OSchip
/
llvm-project
mirror of https://github.com/THU-DSP-LAB/llvm-project.git
13
0
Fork 6
Code Issues Packages Projects Releases Wiki Activity
llvm-project/llvm/lib/Transforms/Scalar/LoopPredication.cpp

1317 lines
52 KiB
C++
Raw Blame History

//===-- LoopPredication.cpp - Guard based loop predication pass -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// The LoopPredication pass tries to convert loop variant range checks to loop
// invariant by widening checks across loop iterations. For example, it will
// convert
//
// for (i = 0; i < n; i++) {
// guard(i < len);
// ...
// }
//
// to
//
// for (i = 0; i < n; i++) {
// guard(n - 1 < len);
// ...
// }
//
// After this transformation the condition of the guard is loop invariant, so
// loop-unswitch can later unswitch the loop by this condition which basically
// predicates the loop by the widened condition:
//
// if (n - 1 < len)
// for (i = 0; i < n; i++) {
// ...
// }
// else
// deoptimize
//
// It's tempting to rely on SCEV here, but it has proven to be problematic.
// Generally the facts SCEV provides about the increment step of add
// recurrences are true if the backedge of the loop is taken, which implicitly
// assumes that the guard doesn't fail. Using these facts to optimize the
// guard results in a circular logic where the guard is optimized under the
// assumption that it never fails.
//
// For example, in the loop below the induction variable will be marked as nuw
// basing on the guard. Basing on nuw the guard predicate will be considered
// monotonic. Given a monotonic condition it's tempting to replace the induction
// variable in the condition with its value on the last iteration. But this
// transformation is not correct, e.g. e = 4, b = 5 breaks the loop.
//
// for (int i = b; i != e; i++)
// guard(i u< len)
//
// One of the ways to reason about this problem is to use an inductive proof
// approach. Given the loop:
//
// if (B(0)) {
// do {
// I = PHI(0, I.INC)
// I.INC = I + Step
// guard(G(I));
// } while (B(I));
// }
//
// where B(x) and G(x) are predicates that map integers to booleans, we want a
// loop invariant expression M such the following program has the same semantics
// as the above:
//
// if (B(0)) {
// do {
// I = PHI(0, I.INC)
// I.INC = I + Step
// guard(G(0) && M);
// } while (B(I));
// }
//
// One solution for M is M = forall X . (G(X) && B(X)) => G(X + Step)
//
// Informal proof that the transformation above is correct:
//
// By the definition of guards we can rewrite the guard condition to:
// G(I) && G(0) && M
//
// Let's prove that for each iteration of the loop:
// G(0) && M => G(I)
// And the condition above can be simplified to G(Start) && M.
//
// Induction base.
// G(0) && M => G(0)
//
// Induction step. Assuming G(0) && M => G(I) on the subsequent
// iteration:
//
// B(I) is true because it's the backedge condition.
// G(I) is true because the backedge is guarded by this condition.
//
// So M = forall X . (G(X) && B(X)) => G(X + Step) implies G(I + Step).
//
// Note that we can use anything stronger than M, i.e. any condition which
// implies M.
//
// When S = 1 (i.e. forward iterating loop), the transformation is supported
// when:
// * The loop has a single latch with the condition of the form:
// B(X) = latchStart + X <pred> latchLimit,
// where <pred> is u<, u<=, s<, or s<=.
// * The guard condition is of the form
// G(X) = guardStart + X u< guardLimit
//
// For the ult latch comparison case M is:
// forall X . guardStart + X u< guardLimit && latchStart + X <u latchLimit =>
// guardStart + X + 1 u< guardLimit
//
// The only way the antecedent can be true and the consequent can be false is
// if
// X == guardLimit - 1 - guardStart
// (and guardLimit is non-zero, but we won't use this latter fact).
// If X == guardLimit - 1 - guardStart then the second half of the antecedent is
// latchStart + guardLimit - 1 - guardStart u< latchLimit
// and its negation is
// latchStart + guardLimit - 1 - guardStart u>= latchLimit
//
// In other words, if
// latchLimit u<= latchStart + guardLimit - 1 - guardStart
// then:
// (the ranges below are written in ConstantRange notation, where [A, B) is the
// set for (I = A; I != B; I++ /*maywrap*/) yield(I);)
//
// forall X . guardStart + X u< guardLimit &&
// latchStart + X u< latchLimit =>
// guardStart + X + 1 u< guardLimit
// == forall X . guardStart + X u< guardLimit &&
// latchStart + X u< latchStart + guardLimit - 1 - guardStart =>
// guardStart + X + 1 u< guardLimit
// == forall X . (guardStart + X) in [0, guardLimit) &&
// (latchStart + X) in [0, latchStart + guardLimit - 1 - guardStart) =>
// (guardStart + X + 1) in [0, guardLimit)
// == forall X . X in [-guardStart, guardLimit - guardStart) &&
// X in [-latchStart, guardLimit - 1 - guardStart) =>
// X in [-guardStart - 1, guardLimit - guardStart - 1)
// == true
//
// So the widened condition is:
// guardStart u< guardLimit &&
// latchStart + guardLimit - 1 - guardStart u>= latchLimit
// Similarly for ule condition the widened condition is:
// guardStart u< guardLimit &&
// latchStart + guardLimit - 1 - guardStart u> latchLimit
// For slt condition the widened condition is:
// guardStart u< guardLimit &&
// latchStart + guardLimit - 1 - guardStart s>= latchLimit
// For sle condition the widened condition is:
// guardStart u< guardLimit &&
// latchStart + guardLimit - 1 - guardStart s> latchLimit
//
// When S = -1 (i.e. reverse iterating loop), the transformation is supported
// when:
// * The loop has a single latch with the condition of the form:
// B(X) = X <pred> latchLimit, where <pred> is u>, u>=, s>, or s>=.
// * The guard condition is of the form
// G(X) = X - 1 u< guardLimit
//
// For the ugt latch comparison case M is:
// forall X. X-1 u< guardLimit and X u> latchLimit => X-2 u< guardLimit
//
// The only way the antecedent can be true and the consequent can be false is if
// X == 1.
// If X == 1 then the second half of the antecedent is
// 1 u> latchLimit, and its negation is latchLimit u>= 1.
//
// So the widened condition is:
// guardStart u< guardLimit && latchLimit u>= 1.
// Similarly for sgt condition the widened condition is:
// guardStart u< guardLimit && latchLimit s>= 1.
// For uge condition the widened condition is:
// guardStart u< guardLimit && latchLimit u> 1.
// For sge condition the widened condition is:
// guardStart u< guardLimit && latchLimit s> 1.
