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//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
// 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 sparse conditional constant propagation and merging:
//
// Specifically, this:
// * Assumes values are constant unless proven otherwise
// * Assumes BasicBlocks are dead unless proven otherwise
// * Proves values to be constant, and replaces them with constants
// * Proves conditional branches to be unconditional
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/SCCP.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SCCPSolver.h"
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "sccp"
STATISTIC(NumInstRemoved, "Number of instructions removed");
STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
STATISTIC(NumInstReplaced,
"Number of instructions replaced with (simpler) instruction");
// runSCCP() - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
static bool runSCCP(Function &F, const DataLayout &DL,
const TargetLibraryInfo *TLI, DomTreeUpdater &DTU) {
LLVM_DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
SCCPSolver Solver(
DL, [TLI](Function &F) -> const TargetLibraryInfo & { return *TLI; },
F.getContext());
// Mark the first block of the function as being executable.
Solver.markBlockExecutable(&F.front());
// Mark all arguments to the function as being overdefined.
for (Argument &AI : F.args())
Solver.markOverdefined(&AI);
// Solve for constants.
bool ResolvedUndefs = true;
while (ResolvedUndefs) {
Solver.solve();
LLVM_DEBUG(dbgs() << "RESOLVING UNDEFs\n");
ResolvedUndefs = Solver.resolvedUndefsIn(F);
}
bool MadeChanges = false;
// If we decided that there are basic blocks that are dead in this function,
// delete their contents now. Note that we cannot actually delete the blocks,
// as we cannot modify the CFG of the function.
SmallPtrSet<Value *, 32> InsertedValues;
SmallVector<BasicBlock *, 8> BlocksToErase;
for (BasicBlock &BB : F) {
if (!Solver.isBlockExecutable(&BB)) {
LLVM_DEBUG(dbgs() << " BasicBlock Dead:" << BB);
++NumDeadBlocks;
BlocksToErase.push_back(&BB);
MadeChanges = true;
continue;
}
MadeChanges |= Solver.simplifyInstsInBlock(BB, InsertedValues,
NumInstRemoved, NumInstReplaced);
}
// Remove unreachable blocks and non-feasible edges.
for (BasicBlock *DeadBB : BlocksToErase)
NumInstRemoved += changeToUnreachable(DeadBB->getFirstNonPHI(),
/*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);
return MadeChanges;
}
PreservedAnalyses SCCPPass::run(Function &F, FunctionAnalysisManager &AM) {
const DataLayout &DL = F.getDataLayout();
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
if (!runSCCP(F, DL, &TLI, DTU))
return PreservedAnalyses::all();
auto PA = PreservedAnalyses();
PA.preserve<DominatorTreeAnalysis>();
return PA;
}
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