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

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//===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the Correlated Value Propagation pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/CorrelatedValuePropagation.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LazyValueInfo.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "correlated-value-propagation"
STATISTIC(NumPhis, "Number of phis propagated");
STATISTIC(NumSelects, "Number of selects propagated");
STATISTIC(NumMemAccess, "Number of memory access targets propagated");
STATISTIC(NumCmps, "Number of comparisons propagated");
STATISTIC(NumReturns, "Number of return values propagated");
STATISTIC(NumDeadCases, "Number of switch cases removed");
STATISTIC(NumSDivs, "Number of sdiv converted to udiv");
STATISTIC(NumAShrs, "Number of ashr converted to lshr");
STATISTIC(NumSRems, "Number of srem converted to urem");
static cl::opt<bool> DontProcessAdds("cvp-dont-process-adds", cl::init(true));
namespace {
class CorrelatedValuePropagation : public FunctionPass {
public:
static char ID;
CorrelatedValuePropagation(): FunctionPass(ID) {
initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<LazyValueInfoWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
}
};
}
char CorrelatedValuePropagation::ID = 0;
INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
"Value Propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
"Value Propagation", false, false)
// Public interface to the Value Propagation pass
Pass *llvm::createCorrelatedValuePropagationPass() {
return new CorrelatedValuePropagation();
}
static bool processSelect(SelectInst *S, LazyValueInfo *LVI) {
if (S->getType()->isVectorTy()) return false;
if (isa<Constant>(S->getOperand(0))) return false;
Constant *C = LVI->getConstant(S->getOperand(0), S->getParent(), S);
if (!C) return false;
ConstantInt *CI = dyn_cast<ConstantInt>(C);
if (!CI) return false;
Value *ReplaceWith = S->getOperand(1);
Value *Other = S->getOperand(2);
if (!CI->isOne()) std::swap(ReplaceWith, Other);
if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());
S->replaceAllUsesWith(ReplaceWith);
S->eraseFromParent();
++NumSelects;
return true;
}
static bool processPHI(PHINode *P, LazyValueInfo *LVI,
const SimplifyQuery &SQ) {
bool Changed = false;
BasicBlock *BB = P->getParent();
for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
Value *Incoming = P->getIncomingValue(i);
if (isa<Constant>(Incoming)) continue;
Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
// Look if the incoming value is a select with a scalar condition for which
// LVI can tells us the value. In that case replace the incoming value with
// the appropriate value of the select. This often allows us to remove the
// select later.
if (!V) {
SelectInst *SI = dyn_cast<SelectInst>(Incoming);
if (!SI) continue;
Value *Condition = SI->getCondition();
if (!Condition->getType()->isVectorTy()) {
if (Constant *C = LVI->getConstantOnEdge(
Condition, P->getIncomingBlock(i), BB, P)) {
if (C->isOneValue()) {
V = SI->getTrueValue();
} else if (C->isZeroValue()) {
V = SI->getFalseValue();
}
// Once LVI learns to handle vector types, we could also add support
// for vector type constants that are not all zeroes or all ones.
}
}
// Look if the select has a constant but LVI tells us that the incoming
// value can never be that constant. In that case replace the incoming
// value with the other value of the select. This often allows us to
// remove the select later.
if (!V) {
Constant *C = dyn_cast<Constant>(SI->getFalseValue());
if (!C) continue;
if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
P->getIncomingBlock(i), BB, P) !=
LazyValueInfo::False)
continue;
V = SI->getTrueValue();
}
DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
}
P->setIncomingValue(i, V);
Changed = true;
}
if (Value *V = SimplifyInstruction(P, SQ)) {
P->replaceAllUsesWith(V);
P->eraseFromParent();
Changed = true;
}
if (Changed)
++NumPhis;
return Changed;
}
static bool processMemAccess(Instruction *I, LazyValueInfo *LVI) {
Value *Pointer = nullptr;
if (LoadInst *L = dyn_cast<LoadInst>(I))
Pointer = L->getPointerOperand();
else
Pointer = cast<StoreInst>(I)->getPointerOperand();
if (isa<Constant>(Pointer)) return false;
Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
if (!C) return false;
++NumMemAccess;
I->replaceUsesOfWith(Pointer, C);
return true;
}
/// See if LazyValueInfo's ability to exploit edge conditions or range
/// information is sufficient to prove this comparison. Even for local
/// conditions, this can sometimes prove conditions instcombine can't by
/// exploiting range information.