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LoopPredication.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/GuardUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#define DEBUG_TYPE "loop-predication"
STATISTIC(TotalConsidered, "Number of guards considered");
STATISTIC(TotalWidened, "Number of checks widened");
using namespace llvm;
static cl::opt<bool> EnableIVTruncation("loop-predication-enable-iv-truncation",
cl::Hidden, cl::init(true));
static cl::opt<bool> EnableCountDownLoop("loop-predication-enable-count-down-loop",
cl::Hidden, cl::init(true));
static cl::opt<bool>
SkipProfitabilityChecks("loop-predication-skip-profitability-checks",
cl::Hidden, cl::init(false));
// This is the scale factor for the latch probability. We use this during
// profitability analysis to find other exiting blocks that have a much higher
// probability of exiting the loop instead of loop exiting via latch.
// This value should be greater than 1 for a sane profitability check.
static cl::opt<float> LatchExitProbabilityScale(
"loop-predication-latch-probability-scale", cl::Hidden, cl::init(2.0),
cl::desc("scale factor for the latch probability. Value should be greater "
"than 1. Lower values are ignored"));
static cl::opt<bool> PredicateWidenableBranchGuards(
"loop-predication-predicate-widenable-branches-to-deopt", cl::Hidden,
cl::desc("Whether or not we should predicate guards "
"expressed as widenable branches to deoptimize blocks"),
cl::init(true));
namespace {
/// Represents an induction variable check:
/// icmp Pred, <induction variable>, <loop invariant limit>
struct LoopICmp {
ICmpInst::Predicate Pred;
const SCEVAddRecExpr *IV;
const SCEV *Limit;
LoopICmp(ICmpInst::Predicate Pred, const SCEVAddRecExpr *IV,
const SCEV *Limit)
: Pred(Pred), IV(IV), Limit(Limit) {}
LoopICmp() = default;
void dump() {
dbgs() << "LoopICmp Pred = " << Pred << ", IV = " << *IV
<< ", Limit = " << *Limit << "\n";
}
};
class LoopPredication {
AliasAnalysis *AA;
DominatorTree *DT;
ScalarEvolution *SE;
LoopInfo *LI;
MemorySSAUpdater *MSSAU;
Loop *L;
const DataLayout *DL;
BasicBlock *Preheader;
LoopICmp LatchCheck;
bool isSupportedStep(const SCEV* Step);
Optional<LoopICmp> parseLoopICmp(ICmpInst *ICI);
Optional<LoopICmp> parseLoopLatchICmp();
/// Return an insertion point suitable for inserting a safe to speculate
/// instruction whose only user will be 'User' which has operands 'Ops'. A
/// trivial result would be the at the User itself, but we try to return a
/// loop invariant location if possible.
Instruction *findInsertPt(Instruction *User, ArrayRef<Value*> Ops);
/// Same as above, *except* that this uses the SCEV definition of invariant
/// which is that an expression *can be made* invariant via SCEVExpander.
/// Thus, this version is only suitable for finding an insert point to be be
/// passed to SCEVExpander!
Instruction *findInsertPt(const SCEVExpander &Expander, Instruction *User,
ArrayRef<const SCEV *> Ops);
/// Return true if the value is known to produce a single fixed value across
/// all iterations on which it executes. Note that this does not imply
/// speculation safety. That must be established separately.
bool isLoopInvariantValue(const SCEV* S);
Value *expandCheck(SCEVExpander &Expander, Instruction *Guard,
ICmpInst::Predicate Pred, const SCEV *LHS,
const SCEV *RHS);
Optional<Value *> widenICmpRangeCheck(ICmpInst *ICI, SCEVExpander &Expander,
Instruction *Guard);
Optional<Value *> widenICmpRangeCheckIncrementingLoop(LoopICmp LatchCheck,
LoopICmp RangeCheck,
SCEVExpander &Expander,
Instruction *Guard);
Optional<Value *> widenICmpRangeCheckDecrementingLoop(LoopICmp LatchCheck,
LoopICmp RangeCheck,
SCEVExpander &Expander,
Instruction *Guard);
unsigned collectChecks(SmallVectorImpl<Value *> &Checks, Value *Condition,
SCEVExpander &Expander, Instruction *Guard);
bool widenGuardConditions(IntrinsicInst *II, SCEVExpander &Expander);
bool widenWidenableBranchGuardConditions(BranchInst *Guard, SCEVExpander &Expander);
// If the loop always exits through another block in the loop, we should not
// predicate based on the latch check. For example, the latch check can be a
// very coarse grained check and there can be more fine grained exit checks
// within the loop.
bool isLoopProfitableToPredicate();
bool predicateLoopExits(Loop *L, SCEVExpander &Rewriter);
public:
LoopPredication(AliasAnalysis *AA, DominatorTree *DT, ScalarEvolution *SE,
LoopInfo *LI, MemorySSAUpdater *MSSAU)
: AA(AA), DT(DT), SE(SE), LI(LI), MSSAU(MSSAU){};
bool runOnLoop(Loop *L);
};
class LoopPredicationLegacyPass : public LoopPass {
public:
static char ID;
LoopPredicationLegacyPass() : LoopPass(ID) {
initializeLoopPredicationLegacyPassPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<BranchProbabilityInfoWrapperPass>();
getLoopAnalysisUsage(AU);
AU.addPreserved<MemorySSAWrapperPass>();
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L))
return false;
auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *MSSAWP = getAnalysisIfAvailable<MemorySSAWrapperPass>();
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (MSSAWP)
MSSAU = std::make_unique<MemorySSAUpdater>(&MSSAWP->getMSSA());
auto *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
LoopPredication LP(AA, DT, SE, LI, MSSAU ? MSSAU.get() : nullptr);
return LP.runOnLoop(L);
}
};
char LoopPredicationLegacyPass::ID = 0;
} // end namespace
INITIALIZE_PASS_BEGIN(LoopPredicationLegacyPass, "loop-predication",
"Loop predication", false, false)
INITIALIZE_PASS_DEPENDENCY(BranchProbabilityInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_END(LoopPredicationLegacyPass, "loop-predication",
"Loop predication", false, false)
Pass *llvm::createLoopPredicationPass() {
return new LoopPredicationLegacyPass();
}
PreservedAnalyses LoopPredicationPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &U) {
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (AR.MSSA)
MSSAU = std::make_unique<MemorySSAUpdater>(AR.MSSA);
LoopPredication LP(&AR.AA, &AR.DT, &AR.SE, &AR.LI,
MSSAU ? MSSAU.get() : nullptr);
if (!LP.runOnLoop(&L))
return PreservedAnalyses::all();
auto PA = getLoopPassPreservedAnalyses();
if (AR.MSSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
