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llvm-project/llvm/lib/Transforms/Scalar/LoopPredication.cpp

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//===-- 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/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.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 {
class LoopPredication {
/// 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() {}
void dump() {
dbgs() << "LoopICmp Pred = " << Pred << ", IV = " << *IV
<< ", Limit = " << *Limit << "\n";
}
};
AliasAnalysis *AA;
ScalarEvolution *SE;
BranchProbabilityInfo *BPI;
Loop *L;
const DataLayout *DL;
BasicBlock *Preheader;
LoopICmp LatchCheck;
bool isSupportedStep(const SCEV* Step);
Optional<LoopICmp> parseLoopICmp(ICmpInst *ICI) {
return parseLoopICmp(ICI->getPredicate(), ICI->getOperand(0),
ICI->getOperand(1));
}
Optional<LoopICmp> parseLoopICmp(ICmpInst::Predicate Pred, Value *LHS,
Value *RHS);
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(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 seperately.
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. We identify such unprofitable loops through BPI.
bool isLoopProfitableToPredicate();
// 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.
bool isSafeToTruncateWideIVType(Type *RangeCheckType);
// Return the loopLatchCheck corresponding to the RangeCheckType if safe to do
// so.
Optional<LoopICmp> generateLoopLatchCheck(Type *RangeCheckType);
public:
LoopPredication(AliasAnalysis *AA, ScalarEvolution *SE,
BranchProbabilityInfo *BPI)
: AA(AA), SE(SE), BPI(BPI){};
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);
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L))
return false;
auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
BranchProbabilityInfo &BPI =
getAnalysis<BranchProbabilityInfoWrapperPass>().getBPI();
auto *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
LoopPredication LP(AA, SE, &BPI);
return LP.runOnLoop(L);
}
};
char LoopPredicationLegacyPass::ID = 0;
} // end namespace llvm
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) {
const auto &FAM =
AM.getResult<FunctionAnalysisManagerLoopProxy>(L, AR).getManager();
Function *F = L.getHeader()->getParent();
auto *BPI = FAM.getCachedResult<BranchProbabilityAnalysis>(*F);
LoopPredication LP(&AR.AA, &AR.SE, BPI);
if (!LP.runOnLoop(&L))
return PreservedAnalyses::all();
return getLoopPassPreservedAnalyses();
}
Optional<LoopPredication::LoopICmp>
LoopPredication::parseLoopICmp(ICmpInst::Predicate Pred, Value *LHS,
Value *RHS) {
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(Guard, {LHS}));
Value *RHSV = Expander.expandCodeFor(RHS, Ty, findInsertPt(Guard, {RHS}));
IRBuilder<> Builder(findInsertPt(Guard, {LHSV, RHSV}));
return Builder.CreateICmp(Pred, LHSV, RHSV);
}
Optional<LoopPredication::LoopICmp>
LoopPredication::generateLoopLatchCheck(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) < DL->getTypeSizeInBits(RangeCheckType))
return None;
if (!isSafeToTruncateWideIVType(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(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) ||
!isSafeToExpandAt(Op, Preheader->getTerminator(), *SE))
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->getMetadata(LLVMContext::MD_invariant_load) != nullptr)
return true;
return false;
}
Optional<Value *> LoopPredication::widenICmpRangeCheckIncrementingLoop(
LoopPredication::LoopICmp LatchCheck, LoopPredication::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 (!isSafeToExpandAt(LatchStart, Guard, *SE) ||
!isSafeToExpandAt(LatchLimit, Guard, *SE)) {
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(
LoopPredication::LoopICmp LatchCheck, LoopPredication::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 (!isSafeToExpandAt(LatchStart, Guard, *SE) ||
!isSafeToExpandAt(LatchLimit, Guard, *SE)) {
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);
}
/// 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(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.getValue());
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 *LastCheck = nullptr;
for (auto *Check : Checks)
if (!LastCheck)
LastCheck = Check;
else
LastCheck = Builder.CreateAnd(LastCheck, Check);
auto *OldCond = Guard->getOperand(0);
Guard->setOperand(0, LastCheck);
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
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 *LastCheck = nullptr;
for (auto *Check : Checks)
if (!LastCheck)
LastCheck = Check;
else
LastCheck = Builder.CreateAnd(LastCheck, Check);
auto *OldCond = BI->getCondition();
BI->setCondition(LastCheck);
assert(isGuardAsWidenableBranch(BI) &&
"Stopped being a guard after transform?");
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
LLVM_DEBUG(dbgs() << "Widened checks = " << NumWidened << "\n");
return true;
}
Optional<LoopPredication::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;
}
ICmpInst::Predicate Pred;
Value *LHS, *RHS;
BasicBlock *TrueDest, *FalseDest;
if (!match(LoopLatch->getTerminator(),
m_Br(m_ICmp(Pred, m_Value(LHS), m_Value(RHS)), TrueDest,
FalseDest))) {
LLVM_DEBUG(dbgs() << "Failed to match the latch terminator!\n");
return None;
}
assert((TrueDest == L->getHeader() || FalseDest == L->getHeader()) &&
"One of the latch's destinations must be the header");
if (TrueDest != L->getHeader())
Pred = ICmpInst::getInversePredicate(Pred);
auto Result = parseLoopICmp(Pred, LHS, RHS);
if (!Result) {
LLVM_DEBUG(dbgs() << "Failed to parse the loop latch condition!\n");
return None;
}
// 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;
}
};
if (IsUnsupportedPredicate(Step, Result->Pred)) {
LLVM_DEBUG(dbgs() << "Unsupported loop latch predicate(" << Result->Pred
<< ")!\n");
return None;
}
return Result;
}
// Returns true if its safe to truncate the IV to RangeCheckType.
bool LoopPredication::isSafeToTruncateWideIVType(Type *RangeCheckType) {
if (!EnableIVTruncation)
return false;
assert(DL->getTypeSizeInBits(LatchCheck.IV->getType()) >
DL->getTypeSizeInBits(RangeCheckType) &&
"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.
bool Increasing;
if (!SE->isMonotonicPredicate(LatchCheck.IV, LatchCheck.Pred, Increasing))
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);
return Start->getAPInt().getActiveBits() < RangeCheckTypeBitSize &&
Limit->getAPInt().getActiveBits() < RangeCheckTypeBitSize;
}
bool LoopPredication::isLoopProfitableToPredicate() {
if (SkipProfitabilityChecks || !BPI)
return true;
SmallVector<std::pair<const BasicBlock *, const 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;
BranchProbability LatchExitProbability =
BPI->getEdgeProbability(LatchBlock, LatchBrExitIdx);
// 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 =
BPI->getEdgeProbability(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;
}
// Using BPI, 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;
}
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()));
}
if (Guards.empty() && GuardsAsWidenableBranches.empty())
return false;
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);
return Changed;
}
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