static bool processCmp(CmpInst *C, LazyValueInfo *LVI) {
Value *Op0 = C->getOperand(0);
Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
if (!Op1) return false;
// As a policy choice, we choose not to waste compile time on anything where
// the comparison is testing local values. While LVI can sometimes reason
// about such cases, it's not its primary purpose. We do make sure to do
// the block local query for uses from terminator instructions, but that's
// handled in the code for each terminator.
auto *I = dyn_cast<Instruction>(Op0);
if (I && I->getParent() == C->getParent())
return false;
LazyValueInfo::Tristate Result =
LVI->getPredicateAt(C->getPredicate(), Op0, Op1, C);
if (Result == LazyValueInfo::Unknown) return false;
++NumCmps;
if (Result == LazyValueInfo::True)
C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
else
C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
C->eraseFromParent();
return true;
}
/// Simplify a switch instruction by removing cases which can never fire. If the
/// uselessness of a case could be determined locally then constant propagation
/// would already have figured it out. Instead, walk the predecessors and
/// statically evaluate cases based on information available on that edge. Cases
/// that cannot fire no matter what the incoming edge can safely be removed. If
/// a case fires on every incoming edge then the entire switch can be removed
/// and replaced with a branch to the case destination.
static bool processSwitch(SwitchInst *SI, LazyValueInfo *LVI) {
Value *Cond = SI->getCondition();
BasicBlock *BB = SI->getParent();
// If the condition was defined in same block as the switch then LazyValueInfo
// currently won't say anything useful about it, though in theory it could.
if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
return false;
// If the switch is unreachable then trying to improve it is a waste of time.
pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
if (PB == PE) return false;
// Analyse each switch case in turn. This is done in reverse order so that
// removing a case doesn't cause trouble for the iteration.
bool Changed = false;
for (auto CI = SI->case_begin(), CE = SI->case_end(); CI != CE;) {
ConstantInt *Case = CI->getCaseValue();
// Check to see if the switch condition is equal to/not equal to the case
// value on every incoming edge, equal/not equal being the same each time.
LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
for (pred_iterator PI = PB; PI != PE; ++PI) {
// Is the switch condition equal to the case value?
LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
Cond, Case, *PI,
BB, SI);
// Give up on this case if nothing is known.
if (Value == LazyValueInfo::Unknown) {
State = LazyValueInfo::Unknown;
break;
}
// If this was the first edge to be visited, record that all other edges
// need to give the same result.
if (PI == PB) {
State = Value;
continue;
}
// If this case is known to fire for some edges and known not to fire for
// others then there is nothing we can do - give up.
if (Value != State) {
State = LazyValueInfo::Unknown;
break;
}
}
if (State == LazyValueInfo::False) {
// This case never fires - remove it.
CI->getCaseSuccessor()->removePredecessor(BB);
CI = SI->removeCase(CI);
CE = SI->case_end();
// The condition can be modified by removePredecessor's PHI simplification
// logic.
Cond = SI->getCondition();
++NumDeadCases;
Changed = true;
continue;
}
if (State == LazyValueInfo::True) {
// This case always fires. Arrange for the switch to be turned into an
// unconditional branch by replacing the switch condition with the case
// value.
SI->setCondition(Case);
NumDeadCases += SI->getNumCases();
Changed = true;
break;
}
// Increment the case iterator sense we didn't delete it.
++CI;
}
if (Changed)
// If the switch has been simplified to the point where it can be replaced
// by a branch then do so now.
ConstantFoldTerminator(BB);
return Changed;
}
/// Infer nonnull attributes for the arguments at the specified callsite.
static bool processCallSite(CallSite CS, LazyValueInfo *LVI) {
SmallVector<unsigned, 4> Indices;
unsigned ArgNo = 0;
for (Value *V : CS.args()) {
PointerType *Type = dyn_cast<PointerType>(V->getType());
// Try to mark pointer typed parameters as non-null. We skip the
// relatively expensive analysis for constants which are obviously either
// null or non-null to start with.