Optional<LoopICmp>
LoopPredication::parseLoopICmp(ICmpInst *ICI) {
auto Pred = ICI->getPredicate();
auto *LHS = ICI->getOperand(0);
auto *RHS = ICI->getOperand(1);
const SCEV *LHSS = SE->getSCEV(LHS);
if (isa<SCEVCouldNotCompute>(LHSS))
return None;
const SCEV *RHSS = SE->getSCEV(RHS);
if (isa<SCEVCouldNotCompute>(RHSS))
return None;
// Canonicalize RHS to be loop invariant bound, LHS - a loop computable IV
if (SE->isLoopInvariant(LHSS, L)) {
std::swap(LHS, RHS);
std::swap(LHSS, RHSS);
Pred = ICmpInst::getSwappedPredicate(Pred);
}
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(LHSS);
if (!AR || AR->getLoop() != L)
return None;
return LoopICmp(Pred, AR, RHSS);
}
Value *LoopPredication::expandCheck(SCEVExpander &Expander,
Instruction *Guard,
ICmpInst::Predicate Pred, const SCEV *LHS,
const SCEV *RHS) {
Type *Ty = LHS->getType();
assert(Ty == RHS->getType() && "expandCheck operands have different types?");
if (SE->isLoopInvariant(LHS, L) && SE->isLoopInvariant(RHS, L)) {
IRBuilder<> Builder(Guard);
if (SE->isLoopEntryGuardedByCond(L, Pred, LHS, RHS))
return Builder.getTrue();
if (SE->isLoopEntryGuardedByCond(L, ICmpInst::getInversePredicate(Pred),
LHS, RHS))
return Builder.getFalse();
}
Value *LHSV =
Expander.expandCodeFor(LHS, Ty, findInsertPt(Expander, Guard, {LHS}));
Value *RHSV =
Expander.expandCodeFor(RHS, Ty, findInsertPt(Expander, Guard, {RHS}));
IRBuilder<> Builder(findInsertPt(Guard, {LHSV, RHSV}));
return Builder.CreateICmp(Pred, LHSV, RHSV);
}
// Returns true if its safe to truncate the IV to RangeCheckType.
// When the IV type is wider than the range operand type, we can still do loop
// predication, by generating SCEVs for the range and latch that are of the
// same type. We achieve this by generating a SCEV truncate expression for the
// latch IV. This is done iff truncation of the IV is a safe operation,
// without loss of information.
// Another way to achieve this is by generating a wider type SCEV for the
// range check operand, however, this needs a more involved check that
// operands do not overflow. This can lead to loss of information when the
// range operand is of the form: add i32 %offset, %iv. We need to prove that
// sext(x + y) is same as sext(x) + sext(y).
// This function returns true if we can safely represent the IV type in
// the RangeCheckType without loss of information.
static bool isSafeToTruncateWideIVType(const DataLayout &DL,
ScalarEvolution &SE,
const LoopICmp LatchCheck,
Type *RangeCheckType) {
if (!EnableIVTruncation)
return false;
assert(DL.getTypeSizeInBits(LatchCheck.IV->getType()).getFixedSize() >
DL.getTypeSizeInBits(RangeCheckType).getFixedSize() &&
"Expected latch check IV type to be larger than range check operand "
"type!");
// The start and end values of the IV should be known. This is to guarantee
// that truncating the wide type will not lose information.
auto *Limit = dyn_cast<SCEVConstant>(LatchCheck.Limit);
auto *Start = dyn_cast<SCEVConstant>(LatchCheck.IV->getStart());
if (!Limit || !Start)
return false;
// This check makes sure that the IV does not change sign during loop
// iterations. Consider latchType = i64, LatchStart = 5, Pred = ICMP_SGE,
// LatchEnd = 2, rangeCheckType = i32. If it's not a monotonic predicate, the
// IV wraps around, and the truncation of the IV would lose the range of
// iterations between 2^32 and 2^64.
if (!SE.getMonotonicPredicateType(LatchCheck.IV, LatchCheck.Pred))
return false;
// The active bits should be less than the bits in the RangeCheckType. This
// guarantees that truncating the latch check to RangeCheckType is a safe
// operation.
auto RangeCheckTypeBitSize =
DL.getTypeSizeInBits(RangeCheckType).getFixedSize();
return Start->getAPInt().getActiveBits() < RangeCheckTypeBitSize &&
Limit->getAPInt().getActiveBits() < RangeCheckTypeBitSize;
}
// Return an LoopICmp describing a latch check equivlent to LatchCheck but with
// the requested type if safe to do so. May involve the use of a new IV.
static Optional<LoopICmp> generateLoopLatchCheck(const DataLayout &DL,
ScalarEvolution &SE,
const LoopICmp LatchCheck,
Type *RangeCheckType) {
auto *LatchType = LatchCheck.IV->getType();
if (RangeCheckType == LatchType)
return LatchCheck;
// For now, bail out if latch type is narrower than range type.
if (DL.getTypeSizeInBits(LatchType).getFixedSize() <
DL.getTypeSizeInBits(RangeCheckType).getFixedSize())
return None;
if (!isSafeToTruncateWideIVType(DL, SE, LatchCheck, RangeCheckType))
return None;
// We can now safely identify the truncated version of the IV and limit for
// RangeCheckType.
LoopICmp NewLatchCheck;
NewLatchCheck.Pred = LatchCheck.Pred;
NewLatchCheck.IV = dyn_cast<SCEVAddRecExpr>(
SE.getTruncateExpr(LatchCheck.IV, RangeCheckType));
if (!NewLatchCheck.IV)
return None;
NewLatchCheck.Limit = SE.getTruncateExpr(LatchCheck.Limit, RangeCheckType);
LLVM_DEBUG(dbgs() << "IV of type: " << *LatchType
<< "can be represented as range check type:"
<< *RangeCheckType << "\n");
LLVM_DEBUG(dbgs() << "LatchCheck.IV: " << *NewLatchCheck.IV << "\n");
LLVM_DEBUG(dbgs() << "LatchCheck.Limit: " << *NewLatchCheck.Limit << "\n");
return NewLatchCheck;
}
bool LoopPredication::isSupportedStep(const SCEV* Step) {
return Step->isOne() || (Step->isAllOnesValue() && EnableCountDownLoop);
}
Instruction *LoopPredication::findInsertPt(Instruction *Use,
ArrayRef<Value*> Ops) {
for (Value *Op : Ops)
if (!L->isLoopInvariant(Op))
return Use;
return Preheader->getTerminator();
}
Instruction *LoopPredication::findInsertPt(const SCEVExpander &Expander,
Instruction *Use,
ArrayRef<const SCEV *> Ops) {
// Subtlety: SCEV considers things to be invariant if the value produced is
// the same across iterations. This is not the same as being able to
// evaluate outside the loop, which is what we actually need here.
for (const SCEV *Op : Ops)
if (!SE->isLoopInvariant(Op, L) ||
!Expander.isSafeToExpandAt(Op, Preheader->getTerminator()))
return Use;
return Preheader->getTerminator();
}
bool LoopPredication::isLoopInvariantValue(const SCEV* S) {
// Handling expressions which produce invariant results, but *haven't* yet
// been removed from the loop serves two important purposes.
