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

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//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
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//
// This file implements constant propagation and merging:
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//
// Specifically, this:
// * Converts instructions like "add int 1, 2" into 3
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//
// Notice that:
// * This pass has a habit of making definitions be dead. It is a good idea
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// to run a DIE pass sometime after running this pass.
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//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include <set>
using namespace llvm;
#define DEBUG_TYPE "constprop"
STATISTIC(NumInstKilled, "Number of instructions killed");
namespace {
struct ConstantPropagation : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
ConstantPropagation() : FunctionPass(ID) {
initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<TargetLibraryInfoWrapperPass>();
}
};
}
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char ConstantPropagation::ID = 0;
INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
"Simple constant propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(ConstantPropagation, "constprop",
"Simple constant propagation", false, false)
FunctionPass *llvm::createConstantPropagationPass() {
return new ConstantPropagation();
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}
bool ConstantPropagation::runOnFunction(Function &F) {
// Initialize the worklist to all of the instructions ready to process...
std::set<Instruction*> WorkList;
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for (Instruction &I: instructions(&F))
WorkList.insert(&I);
bool Changed = false;
const DataLayout &DL = F.getParent()->getDataLayout();
TargetLibraryInfo *TLI =
&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
while (!WorkList.empty()) {
Instruction *I = *WorkList.begin();
WorkList.erase(WorkList.begin()); // Get an element from the worklist...
if (!I->use_empty()) // Don't muck with dead instructions...
if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
// Add all of the users of this instruction to the worklist, they might
// be constant propagatable now...
[C++11] Add range based accessors for the Use-Def chain of a Value. This requires a number of steps. 1) Move value_use_iterator into the Value class as an implementation detail 2) Change it to actually be a *Use* iterator rather than a *User* iterator. 3) Add an adaptor which is a User iterator that always looks through the Use to the User. 4) Wrap these in Value::use_iterator and Value::user_iterator typedefs. 5) Add the range adaptors as Value::uses() and Value::users(). 6) Update *all* of the callers to correctly distinguish between whether they wanted a use_iterator (and to explicitly dig out the User when needed), or a user_iterator which makes the Use itself totally opaque. Because #6 requires churning essentially everything that walked the Use-Def chains, I went ahead and added all of the range adaptors and switched them to range-based loops where appropriate. Also because the renaming requires at least churning every line of code, it didn't make any sense to split these up into multiple commits -- all of which would touch all of the same lies of code. The result is still not quite optimal. The Value::use_iterator is a nice regular iterator, but Value::user_iterator is an iterator over User*s rather than over the User objects themselves. As a consequence, it fits a bit awkwardly into the range-based world and it has the weird extra-dereferencing 'operator->' that so many of our iterators have. I think this could be fixed by providing something which transforms a range of T&s into a range of T*s, but that *can* be separated into another patch, and it isn't yet 100% clear whether this is the right move. However, this change gets us most of the benefit and cleans up a substantial amount of code around Use and User. =] llvm-svn: 203364
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for (User *U : I->users())
WorkList.insert(cast<Instruction>(U));
// Replace all of the uses of a variable with uses of the constant.
I->replaceAllUsesWith(C);
// Remove the dead instruction.
WorkList.erase(I);
I->eraseFromParent();
// We made a change to the function...
Changed = true;
++NumInstKilled;
}
}
return Changed;
}