if (Type && !CS.paramHasAttr(ArgNo, Attribute::NonNull) &&
!isa<Constant>(V) &&
LVI->getPredicateAt(ICmpInst::ICMP_EQ, V,
ConstantPointerNull::get(Type),
CS.getInstruction()) == LazyValueInfo::False)
Indices.push_back(ArgNo + 1);
ArgNo++;
}
assert(ArgNo == CS.arg_size() && "sanity check");
if (Indices.empty())
return false;
AttributeList AS = CS.getAttributes();
LLVMContext &Ctx = CS.getInstruction()->getContext();
AS = AS.addAttribute(Ctx, Indices, Attribute::get(Ctx, Attribute::NonNull));
CS.setAttributes(AS);
return true;
}
// Helper function to rewrite srem and sdiv. As a policy choice, we choose not
// to waste compile time on anything where the operands are local defs. While
// LVI can sometimes reason about such cases, it's not its primary purpose.
static bool hasLocalDefs(BinaryOperator *SDI) {
for (Value *O : SDI->operands()) {
auto *I = dyn_cast<Instruction>(O);
if (I && I->getParent() == SDI->getParent())
return true;
}
return false;
}
static bool hasPositiveOperands(BinaryOperator *SDI, LazyValueInfo *LVI) {
Constant *Zero = ConstantInt::get(SDI->getType(), 0);
for (Value *O : SDI->operands()) {
auto Result = LVI->getPredicateAt(ICmpInst::ICMP_SGE, O, Zero, SDI);
if (Result != LazyValueInfo::True)
return false;
}
return true;
}
static bool processSRem(BinaryOperator *SDI, LazyValueInfo *LVI) {
if (SDI->getType()->isVectorTy() || hasLocalDefs(SDI) ||
!hasPositiveOperands(SDI, LVI))
return false;
++NumSRems;
auto *BO = BinaryOperator::CreateURem(SDI->getOperand(0), SDI->getOperand(1),
SDI->getName(), SDI);
SDI->replaceAllUsesWith(BO);
SDI->eraseFromParent();
return true;
}
/// See if LazyValueInfo's ability to exploit edge conditions or range
/// information is sufficient to prove the both operands of this SDiv are
/// positive. If this is the case, replace the SDiv with a UDiv. Even for local
/// conditions, this can sometimes prove conditions instcombine can't by
/// exploiting range information.
static bool processSDiv(BinaryOperator *SDI, LazyValueInfo *LVI) {
if (SDI->getType()->isVectorTy() || hasLocalDefs(SDI) ||
!hasPositiveOperands(SDI, LVI))
return false;
++NumSDivs;
auto *BO = BinaryOperator::CreateUDiv(SDI->getOperand(0), SDI->getOperand(1),
SDI->getName(), SDI);
BO->setIsExact(SDI->isExact());
SDI->replaceAllUsesWith(BO);
SDI->eraseFromParent();
return true;
}
static bool processAShr(BinaryOperator *SDI, LazyValueInfo *LVI) {
if (SDI->getType()->isVectorTy() || hasLocalDefs(SDI))
return false;
Constant *Zero = ConstantInt::get(SDI->getType(), 0);
if (LVI->getPredicateAt(ICmpInst::ICMP_SGE, SDI->getOperand(0), Zero, SDI) !=
LazyValueInfo::True)
return false;
++NumAShrs;
auto *BO = BinaryOperator::CreateLShr(SDI->getOperand(0), SDI->getOperand(1),
SDI->getName(), SDI);
BO->setIsExact(SDI->isExact());
SDI->replaceAllUsesWith(BO);
SDI->eraseFromParent();
return true;
}
static bool processAdd(BinaryOperator *AddOp, LazyValueInfo *LVI) {
typedef OverflowingBinaryOperator OBO;
if (DontProcessAdds)
return false;
if (AddOp->getType()->isVectorTy() || hasLocalDefs(AddOp))
return false;
bool NSW = AddOp->hasNoSignedWrap();
bool NUW = AddOp->hasNoUnsignedWrap();
if (NSW && NUW)
return false;
BasicBlock *BB = AddOp->getParent();
Value *LHS = AddOp->getOperand(0);
Value *RHS = AddOp->getOperand(1);
ConstantRange LRange = LVI->getConstantRange(LHS, BB, AddOp);
// Initialize RRange only if we need it. If we know that guaranteed no wrap
// range for the given LHS range is empty don't spend time calculating the
// range for the RHS.