// 1) Most importantly, it resolves a pass ordering cycle which would
// otherwise need us to iteration licm, loop-predication, and either
// loop-unswitch or loop-peeling to make progress on examples with lots of
// predicable range checks in a row. (Since, in the general case, we can't
// hoist the length checks until the dominating checks have been discharged
// as we can't prove doing so is safe.)
// 2) As a nice side effect, this exposes the value of peeling or unswitching
// much more obviously in the IR. Otherwise, the cost modeling for other
// transforms would end up needing to duplicate all of this logic to model a
// check which becomes predictable based on a modeled peel or unswitch.
//
// The cost of doing so in the worst case is an extra fill from the stack in
// the loop to materialize the loop invariant test value instead of checking
// against the original IV which is presumable in a register inside the loop.
// Such cases are presumably rare, and hint at missing oppurtunities for
// other passes.
if (SE->isLoopInvariant(S, L))
// Note: This the SCEV variant, so the original Value* may be within the
// loop even though SCEV has proven it is loop invariant.
return true;
// Handle a particular important case which SCEV doesn't yet know about which
// shows up in range checks on arrays with immutable lengths.
// TODO: This should be sunk inside SCEV.
if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S))
if (const auto *LI = dyn_cast<LoadInst>(U->getValue()))
if (LI->isUnordered() && L->hasLoopInvariantOperands(LI))
if (AA->pointsToConstantMemory(LI->getOperand(0)) ||
LI->hasMetadata(LLVMContext::MD_invariant_load))
return true;
return false;
}
Optional<Value *> LoopPredication::widenICmpRangeCheckIncrementingLoop(
LoopICmp LatchCheck, LoopICmp RangeCheck,
SCEVExpander &Expander, Instruction *Guard) {
auto *Ty = RangeCheck.IV->getType();
// Generate the widened condition for the forward loop:
// guardStart u< guardLimit &&
// latchLimit <pred> guardLimit - 1 - guardStart + latchStart
// where <pred> depends on the latch condition predicate. See the file
// header comment for the reasoning.
// guardLimit - guardStart + latchStart - 1
const SCEV *GuardStart = RangeCheck.IV->getStart();
const SCEV *GuardLimit = RangeCheck.Limit;
const SCEV *LatchStart = LatchCheck.IV->getStart();
const SCEV *LatchLimit = LatchCheck.Limit;
// Subtlety: We need all the values to be *invariant* across all iterations,
// but we only need to check expansion safety for those which *aren't*
// already guaranteed to dominate the guard.
if (!isLoopInvariantValue(GuardStart) ||
!isLoopInvariantValue(GuardLimit) ||
!isLoopInvariantValue(LatchStart) ||
!isLoopInvariantValue(LatchLimit)) {
LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
return None;
}
if (!Expander.isSafeToExpandAt(LatchStart, Guard) ||
!Expander.isSafeToExpandAt(LatchLimit, Guard)) {
LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
return None;
}
// guardLimit - guardStart + latchStart - 1
const SCEV *RHS =
SE->getAddExpr(SE->getMinusSCEV(GuardLimit, GuardStart),
SE->getMinusSCEV(LatchStart, SE->getOne(Ty)));
auto LimitCheckPred =
ICmpInst::getFlippedStrictnessPredicate(LatchCheck.Pred);
LLVM_DEBUG(dbgs() << "LHS: " << *LatchLimit << "\n");
LLVM_DEBUG(dbgs() << "RHS: " << *RHS << "\n");
LLVM_DEBUG(dbgs() << "Pred: " << LimitCheckPred << "\n");
auto *LimitCheck =
expandCheck(Expander, Guard, LimitCheckPred, LatchLimit, RHS);
auto *FirstIterationCheck = expandCheck(Expander, Guard, RangeCheck.Pred,
GuardStart, GuardLimit);
IRBuilder<> Builder(findInsertPt(Guard, {FirstIterationCheck, LimitCheck}));
return Builder.CreateAnd(FirstIterationCheck, LimitCheck);
}
Optional<Value *> LoopPredication::widenICmpRangeCheckDecrementingLoop(
LoopICmp LatchCheck, LoopICmp RangeCheck,
SCEVExpander &Expander, Instruction *Guard) {
auto *Ty = RangeCheck.IV->getType();
const SCEV *GuardStart = RangeCheck.IV->getStart();
const SCEV *GuardLimit = RangeCheck.Limit;
const SCEV *LatchStart = LatchCheck.IV->getStart();
const SCEV *LatchLimit = LatchCheck.Limit;
// Subtlety: We need all the values to be *invariant* across all iterations,
// but we only need to check expansion safety for those which *aren't*
// already guaranteed to dominate the guard.
if (!isLoopInvariantValue(GuardStart) ||
!isLoopInvariantValue(GuardLimit) ||
!isLoopInvariantValue(LatchStart) ||
!isLoopInvariantValue(LatchLimit)) {
LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
return None;
}
if (!Expander.isSafeToExpandAt(LatchStart, Guard) ||
!Expander.isSafeToExpandAt(LatchLimit, Guard)) {
LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
return None;
}
// The decrement of the latch check IV should be the same as the
// rangeCheckIV.
auto *PostDecLatchCheckIV = LatchCheck.IV->getPostIncExpr(*SE);
if (RangeCheck.IV != PostDecLatchCheckIV) {
LLVM_DEBUG(dbgs() << "Not the same. PostDecLatchCheckIV: "
<< *PostDecLatchCheckIV
<< " and RangeCheckIV: " << *RangeCheck.IV << "\n");
return None;
}
// Generate the widened condition for CountDownLoop:
// guardStart u< guardLimit &&
// latchLimit <pred> 1.
