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//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements an _extremely_ simple interprocedural constant
// propagation pass. It could certainly be improved in many different ways,
// like using a worklist. This pass makes arguments dead, but does not remove
// them. The existing dead argument elimination pass should be run after this
// to clean up the mess.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Constants.h"
#include "llvm/Support/CallSite.h"
#include "Support/Statistic.h"
using namespace llvm;
namespace {
Statistic<> NumArgumentsProped("ipconstprop",
"Number of args turned into constants");
/// IPCP - The interprocedural constant propagation pass
///
struct IPCP : public Pass {
bool run(Module &M);
private:
bool processFunction(Function &F);
};
RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation");
}
Pass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
bool IPCP::run(Module &M) {
bool Changed = false;
bool LocalChange = true;
// FIXME: instead of using smart algorithms, we just iterate until we stop
// making changes.
while (LocalChange) {
LocalChange = false;
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
if (!I->isExternal() && I->hasInternalLinkage())
LocalChange |= processFunction(*I);
Changed |= LocalChange;
}
return Changed;
}
/// processFunction - Look at all uses of the specified function. If all uses
/// are direct call sites, and all pass a particular constant in for an
/// argument, propagate that constant in as the argument.
///
bool IPCP::processFunction(Function &F) {
if (F.aempty() || F.use_empty()) return false; // No arguments? Early exit.
std::vector<std::pair<Constant*, bool> > ArgumentConstants;
ArgumentConstants.resize(F.asize());
unsigned NumNonconstant = 0;
for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
if (!isa<Instruction>(*I))
return false; // Used by a non-instruction, do not transform
else {
CallSite CS = CallSite::get(cast<Instruction>(*I));
if (CS.getInstruction() == 0 ||
CS.getCalledFunction() != &F)
return false; // Not a direct call site?
// Check out all of the potentially constant arguments
CallSite::arg_iterator AI = CS.arg_begin();
for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
if (*AI == &F) return false; // Passes the function into itself
if (!ArgumentConstants[i].second) {
if (Constant *C = dyn_cast<Constant>(*AI)) {
if (!ArgumentConstants[i].first)
ArgumentConstants[i].first = C;
else if (ArgumentConstants[i].first != C) {
// Became non-constant
ArgumentConstants[i].second = true;
++NumNonconstant;
if (NumNonconstant == ArgumentConstants.size()) return false;
}
} else {
// This is not a constant argument. Mark the argument as
// non-constant.
ArgumentConstants[i].second = true;
++NumNonconstant;
if (NumNonconstant == ArgumentConstants.size()) return false;
}
}
}
}
// If we got to this point, there is a constant argument!
assert(NumNonconstant != ArgumentConstants.size());
Function::aiterator AI = F.abegin();
bool MadeChange = false;
for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI)
// Do we have a constant argument!?
if (!ArgumentConstants[i].second && !AI->use_empty()) {
assert(ArgumentConstants[i].first && "Unknown constant value!");
Value *V = ArgumentConstants[i].first;
AI->replaceAllUsesWith(V);
++NumArgumentsProped;
MadeChange = true;
}
return MadeChange;
}
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