front());
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// Add inputs from inside the loop for this PHI.
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for (BasicBlock *Pred : PredCache.get(ExitBB)) {
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PN->addIncoming(I, Pred);
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// If the exit block has a predecessor not within the loop, arrange for
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// the incoming value use corresponding to that predecessor to be
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// rewritten in terms of a different LCSSA PHI.
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if (!L->contains(Pred))
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UsesToRewrite.push_back(
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&PN->getOperandUse(PN->getOperandNumForIncomingValue(
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PN->getNumIncomingValues() - 1)));
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}
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AddedPHIs.push_back(PN);
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// Remember that this phi makes the value alive in this block.
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SSAUpdate.AddAvailableValue(ExitBB, PN);
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// LoopSimplify might fail to simplify some loops (e.g. when indirect
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// branches are involved). In such situations, it might happen that an
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// exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
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// create PHIs in such an exit block, we are also inserting PHIs into L2's
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// header. This could break LCSSA form for L2 because these inserted PHIs
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// can also have uses outside of L2. Remember all PHIs in such situation
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// as to revisit than later on. FIXME: Remove this if indirectbr support
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// into LoopSimplify gets improved.
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if (auto *OtherLoop = LI.getLoopFor(ExitBB))
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if (!L->contains(OtherLoop))
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PostProcessPHIs.push_back(PN);
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}
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// Rewrite all uses outside the loop in terms of the new PHIs we just
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// inserted.
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for (Use *UseToRewrite : UsesToRewrite) {
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// If this use is in an exit block, rewrite to use the newly inserted PHI.
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// This is required for correctness because SSAUpdate doesn't handle uses
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// in the same block. It assumes the PHI we inserted is at the end of the
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// block.
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Instruction *User = cast<Instruction>(UseToRewrite->getUser());
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BasicBlock *UserBB = User->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(*UseToRewrite);
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if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
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// Tell the VHs that the uses changed. This updates SCEV's caches.
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if (UseToRewrite->get()->hasValueHandle())
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ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front());
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UseToRewrite->set(&UserBB->front());
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continue;
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}
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// Otherwise, do full PHI insertion.
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SSAUpdate.RewriteUse(*UseToRewrite);
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}
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// SSAUpdater might have inserted phi-nodes inside other loops. We'll need
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// to post-process them to keep LCSSA form.
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for (PHINode *InsertedPN : InsertedPHIs) {
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if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
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if (!L->contains(OtherLoop))
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PostProcessPHIs.push_back(InsertedPN);
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}
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// Post process PHI instructions that were inserted into another disjoint
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// loop and update their exits properly.
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for (auto *PostProcessPN : PostProcessPHIs) {
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if (PostProcessPN->use_empty())
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continue;
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// Reprocess each PHI instruction.
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Worklist.push_back(PostProcessPN);
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}
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// Keep track of PHI nodes that we want to remove because they did not have
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// any uses rewritten.
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for (PHINode *PN : AddedPHIs)
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if (PN->use_empty())
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PHIsToRemove.insert(PN);
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Changed = true;
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}
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// Remove PHI nodes that did not have any uses rewritten.
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for (PHINode *PN : PHIsToRemove) {
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assert (PN->use_empty() && "Trying to remove a phi with uses.");
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PN->eraseFromParent();
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}
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return Changed;
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}
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/// Return true if the specified block dominates at least
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/// one of the blocks in the specified list.
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static bool
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blockDominatesAnExit(BasicBlock *BB,
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DominatorTree &DT,
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const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
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DomTreeNode *DomNode = DT.getNode(BB);
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return any_of(ExitBlocks, [&](BasicBlock *EB) {
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return DT.dominates(DomNode, DT.getNode(EB));
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});
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}
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bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
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ScalarEvolution *SE) {
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bool Changed = false;
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// Get the set of exiting blocks.
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SmallVector<BasicBlock *, 8> ExitBlocks;
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L.getExitBlocks(ExitBlocks);
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if (ExitBlocks.empty())
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return false;
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SmallVector<Instruction *, 8> Worklist;
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// Look at all the instructions in the loop, checking to see if they have uses
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// outside the loop. If so, put them into the worklist to rewrite those uses.
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for (BasicBlock *BB : L.blocks()) {
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// For large loops, avoid use-scanning by using dominance information: In
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// particular, if a block does not dominate any of the loop exits, then none
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// of the values defined in the block could be used outside the loop.
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if (!blockDominatesAnExit(BB, DT, ExitBlocks))
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continue;
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for (Instruction &I : *BB) {
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// Reject two common cases fast: instructions with no uses (like stores)
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// and instructions with one use that is in the same block as this.
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if (I.use_empty() ||
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(I.hasOneUse() && I.user_back()->getParent() == BB &&
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!isa<PHINode>(I.user_back())))
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continue;
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Worklist.push_back(&I);
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}
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}
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Changed = formLCSSAForInstructions(Worklist, DT, *LI);
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// If we modified the code, remove any caches about the loop from SCEV to
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// avoid dangling entries.
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// FIXME: This is a big hammer, can we clear the cache more selectively?
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if (SE && Changed)
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SE->forgetLoop(&L);
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assert(L.isLCSSAForm(DT));
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return Changed;
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}
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/// Process a loop nest depth first.
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bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
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ScalarEvolution *SE) {
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bool Changed = false;
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// Recurse depth-first through inner loops.
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for (Loop *SubLoop : L.getSubLoops())
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Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
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Changed |= formLCSSA(L, DT, LI, SE);
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return Changed;
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}
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/// Process all loops in the function, inner-most out.
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static bool formLCSSAOnAllLoops(LoopInfo *LI, DominatorTree &DT,
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ScalarEvolution *SE) {
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bool Changed = false;
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for (auto &L : *LI)
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Changed |= formLCSSARecursively(*L, DT, LI, SE);
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return Changed;
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}
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namespace {
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struct LCSSAWrapperPass : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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LCSSAWrapperPass() : FunctionPass(ID) {
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initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
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}
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// Cached analysis information for the current function.
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DominatorTree *DT;
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LoopInfo *LI;
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ScalarEvolution *SE;
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bool runOnFunction(Function &F) override;
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void verifyAnalysis() const override {
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assert(
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all_of(*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT); }) &&
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"LCSSA form is broken!");
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};
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG. It maintains both of these,
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/// as well as the CFG. It also requires dominator information.
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreservedID(LoopSimplifyID);
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AU.addPreserved<AAResultsWrapperPass>();
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AU.addPreserved<BasicAAWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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AU.addPreserved<ScalarEvolutionWrapperPass>();
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AU.addPreserved<SCEVAAWrapperPass>();
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}
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};
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}
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char LCSSAWrapperPass::ID = 0;
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INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
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false, false)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
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false, false)
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Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
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char &llvm::LCSSAID = LCSSAWrapperPass::ID;
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/// Transform \p F into loop-closed SSA form.
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bool LCSSAWrapperPass::runOnFunction(Function &F) {
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LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
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SE = SEWP ? &SEWP->getSE() : nullptr;
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return formLCSSAOnAllLoops(LI, *DT, SE);
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}
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PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
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auto &LI = AM.getResult<LoopAnalysis>(F);
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auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
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auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
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if (!formLCSSAOnAllLoops(&LI, DT, SE))
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return PreservedAnalyses::all();
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// FIXME: This should also 'preserve the CFG'.
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PreservedAnalyses PA;
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PA.preserve<BasicAA>();
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PA.preserve<GlobalsAA>();
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PA.preserve<SCEVAA>();
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PA.preserve<ScalarEvolutionAnalysis>();
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return PA;
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}
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