// See the header comment for reasoning of the checks.
auto LimitCheckPred =
ICmpInst::getFlippedStrictnessPredicate(LatchCheck.Pred);
auto *FirstIterationCheck = expandCheck(Expander, Guard,
ICmpInst::ICMP_ULT,
GuardStart, GuardLimit);
auto *LimitCheck = expandCheck(Expander, Guard, LimitCheckPred, LatchLimit,
SE->getOne(Ty));
IRBuilder<> Builder(findInsertPt(Guard, {FirstIterationCheck, LimitCheck}));
return Builder.CreateAnd(FirstIterationCheck, LimitCheck);
}
static void normalizePredicate(ScalarEvolution *SE, Loop *L,
LoopICmp& RC) {
// LFTR canonicalizes checks to the ICMP_NE/EQ form; normalize back to the
// ULT/UGE form for ease of handling by our caller.
if (ICmpInst::isEquality(RC.Pred) &&
RC.IV->getStepRecurrence(*SE)->isOne() &&
SE->isKnownPredicate(ICmpInst::ICMP_ULE, RC.IV->getStart(), RC.Limit))
RC.Pred = RC.Pred == ICmpInst::ICMP_NE ?
ICmpInst::ICMP_ULT : ICmpInst::ICMP_UGE;
}
/// If ICI can be widened to a loop invariant condition emits the loop
/// invariant condition in the loop preheader and return it, otherwise
/// returns None.
Optional<Value *> LoopPredication::widenICmpRangeCheck(ICmpInst *ICI,
SCEVExpander &Expander,
Instruction *Guard) {
LLVM_DEBUG(dbgs() << "Analyzing ICmpInst condition:\n");
LLVM_DEBUG(ICI->dump());
// parseLoopStructure guarantees that the latch condition is:
// ++i <pred> latchLimit, where <pred> is u<, u<=, s<, or s<=.
// We are looking for the range checks of the form:
// i u< guardLimit
auto RangeCheck = parseLoopICmp(ICI);
if (!RangeCheck) {
LLVM_DEBUG(dbgs() << "Failed to parse the loop latch condition!\n");
return None;
}
LLVM_DEBUG(dbgs() << "Guard check:\n");
LLVM_DEBUG(RangeCheck->dump());
if (RangeCheck->Pred != ICmpInst::ICMP_ULT) {
LLVM_DEBUG(dbgs() << "Unsupported range check predicate("
<< RangeCheck->Pred << ")!\n");
return None;
}
auto *RangeCheckIV = RangeCheck->IV;
if (!RangeCheckIV->isAffine()) {
LLVM_DEBUG(dbgs() << "Range check IV is not affine!\n");
return None;
}
auto *Step = RangeCheckIV->getStepRecurrence(*SE);
// We cannot just compare with latch IV step because the latch and range IVs
// may have different types.
if (!isSupportedStep(Step)) {
LLVM_DEBUG(dbgs() << "Range check and latch have IVs different steps!\n");
return None;
}
auto *Ty = RangeCheckIV->getType();
auto CurrLatchCheckOpt = generateLoopLatchCheck(*DL, *SE, LatchCheck, Ty);
if (!CurrLatchCheckOpt) {
LLVM_DEBUG(dbgs() << "Failed to generate a loop latch check "
"corresponding to range type: "
<< *Ty << "\n");
return None;
}
LoopICmp CurrLatchCheck = *CurrLatchCheckOpt;
// At this point, the range and latch step should have the same type, but need
// not have the same value (we support both 1 and -1 steps).
assert(Step->getType() ==
CurrLatchCheck.IV->getStepRecurrence(*SE)->getType() &&
"Range and latch steps should be of same type!");
if (Step != CurrLatchCheck.IV->getStepRecurrence(*SE)) {
LLVM_DEBUG(dbgs() << "Range and latch have different step values!\n");
return None;
}
if (Step->isOne())
return widenICmpRangeCheckIncrementingLoop(CurrLatchCheck, *RangeCheck,
Expander, Guard);
else {
assert(Step->isAllOnesValue() && "Step should be -1!");
return widenICmpRangeCheckDecrementingLoop(CurrLatchCheck, *RangeCheck,
Expander, Guard);
}
}
unsigned LoopPredication::collectChecks(SmallVectorImpl<Value *> &Checks,
Value *Condition,
SCEVExpander &Expander,
Instruction *Guard) {
unsigned NumWidened = 0;
// The guard condition is expected to be in form of:
// cond1 && cond2 && cond3 ...
// Iterate over subconditions looking for icmp conditions which can be
// widened across loop iterations. Widening these conditions remember the
// resulting list of subconditions in Checks vector.
SmallVector<Value *, 4> Worklist(1, Condition);
SmallPtrSet<Value *, 4> Visited;
Value *WideableCond = nullptr;
do {
Value *Condition = Worklist.pop_back_val();
if (!Visited.insert(Condition).second)
continue;
Value *LHS, *RHS;
using namespace llvm::PatternMatch;
if (match(Condition, m_And(m_Value(LHS), m_Value(RHS)))) {
Worklist.push_back(LHS);
Worklist.push_back(RHS);
continue;
}
if (match(Condition,
m_Intrinsic<Intrinsic::experimental_widenable_condition>())) {
// Pick any, we don't care which
WideableCond = Condition;
continue;
}
if (ICmpInst *ICI = dyn_cast<ICmpInst>(Condition)) {
if (auto NewRangeCheck = widenICmpRangeCheck(ICI, Expander,
Guard)) {
Checks.push_back(*NewRangeCheck);
NumWidened++;
continue;
}
}
// Save the condition as is if we can't widen it
Checks.push_back(Condition);
} while (!Worklist.empty());
// At the moment, our matching logic for wideable conditions implicitly
// assumes we preserve the form: (br (and Cond, WC())). FIXME
// Note that if there were multiple calls to wideable condition in the
// traversal, we only need to keep one, and which one is arbitrary.
