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//===-- SCCP.cpp ----------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements Interprocedural Sparse Conditional Constant Propagation.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/SCCP.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueLattice.h"
#include "llvm/Analysis/ValueLatticeUtils.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/AttributeMask.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/ModRef.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/IPO/FunctionSpecialization.h"
#include "llvm/Transforms/Scalar/SCCP.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SCCPSolver.h"
using namespace llvm;
#define DEBUG_TYPE "sccp"
STATISTIC(NumInstRemoved, "Number of instructions removed");
STATISTIC(NumArgsElimed ,"Number of arguments constant propagated");
STATISTIC(NumGlobalConst, "Number of globals found to be constant");
STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
STATISTIC(NumInstReplaced,
"Number of instructions replaced with (simpler) instruction");
static cl::opt<unsigned> FuncSpecMaxIters(
"funcspec-max-iters", cl::init(10), cl::Hidden, cl::desc(
"The maximum number of iterations function specialization is run"));
static void findReturnsToZap(Function &F,
SmallVector<ReturnInst *, 8> &ReturnsToZap,
SCCPSolver &Solver) {
// We can only do this if we know that nothing else can call the function.
if (!Solver.isArgumentTrackedFunction(&F))
return;
if (Solver.mustPreserveReturn(&F)) {
LLVM_DEBUG(
dbgs()
<< "Can't zap returns of the function : " << F.getName()
<< " due to present musttail or \"clang.arc.attachedcall\" call of "
"it\n");
return;
}
assert(
all_of(F.users(),
[&Solver](User *U) {
if (isa<Instruction>(U) &&
!Solver.isBlockExecutable(cast<Instruction>(U)->getParent()))
return true;
// Non-callsite uses are not impacted by zapping. Also, constant
// uses (like blockaddresses) could stuck around, without being
// used in the underlying IR, meaning we do not have lattice
// values for them.
if (!isa<CallBase>(U))
return true;
if (U->getType()->isStructTy()) {
return all_of(Solver.getStructLatticeValueFor(U),
[](const ValueLatticeElement &LV) {
return !SCCPSolver::isOverdefined(LV);
});
}
// We don't consider assume-like intrinsics to be actual address
// captures.
if (auto *II = dyn_cast<IntrinsicInst>(U)) {
if (II->isAssumeLikeIntrinsic())
return true;
}
return !SCCPSolver::isOverdefined(Solver.getLatticeValueFor(U));
}) &&
"We can only zap functions where all live users have a concrete value");
for (BasicBlock &BB : F) {
if (CallInst *CI = BB.getTerminatingMustTailCall()) {
LLVM_DEBUG(dbgs() << "Can't zap return of the block due to present "
<< "musttail call : " << *CI << "\n");
(void)CI;
return;
}
if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator()))
if (!isa<UndefValue>(RI->getOperand(0)))
ReturnsToZap.push_back(RI);
}
}
static bool runIPSCCP(
Module &M, const DataLayout &DL, FunctionAnalysisManager *FAM,
std::function<const TargetLibraryInfo &(Function &)> GetTLI,
std::function<TargetTransformInfo &(Function &)> GetTTI,
std::function<AssumptionCache &(Function &)> GetAC,
std::function<DominatorTree &(Function &)> GetDT,
std::function<BlockFrequencyInfo &(Function &)> GetBFI,
bool IsFuncSpecEnabled) {
SCCPSolver Solver(DL, GetTLI, M.getContext());
FunctionSpecializer Specializer(Solver, M, FAM, GetBFI, GetTLI, GetTTI,
GetAC);
// Loop over all functions, marking arguments to those with their addresses
// taken or that are external as overdefined.
for (Function &F : M) {
if (F.isDeclaration())
continue;
DominatorTree &DT = GetDT(F);
AssumptionCache &AC = GetAC(F);
Solver.addPredicateInfo(F, DT, AC);
// Determine if we can track the function's return values. If so, add the
// function to the solver's set of return-tracked functions.
if (canTrackReturnsInterprocedurally(&F))
Solver.addTrackedFunction(&F);
// Determine if we can track the function's arguments. If so, add the
// function to the solver's set of argument-tracked functions.
if (canTrackArgumentsInterprocedurally(&F)) {
Solver.addArgumentTrackedFunction(&F);
continue;
}
// Assume the function is called.
Solver.markBlockExecutable(&F.front());
for (Argument &AI : F.args())
Solver.trackValueOfArgument(&AI);
}
// Determine if we can track any of the module's global variables. If so, add
// the global variables we can track to the solver's set of tracked global
// variables.
for (GlobalVariable &G : M.globals()) {
G.removeDeadConstantUsers();
if (canTrackGlobalVariableInterprocedurally(&G))
Solver.trackValueOfGlobalVariable(&G);
}
// Solve for constants.