Optional<ConstantRange> RRange;
auto LazyRRange = [&] () {
if (!RRange)
RRange = LVI->getConstantRange(RHS, BB, AddOp);
return RRange.getValue();
};
bool Changed = false;
if (!NUW) {
ConstantRange NUWRange =
LRange.makeGuaranteedNoWrapRegion(BinaryOperator::Add, LRange,
OBO::NoUnsignedWrap);
if (!NUWRange.isEmptySet()) {
bool NewNUW = NUWRange.contains(LazyRRange());
AddOp->setHasNoUnsignedWrap(NewNUW);
Changed |= NewNUW;
}
}
if (!NSW) {
ConstantRange NSWRange =
LRange.makeGuaranteedNoWrapRegion(BinaryOperator::Add, LRange,
OBO::NoSignedWrap);
if (!NSWRange.isEmptySet()) {
bool NewNSW = NSWRange.contains(LazyRRange());
AddOp->setHasNoSignedWrap(NewNSW);
Changed |= NewNSW;
}
}
return Changed;
}
static Constant *getConstantAt(Value *V, Instruction *At, LazyValueInfo *LVI) {
if (Constant *C = LVI->getConstant(V, At->getParent(), At))
return C;
// TODO: The following really should be sunk inside LVI's core algorithm, or
// at least the outer shims around such.
auto *C = dyn_cast<CmpInst>(V);
if (!C) return nullptr;
Value *Op0 = C->getOperand(0);
Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
if (!Op1) return nullptr;
LazyValueInfo::Tristate Result =
LVI->getPredicateAt(C->getPredicate(), Op0, Op1, At);
if (Result == LazyValueInfo::Unknown)
return nullptr;
return (Result == LazyValueInfo::True) ?
ConstantInt::getTrue(C->getContext()) :
ConstantInt::getFalse(C->getContext());
}
static bool runImpl(Function &F, LazyValueInfo *LVI, const SimplifyQuery &SQ) {
bool FnChanged = false;
// Visiting in a pre-order depth-first traversal causes us to simplify early
// blocks before querying later blocks (which require us to analyze early
// blocks). Eagerly simplifying shallow blocks means there is strictly less
// work to do for deep blocks. This also means we don't visit unreachable
// blocks.
for (BasicBlock *BB : depth_first(&F.getEntryBlock())) {
bool BBChanged = false;
for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
Instruction *II = &*BI++;
switch (II->getOpcode()) {
case Instruction::Select:
BBChanged |= processSelect(cast<SelectInst>(II), LVI);
break;
case Instruction::PHI:
BBChanged |= processPHI(cast<PHINode>(II), LVI, SQ);
break;
case Instruction::ICmp:
case Instruction::FCmp:
BBChanged |= processCmp(cast<CmpInst>(II), LVI);
break;
case Instruction::Load:
case Instruction::Store:
BBChanged |= processMemAccess(II, LVI);
break;
case Instruction::Call:
case Instruction::Invoke:
BBChanged |= processCallSite(CallSite(II), LVI);
break;
case Instruction::SRem:
BBChanged |= processSRem(cast<BinaryOperator>(II), LVI);
break;
case Instruction::SDiv:
BBChanged |= processSDiv(cast<BinaryOperator>(II), LVI);
break;
case Instruction::AShr:
BBChanged |= processAShr(cast<BinaryOperator>(II), LVI);
break;
case Instruction::Add:
BBChanged |= processAdd(cast<BinaryOperator>(II), LVI);
break;
}
}
Instruction *Term = BB->getTerminator();
switch (Term->getOpcode()) {
case Instruction::Switch:
BBChanged |= processSwitch(cast<SwitchInst>(Term), LVI);
break;
case Instruction::Ret: {
auto *RI = cast<ReturnInst>(Term);
// Try to determine the return value if we can. This is mainly here to
// simplify the writing of unit tests, but also helps to enable IPO by
// constant folding the return values of callees.
auto *RetVal = RI->getReturnValue();
if (!RetVal) break; // handle "ret void"
if (isa<Constant>(RetVal)) break; // nothing to do
if (auto *C = getConstantAt(RetVal, RI, LVI)) {
++NumReturns;
RI->replaceUsesOfWith(RetVal, C);
BBChanged = true;
}
}
};
FnChanged |= BBChanged;
}
return FnChanged;
}
bool CorrelatedValuePropagation::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
LazyValueInfo *LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
return runImpl(F, LVI, getBestSimplifyQuery(*this, F));
}
PreservedAnalyses
CorrelatedValuePropagationPass::run(Function &F, FunctionAnalysisManager &AM) {
LazyValueInfo *LVI = &AM.getResult<LazyValueAnalysis>(F);
bool Changed = runImpl(F, LVI, getBestSimplifyQuery(AM, F));
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<GlobalsAA>();
return PA;
}
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