if (WideableCond)
Checks.push_back(WideableCond);
return NumWidened;
}
bool LoopPredication::widenGuardConditions(IntrinsicInst *Guard,
SCEVExpander &Expander) {
LLVM_DEBUG(dbgs() << "Processing guard:\n");
LLVM_DEBUG(Guard->dump());
TotalConsidered++;
SmallVector<Value *, 4> Checks;
unsigned NumWidened = collectChecks(Checks, Guard->getOperand(0), Expander,
Guard);
if (NumWidened == 0)
return false;
TotalWidened += NumWidened;
// Emit the new guard condition
IRBuilder<> Builder(findInsertPt(Guard, Checks));
Value *AllChecks = Builder.CreateAnd(Checks);
auto *OldCond = Guard->getOperand(0);
Guard->setOperand(0, AllChecks);
RecursivelyDeleteTriviallyDeadInstructions(OldCond, nullptr /* TLI */, MSSAU);
LLVM_DEBUG(dbgs() << "Widened checks = " << NumWidened << "\n");
return true;
}
bool LoopPredication::widenWidenableBranchGuardConditions(
BranchInst *BI, SCEVExpander &Expander) {
assert(isGuardAsWidenableBranch(BI) && "Must be!");
LLVM_DEBUG(dbgs() << "Processing guard:\n");
LLVM_DEBUG(BI->dump());
TotalConsidered++;
SmallVector<Value *, 4> Checks;
unsigned NumWidened = collectChecks(Checks, BI->getCondition(),
Expander, BI);
if (NumWidened == 0)
return false;
TotalWidened += NumWidened;
// Emit the new guard condition
IRBuilder<> Builder(findInsertPt(BI, Checks));
Value *AllChecks = Builder.CreateAnd(Checks);
auto *OldCond = BI->getCondition();
BI->setCondition(AllChecks);
RecursivelyDeleteTriviallyDeadInstructions(OldCond, nullptr /* TLI */, MSSAU);
assert(isGuardAsWidenableBranch(BI) &&
"Stopped being a guard after transform?");
LLVM_DEBUG(dbgs() << "Widened checks = " << NumWidened << "\n");
return true;
}
Optional<LoopICmp> LoopPredication::parseLoopLatchICmp() {
using namespace PatternMatch;
BasicBlock *LoopLatch = L->getLoopLatch();
if (!LoopLatch) {
LLVM_DEBUG(dbgs() << "The loop doesn't have a single latch!\n");
return None;
}
auto *BI = dyn_cast<BranchInst>(LoopLatch->getTerminator());
if (!BI || !BI->isConditional()) {
LLVM_DEBUG(dbgs() << "Failed to match the latch terminator!\n");
return None;
}
BasicBlock *TrueDest = BI->getSuccessor(0);
assert(
(TrueDest == L->getHeader() || BI->getSuccessor(1) == L->getHeader()) &&
"One of the latch's destinations must be the header");
auto *ICI = dyn_cast<ICmpInst>(BI->getCondition());
if (!ICI) {
LLVM_DEBUG(dbgs() << "Failed to match the latch condition!\n");
return None;
}
auto Result = parseLoopICmp(ICI);
if (!Result) {
LLVM_DEBUG(dbgs() << "Failed to parse the loop latch condition!\n");
return None;
}
if (TrueDest != L->getHeader())
Result->Pred = ICmpInst::getInversePredicate(Result->Pred);
// Check affine first, so if it's not we don't try to compute the step
// recurrence.
if (!Result->IV->isAffine()) {
LLVM_DEBUG(dbgs() << "The induction variable is not affine!\n");
return None;
}
auto *Step = Result->IV->getStepRecurrence(*SE);
if (!isSupportedStep(Step)) {
LLVM_DEBUG(dbgs() << "Unsupported loop stride(" << *Step << ")!\n");
return None;
}
auto IsUnsupportedPredicate = [](const SCEV *Step, ICmpInst::Predicate Pred) {
if (Step->isOne()) {
return Pred != ICmpInst::ICMP_ULT && Pred != ICmpInst::ICMP_SLT &&
Pred != ICmpInst::ICMP_ULE && Pred != ICmpInst::ICMP_SLE;
} else {
assert(Step->isAllOnesValue() && "Step should be -1!");
return Pred != ICmpInst::ICMP_UGT && Pred != ICmpInst::ICMP_SGT &&
Pred != ICmpInst::ICMP_UGE && Pred != ICmpInst::ICMP_SGE;
}
};
normalizePredicate(SE, L, *Result);
if (IsUnsupportedPredicate(Step, Result->Pred)) {
LLVM_DEBUG(dbgs() << "Unsupported loop latch predicate(" << Result->Pred
<< ")!\n");
return None;
}
return Result;
}
bool LoopPredication::isLoopProfitableToPredicate() {
if (SkipProfitabilityChecks)
return true;
SmallVector<std::pair<BasicBlock *, BasicBlock *>, 8> ExitEdges;
L->getExitEdges(ExitEdges);
// If there is only one exiting edge in the loop, it is always profitable to
// predicate the loop.
if (ExitEdges.size() == 1)
return true;
// Calculate the exiting probabilities of all exiting edges from the loop,
// starting with the LatchExitProbability.
// Heuristic for profitability: If any of the exiting blocks' probability of
// exiting the loop is larger than exiting through the latch block, it's not
// profitable to predicate the loop.
auto *LatchBlock = L->getLoopLatch();
assert(LatchBlock && "Should have a single latch at this point!");
auto *LatchTerm = LatchBlock->getTerminator();
assert(LatchTerm->getNumSuccessors() == 2 &&
"expected to be an exiting block with 2 succs!");
unsigned LatchBrExitIdx =
LatchTerm->getSuccessor(0) == L->getHeader() ? 1 : 0;
// We compute branch probabilities without BPI. We do not rely on BPI since
// Loop predication is usually run in an LPM and BPI is only preserved
// lossily within loop pass managers, while BPI has an inherent notion of
// being complete for an entire function.
// If the latch exits into a deoptimize or an unreachable block, do not
// predicate on that latch check.
auto *LatchExitBlock = LatchTerm->getSuccessor(LatchBrExitIdx);
if (isa<UnreachableInst>(LatchTerm) ||
LatchExitBlock->getTerminatingDeoptimizeCall())
return false;
auto IsValidProfileData = [](MDNode *ProfileData, const Instruction *Term) {
if (!ProfileData || !ProfileData->getOperand(0))
return false;
if (MDString *MDS = dyn_cast<MDString>(ProfileData->getOperand(0)))
if (!MDS->getString().equals("branch_weights"))
return false;
if (ProfileData->getNumOperands() != 1 + Term->getNumSuccessors())
return false;
return true;
};
MDNode *LatchProfileData = LatchTerm->getMetadata(LLVMContext::MD_prof);
// Latch terminator has no valid profile data, so nothing to check
// profitability on.
if (!IsValidProfileData(LatchProfileData, LatchTerm))
return true;
auto ComputeBranchProbability =
[&](const BasicBlock *ExitingBlock,
const BasicBlock *ExitBlock) -> BranchProbability {
auto *Term = ExitingBlock->getTerminator();
MDNode *ProfileData = Term->getMetadata(LLVMContext::MD_prof);
unsigned NumSucc = Term->getNumSuccessors();
if (IsValidProfileData(ProfileData, Term)) {
uint64_t Numerator = 0, Denominator = 0, ProfVal = 0;
for (unsigned i = 0; i < NumSucc; i++) {
ConstantInt *CI =
mdconst::extract<ConstantInt>(ProfileData->getOperand(i + 1));
ProfVal = CI->getValue().getZExtValue();
if (Term->getSuccessor(i) == ExitBlock)
Numerator += ProfVal;
Denominator += ProfVal;
}
return BranchProbability::getBranchProbability(Numerator, Denominator);
} else {
assert(LatchBlock != ExitingBlock &&
"Latch term should always have profile data!");
// No profile data, so we choose the weight as 1/num_of_succ(Src)
return BranchProbability::getBranchProbability(1, NumSucc);
}
};
BranchProbability LatchExitProbability =
ComputeBranchProbability(LatchBlock, LatchExitBlock);
// Protect against degenerate inputs provided by the user. Providing a value
// less than one, can invert the definition of profitable loop predication.