Solver.solveWhileResolvedUndefsIn(M);
if (IsFuncSpecEnabled) {
unsigned Iters = 0;
while (Iters++ < FuncSpecMaxIters && Specializer.run());
}
// Iterate over all of the instructions in the module, replacing them with
// constants if we have found them to be of constant values.
bool MadeChanges = false;
for (Function &F : M) {
if (F.isDeclaration())
continue;
SmallVector<BasicBlock *, 512> BlocksToErase;
if (Solver.isBlockExecutable(&F.front())) {
bool ReplacedPointerArg = false;
for (Argument &Arg : F.args()) {
if (!Arg.use_empty() && Solver.tryToReplaceWithConstant(&Arg)) {
ReplacedPointerArg |= Arg.getType()->isPointerTy();
++NumArgsElimed;
}
}
// If we replaced an argument, we may now also access a global (currently
// classified as "other" memory). Update memory attribute to reflect this.
if (ReplacedPointerArg) {
auto UpdateAttrs = [&](AttributeList AL) {
MemoryEffects ME = AL.getMemoryEffects();
if (ME == MemoryEffects::unknown())
return AL;
ME |= MemoryEffects(IRMemLocation::Other,
ME.getModRef(IRMemLocation::ArgMem));
return AL.addFnAttribute(
F.getContext(),
Attribute::getWithMemoryEffects(F.getContext(), ME));
};
F.setAttributes(UpdateAttrs(F.getAttributes()));
for (User *U : F.users()) {
auto *CB = dyn_cast<CallBase>(U);
if (!CB || CB->getCalledFunction() != &F)
continue;
CB->setAttributes(UpdateAttrs(CB->getAttributes()));
}
}
MadeChanges |= ReplacedPointerArg;
}
SmallPtrSet<Value *, 32> InsertedValues;
for (BasicBlock &BB : F) {
if (!Solver.isBlockExecutable(&BB)) {
LLVM_DEBUG(dbgs() << " BasicBlock Dead:" << BB);
++NumDeadBlocks;
MadeChanges = true;
if (&BB != &F.front())
BlocksToErase.push_back(&BB);
continue;
}
MadeChanges |= Solver.simplifyInstsInBlock(
BB, InsertedValues, NumInstRemoved, NumInstReplaced);
}
DominatorTree *DT = FAM->getCachedResult<DominatorTreeAnalysis>(F);
PostDominatorTree *PDT = FAM->getCachedResult<PostDominatorTreeAnalysis>(F);
DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
// Change dead blocks to unreachable. We do it after replacing constants
// in all executable blocks, because changeToUnreachable may remove PHI
// nodes in executable blocks we found values for. The function's entry
// block is not part of BlocksToErase, so we have to handle it separately.
for (BasicBlock *BB : BlocksToErase) {
NumInstRemoved += changeToUnreachable(BB->getFirstNonPHIOrDbg(),
/*PreserveLCSSA=*/false, &DTU);
}
if (!Solver.isBlockExecutable(&F.front()))
NumInstRemoved += changeToUnreachable(F.front().getFirstNonPHIOrDbg(),
/*PreserveLCSSA=*/false, &DTU);
BasicBlock *NewUnreachableBB = nullptr;
for (BasicBlock &BB : F)
MadeChanges |= Solver.removeNonFeasibleEdges(&BB, DTU, NewUnreachableBB);
for (BasicBlock *DeadBB : BlocksToErase)
if (!DeadBB->hasAddressTaken())
DTU.deleteBB(DeadBB);
for (BasicBlock &BB : F) {
for (Instruction &Inst : llvm::make_early_inc_range(BB)) {
if (Solver.getPredicateInfoFor(&Inst)) {
if (auto *II = dyn_cast<IntrinsicInst>(&Inst)) {
if (II->getIntrinsicID() == Intrinsic::ssa_copy) {
Value *Op = II->getOperand(0);
Inst.replaceAllUsesWith(Op);
Inst.eraseFromParent();
}
}
}
}
}
}
// If we inferred constant or undef return values for a function, we replaced
// all call uses with the inferred value. This means we don't need to bother
// actually returning anything from the function. Replace all return
// instructions with return undef.
//
// Do this in two stages: first identify the functions we should process, then
// actually zap their returns. This is important because we can only do this
// if the address of the function isn't taken. In cases where a return is the
// last use of a function, the order of processing functions would affect
// whether other functions are optimizable.
SmallVector<ReturnInst*, 8> ReturnsToZap;
for (const auto &I : Solver.getTrackedRetVals()) {
Function *F = I.first;
const ValueLatticeElement &ReturnValue = I.second;
// If there is a known constant range for the return value, add range
// attribute to the return value.
if (ReturnValue.isConstantRange() &&
!ReturnValue.getConstantRange().isSingleElement()) {
// Do not add range metadata if the return value may include undef.
if (ReturnValue.isConstantRangeIncludingUndef())
continue;
// Do not touch existing attribute for now.