float ScaleFactor = LatchExitProbabilityScale;
if (ScaleFactor < 1) {
LLVM_DEBUG(
dbgs()
<< "Ignored user setting for loop-predication-latch-probability-scale: "
<< LatchExitProbabilityScale << "\n");
LLVM_DEBUG(dbgs() << "The value is set to 1.0\n");
ScaleFactor = 1.0;
}
const auto LatchProbabilityThreshold = LatchExitProbability * ScaleFactor;
for (const auto &ExitEdge : ExitEdges) {
BranchProbability ExitingBlockProbability =
ComputeBranchProbability(ExitEdge.first, ExitEdge.second);
// Some exiting edge has higher probability than the latch exiting edge.
// No longer profitable to predicate.
if (ExitingBlockProbability > LatchProbabilityThreshold)
return false;
}
// We have concluded that the most probable way to exit from the
// loop is through the latch (or there's no profile information and all
// exits are equally likely).
return true;
}
/// If we can (cheaply) find a widenable branch which controls entry into the
/// loop, return it.
static BranchInst *FindWidenableTerminatorAboveLoop(Loop *L, LoopInfo &LI) {
// Walk back through any unconditional executed blocks and see if we can find
// a widenable condition which seems to control execution of this loop. Note
// that we predict that maythrow calls are likely untaken and thus that it's
// profitable to widen a branch before a maythrow call with a condition
// afterwards even though that may cause the slow path to run in a case where
// it wouldn't have otherwise.
BasicBlock *BB = L->getLoopPreheader();
if (!BB)
return nullptr;
do {
if (BasicBlock *Pred = BB->getSinglePredecessor())
if (BB == Pred->getSingleSuccessor()) {
BB = Pred;
continue;
}
break;
} while (true);
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
auto *Term = Pred->getTerminator();
Value *Cond, *WC;
BasicBlock *IfTrueBB, *IfFalseBB;
if (parseWidenableBranch(Term, Cond, WC, IfTrueBB, IfFalseBB) &&
IfTrueBB == BB)
return cast<BranchInst>(Term);
}
return nullptr;
}
/// Return the minimum of all analyzeable exit counts. This is an upper bound
/// on the actual exit count. If there are not at least two analyzeable exits,
/// returns SCEVCouldNotCompute.
static const SCEV *getMinAnalyzeableBackedgeTakenCount(ScalarEvolution &SE,
DominatorTree &DT,
Loop *L) {
SmallVector<BasicBlock *, 16> ExitingBlocks;
L->getExitingBlocks(ExitingBlocks);
SmallVector<const SCEV *, 4> ExitCounts;
for (BasicBlock *ExitingBB : ExitingBlocks) {
const SCEV *ExitCount = SE.getExitCount(L, ExitingBB);
if (isa<SCEVCouldNotCompute>(ExitCount))
continue;
assert(DT.dominates(ExitingBB, L->getLoopLatch()) &&
"We should only have known counts for exiting blocks that "
"dominate latch!");
ExitCounts.push_back(ExitCount);
}
if (ExitCounts.size() < 2)
return SE.getCouldNotCompute();
return SE.getUMinFromMismatchedTypes(ExitCounts);
}
/// This implements an analogous, but entirely distinct transform from the main
/// loop predication transform. This one is phrased in terms of using a
/// widenable branch *outside* the loop to allow us to simplify loop exits in a
/// following loop. This is close in spirit to the IndVarSimplify transform
/// of the same name, but is materially different widening loosens legality
/// sharply.
bool LoopPredication::predicateLoopExits(Loop *L, SCEVExpander &Rewriter) {
// The transformation performed here aims to widen a widenable condition
// above the loop such that all analyzeable exit leading to deopt are dead.
// It assumes that the latch is the dominant exit for profitability and that
// exits branching to deoptimizing blocks are rarely taken. It relies on the
// semantics of widenable expressions for legality. (i.e. being able to fall
// down the widenable path spuriously allows us to ignore exit order,
// unanalyzeable exits, side effects, exceptional exits, and other challenges
// which restrict the applicability of the non-WC based version of this
// transform in IndVarSimplify.)
//
// NOTE ON POISON/UNDEF - We're hoisting an expression above guards which may
// imply flags on the expression being hoisted and inserting new uses (flags
// are only correct for current uses). The result is that we may be
// inserting a branch on the value which can be either poison or undef. In
// this case, the branch can legally go either way; we just need to avoid
// introducing UB. This is achieved through the use of the freeze
// instruction.
SmallVector<BasicBlock *, 16> ExitingBlocks;
L->getExitingBlocks(ExitingBlocks);
if (ExitingBlocks.empty())
return false; // Nothing to do.
auto *Latch = L->getLoopLatch();
if (!Latch)
return false;
auto *WidenableBR = FindWidenableTerminatorAboveLoop(L, *LI);
if (!WidenableBR)
return false;
const SCEV *LatchEC = SE->getExitCount(L, Latch);
if (isa<SCEVCouldNotCompute>(LatchEC))
return false; // profitability - want hot exit in analyzeable set
// At this point, we have found an analyzeable latch, and a widenable
// condition above the loop. If we have a widenable exit within the loop
// (for which we can't compute exit counts), drop the ability to further
// widen so that we gain ability to analyze it's exit count and perform this
// transform. TODO: It'd be nice to know for sure the exit became
// analyzeable after dropping widenability.
bool ChangedLoop = false;
for (auto *ExitingBB : ExitingBlocks) {
if (LI->getLoopFor(ExitingBB) != L)
continue;
auto *BI = dyn_cast<BranchInst>(ExitingBB->getTerminator());
if (!BI)
continue;
Use *Cond, *WC;
BasicBlock *IfTrueBB, *IfFalseBB;
if (parseWidenableBranch(BI, Cond, WC, IfTrueBB, IfFalseBB) &&
L->contains(IfTrueBB)) {
WC->set(ConstantInt::getTrue(IfTrueBB->getContext()));
ChangedLoop = true;
}
}
if (ChangedLoop)
SE->forgetLoop(L);
// The use of umin(all analyzeable exits) instead of latch is subtle, but
// important for profitability. We may have a loop which hasn't been fully
// canonicalized just yet. If the exit we chose to widen is provably never
// taken, we want the widened form to *also* be provably never taken. We
// can't guarantee this as a current unanalyzeable exit may later become
// analyzeable, but we can at least avoid the obvious cases.
const SCEV *MinEC = getMinAnalyzeableBackedgeTakenCount(*SE, *DT, L);
if (isa<SCEVCouldNotCompute>(MinEC) || MinEC->getType()->isPointerTy() ||
!SE->isLoopInvariant(MinEC, L) ||
!Rewriter.isSafeToExpandAt(MinEC, WidenableBR))
return ChangedLoop;
// Subtlety: We need to avoid inserting additional uses of the WC. We know
// that it can only have one transitive use at the moment, and thus moving
// that use to just before the branch and inserting code before it and then
// modifying the operand is legal.
auto *IP = cast<Instruction>(WidenableBR->getCondition());
// Here we unconditionally modify the IR, so after this point we should return
// only `true`!
IP->moveBefore(WidenableBR);
if (MSSAU)
if (auto *MUD = MSSAU->getMemorySSA()->getMemoryAccess(IP))
MSSAU->moveToPlace(MUD, WidenableBR->getParent(),
MemorySSA::BeforeTerminator);
Rewriter.setInsertPoint(IP);
IRBuilder<> B(IP);
bool InvalidateLoop = false;
Value *MinECV = nullptr; // lazily generated if needed
for (BasicBlock *ExitingBB : ExitingBlocks) {
// If our exiting block exits multiple loops, we can only rewrite the
// innermost one. Otherwise, we're changing how many times the innermost
// loop runs before it exits.
if (LI->getLoopFor(ExitingBB) != L)
continue;
// Can't rewrite non-branch yet.
auto *BI = dyn_cast<BranchInst>(ExitingBB->getTerminator());
if (!BI)
continue;
// If already constant, nothing to do.
if (isa<Constant>(BI->getCondition()))
continue;
const SCEV *ExitCount = SE->getExitCount(L, ExitingBB);
if (isa<SCEVCouldNotCompute>(ExitCount) ||
ExitCount->getType()->isPointerTy() ||
!Rewriter.isSafeToExpandAt(ExitCount, WidenableBR))
continue;
const bool ExitIfTrue = !L->contains(*succ_begin(ExitingBB));
BasicBlock *ExitBB = BI->getSuccessor(ExitIfTrue ? 0 : 1);
if (!ExitBB->getPostdominatingDeoptimizeCall())
continue;
/// Here we can be fairly sure that executing this exit will most likely
/// lead to executing llvm.experimental.deoptimize.
/// This is a profitability heuristic, not a legality constraint.
// If we found a widenable exit condition, do two things:
// 1) fold the widened exit test into the widenable condition
// 2) fold the branch to untaken - avoids infinite looping
Value *ECV = Rewriter.expandCodeFor(ExitCount);
if (!MinECV)
MinECV = Rewriter.expandCodeFor(MinEC);
Value *RHS = MinECV;
if (ECV->getType() != RHS->getType()) {
Type *WiderTy = SE->getWiderType(ECV->getType(), RHS->getType());
ECV = B.CreateZExt(ECV, WiderTy);
RHS = B.CreateZExt(RHS, WiderTy);
}
assert(!Latch || DT->dominates(ExitingBB, Latch));
Value *NewCond = B.CreateICmp(ICmpInst::ICMP_UGT, ECV, RHS);
// Freeze poison or undef to an arbitrary bit pattern to ensure we can
// branch without introducing UB. See NOTE ON POISON/UNDEF above for
// context.
NewCond = B.CreateFreeze(NewCond);
widenWidenableBranch(WidenableBR, NewCond);
Value *OldCond = BI->getCondition();
BI->setCondition(ConstantInt::get(OldCond->getType(), !ExitIfTrue));
InvalidateLoop = true;
}
if (InvalidateLoop)
// We just mutated a bunch of loop exits changing there exit counts
// widely. We need to force recomputation of the exit counts given these
// changes. Note that all of the inserted exits are never taken, and
// should be removed next time the CFG is modified.
SE->forgetLoop(L);
// Always return `true` since we have moved the WidenableBR's condition.
return true;
}
bool LoopPredication::runOnLoop(Loop *Loop) {
L = Loop;
LLVM_DEBUG(dbgs() << "Analyzing ");
LLVM_DEBUG(L->dump());
Module *M = L->getHeader()->getModule();
// There is nothing to do if the module doesn't use guards
auto *GuardDecl =
M->getFunction(Intrinsic::getName(Intrinsic::experimental_guard));
bool HasIntrinsicGuards = GuardDecl && !GuardDecl->use_empty();
auto *WCDecl = M->getFunction(
Intrinsic::getName(Intrinsic::experimental_widenable_condition));
bool HasWidenableConditions =
PredicateWidenableBranchGuards && WCDecl && !WCDecl->use_empty();
if (!HasIntrinsicGuards && !HasWidenableConditions)
return false;
DL = &M->getDataLayout();
Preheader = L->getLoopPreheader();
if (!Preheader)
return false;
auto LatchCheckOpt = parseLoopLatchICmp();
if (!LatchCheckOpt)
return false;
LatchCheck = *LatchCheckOpt;
LLVM_DEBUG(dbgs() << "Latch check:\n");
LLVM_DEBUG(LatchCheck.dump());
if (!isLoopProfitableToPredicate()) {
LLVM_DEBUG(dbgs() << "Loop not profitable to predicate!\n");
return false;
}
// Collect all the guards into a vector and process later, so as not
// to invalidate the instruction iterator.
SmallVector<IntrinsicInst *, 4> Guards;
SmallVector<BranchInst *, 4> GuardsAsWidenableBranches;
for (const auto BB : L->blocks()) {
for (auto &I : *BB)
if (isGuard(&I))
Guards.push_back(cast<IntrinsicInst>(&I));
if (PredicateWidenableBranchGuards &&
isGuardAsWidenableBranch(BB->getTerminator()))
GuardsAsWidenableBranches.push_back(
cast<BranchInst>(BB->getTerminator()));
}
SCEVExpander Expander(*SE, *DL, "loop-predication");
bool Changed = false;
for (auto *Guard : Guards)
Changed |= widenGuardConditions(Guard, Expander);
for (auto *Guard : GuardsAsWidenableBranches)
Changed |= widenWidenableBranchGuardConditions(Guard, Expander);
Changed |= predicateLoopExits(L, Expander);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
return Changed;
}
Reference in New Issue View Git Blame Copy Permalink