// TODO: We should be able to take the intersection of the existing
// attribute and the inferred range.
if (F->hasRetAttribute(Attribute::Range))
continue;
auto &CR = ReturnValue.getConstantRange();
F->addRangeRetAttr(CR);
continue;
}
if (F->getReturnType()->isVoidTy())
continue;
if (SCCPSolver::isConstant(ReturnValue) || ReturnValue.isUnknownOrUndef())
findReturnsToZap(*F, ReturnsToZap, Solver);
}
for (auto *F : Solver.getMRVFunctionsTracked()) {
assert(F->getReturnType()->isStructTy() &&
"The return type should be a struct");
StructType *STy = cast<StructType>(F->getReturnType());
if (Solver.isStructLatticeConstant(F, STy))
findReturnsToZap(*F, ReturnsToZap, Solver);
}
// Zap all returns which we've identified as zap to change.
SmallSetVector<Function *, 8> FuncZappedReturn;
for (ReturnInst *RI : ReturnsToZap) {
Function *F = RI->getParent()->getParent();
RI->setOperand(0, UndefValue::get(F->getReturnType()));
// Record all functions that are zapped.
FuncZappedReturn.insert(F);
}
// Remove the returned attribute for zapped functions and the
// corresponding call sites.
// Also remove any attributes that convert an undef return value into
// immediate undefined behavior
AttributeMask UBImplyingAttributes =
AttributeFuncs::getUBImplyingAttributes();
for (Function *F : FuncZappedReturn) {
for (Argument &A : F->args())
F->removeParamAttr(A.getArgNo(), Attribute::Returned);
F->removeRetAttrs(UBImplyingAttributes);
for (Use &U : F->uses()) {
CallBase *CB = dyn_cast<CallBase>(U.getUser());
if (!CB) {
assert(isa<BlockAddress>(U.getUser()) ||
(isa<Constant>(U.getUser()) &&
all_of(U.getUser()->users(), [](const User *UserUser) {
return cast<IntrinsicInst>(UserUser)->isAssumeLikeIntrinsic();
})));
continue;
}
for (Use &Arg : CB->args())
CB->removeParamAttr(CB->getArgOperandNo(&Arg), Attribute::Returned);
CB->removeRetAttrs(UBImplyingAttributes);
}
}
// If we inferred constant or undef values for globals variables, we can
// delete the global and any stores that remain to it.
for (const auto &I : make_early_inc_range(Solver.getTrackedGlobals())) {
GlobalVariable *GV = I.first;
if (SCCPSolver::isOverdefined(I.second))
continue;
LLVM_DEBUG(dbgs() << "Found that GV '" << GV->getName()
<< "' is constant!\n");
while (!GV->use_empty()) {
StoreInst *SI = cast<StoreInst>(GV->user_back());
SI->eraseFromParent();
}
// Try to create a debug constant expression for the global variable
// initializer value.
SmallVector<DIGlobalVariableExpression *, 1> GVEs;
GV->getDebugInfo(GVEs);
if (GVEs.size() == 1) {
DIBuilder DIB(M);
if (DIExpression *InitExpr = getExpressionForConstant(
DIB, *GV->getInitializer(), *GV->getValueType()))
GVEs[0]->replaceOperandWith(1, InitExpr);
}
MadeChanges = true;
M.eraseGlobalVariable(GV);
++NumGlobalConst;
}
return MadeChanges;
}
PreservedAnalyses IPSCCPPass::run(Module &M, ModuleAnalysisManager &AM) {
const DataLayout &DL = M.getDataLayout();
auto &FAM = AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
auto GetTLI = [&FAM](Function &F) -> const TargetLibraryInfo & {
return FAM.getResult<TargetLibraryAnalysis>(F);
};
auto GetTTI = [&FAM](Function &F) -> TargetTransformInfo & {
return FAM.getResult<TargetIRAnalysis>(F);
};
auto GetAC = [&FAM](Function &F) -> AssumptionCache & {
return FAM.getResult<AssumptionAnalysis>(F);
};
auto GetDT = [&FAM](Function &F) -> DominatorTree & {
return FAM.getResult<DominatorTreeAnalysis>(F);
};
auto GetBFI = [&FAM](Function &F) -> BlockFrequencyInfo & {
return FAM.getResult<BlockFrequencyAnalysis>(F);
};
if (!runIPSCCP(M, DL, &FAM, GetTLI, GetTTI, GetAC, GetDT, GetBFI,
isFuncSpecEnabled()))
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<DominatorTreeAnalysis>();
PA.preserve<PostDominatorTreeAnalysis>();
PA.preserve<FunctionAnalysisManagerModuleProxy>();
return PA;
}