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281 lines
9.3 KiB
281 lines
9.3 KiB
//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass implements an _extremely_ simple interprocedural constant
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// propagation pass. It could certainly be improved in many different ways,
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// like using a worklist. This pass makes arguments dead, but does not remove
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// them. The existing dead argument elimination pass should be run after this
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// to clean up the mess.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/IPO.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/CallSite.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Pass.h"
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using namespace llvm;
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#define DEBUG_TYPE "ipconstprop"
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STATISTIC(NumArgumentsProped, "Number of args turned into constants");
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STATISTIC(NumReturnValProped, "Number of return values turned into constants");
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namespace {
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/// IPCP - The interprocedural constant propagation pass
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///
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struct IPCP : public ModulePass {
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static char ID; // Pass identification, replacement for typeid
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IPCP() : ModulePass(ID) {
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initializeIPCPPass(*PassRegistry::getPassRegistry());
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}
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bool runOnModule(Module &M) override;
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};
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}
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/// PropagateConstantsIntoArguments - Look at all uses of the specified
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/// function. If all uses are direct call sites, and all pass a particular
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/// constant in for an argument, propagate that constant in as the argument.
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///
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static bool PropagateConstantsIntoArguments(Function &F) {
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if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.
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// For each argument, keep track of its constant value and whether it is a
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// constant or not. The bool is driven to true when found to be non-constant.
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SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
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ArgumentConstants.resize(F.arg_size());
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unsigned NumNonconstant = 0;
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for (Use &U : F.uses()) {
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User *UR = U.getUser();
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// Ignore blockaddress uses.
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if (isa<BlockAddress>(UR)) continue;
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// Used by a non-instruction, or not the callee of a function, do not
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// transform.
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if (!isa<CallInst>(UR) && !isa<InvokeInst>(UR))
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return false;
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CallSite CS(cast<Instruction>(UR));
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if (!CS.isCallee(&U))
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return false;
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// Check out all of the potentially constant arguments. Note that we don't
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// inspect varargs here.
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CallSite::arg_iterator AI = CS.arg_begin();
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Function::arg_iterator Arg = F.arg_begin();
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for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
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++i, ++AI, ++Arg) {
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// If this argument is known non-constant, ignore it.
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if (ArgumentConstants[i].second)
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continue;
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Constant *C = dyn_cast<Constant>(*AI);
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if (C && ArgumentConstants[i].first == nullptr) {
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ArgumentConstants[i].first = C; // First constant seen.
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} else if (C && ArgumentConstants[i].first == C) {
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// Still the constant value we think it is.
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} else if (*AI == &*Arg) {
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// Ignore recursive calls passing argument down.
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} else {
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// Argument became non-constant. If all arguments are non-constant now,
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// give up on this function.
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if (++NumNonconstant == ArgumentConstants.size())
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return false;
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ArgumentConstants[i].second = true;
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}
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}
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}
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// If we got to this point, there is a constant argument!
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assert(NumNonconstant != ArgumentConstants.size());
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bool MadeChange = false;
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Function::arg_iterator AI = F.arg_begin();
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for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
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// Do we have a constant argument?
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if (ArgumentConstants[i].second || AI->use_empty() ||
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AI->hasInAllocaAttr() || (AI->hasByValAttr() && !F.onlyReadsMemory()))
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continue;
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Value *V = ArgumentConstants[i].first;
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if (!V) V = UndefValue::get(AI->getType());
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AI->replaceAllUsesWith(V);
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++NumArgumentsProped;
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MadeChange = true;
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}
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return MadeChange;
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}
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// Check to see if this function returns one or more constants. If so, replace
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// all callers that use those return values with the constant value. This will
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// leave in the actual return values and instructions, but deadargelim will
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// clean that up.
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//
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// Additionally if a function always returns one of its arguments directly,
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// callers will be updated to use the value they pass in directly instead of
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// using the return value.
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static bool PropagateConstantReturn(Function &F) {
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if (F.getReturnType()->isVoidTy())
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return false; // No return value.
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// We can infer and propagate the return value only when we know that the
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// definition we'll get at link time is *exactly* the definition we see now.
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// For more details, see GlobalValue::mayBeDerefined.
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if (!F.isDefinitionExact())
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return false;
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// Check to see if this function returns a constant.
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SmallVector<Value *,4> RetVals;
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StructType *STy = dyn_cast<StructType>(F.getReturnType());
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if (STy)
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for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i)
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RetVals.push_back(UndefValue::get(STy->getElementType(i)));
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else
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RetVals.push_back(UndefValue::get(F.getReturnType()));
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unsigned NumNonConstant = 0;
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for (BasicBlock &BB : F)
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if (ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
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for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
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// Already found conflicting return values?
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Value *RV = RetVals[i];
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if (!RV)
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continue;
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// Find the returned value
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Value *V;
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if (!STy)
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V = RI->getOperand(0);
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else
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V = FindInsertedValue(RI->getOperand(0), i);
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if (V) {
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// Ignore undefs, we can change them into anything
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if (isa<UndefValue>(V))
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continue;
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// Try to see if all the rets return the same constant or argument.
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if (isa<Constant>(V) || isa<Argument>(V)) {
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if (isa<UndefValue>(RV)) {
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// No value found yet? Try the current one.
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RetVals[i] = V;
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continue;
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}
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// Returning the same value? Good.
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if (RV == V)
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continue;
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}
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}
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// Different or no known return value? Don't propagate this return
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// value.
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RetVals[i] = nullptr;
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// All values non-constant? Stop looking.
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if (++NumNonConstant == RetVals.size())
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return false;
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}
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}
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// If we got here, the function returns at least one constant value. Loop
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// over all users, replacing any uses of the return value with the returned
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// constant.
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bool MadeChange = false;
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for (Use &U : F.uses()) {
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CallSite CS(U.getUser());
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Instruction* Call = CS.getInstruction();
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// Not a call instruction or a call instruction that's not calling F
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// directly?
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if (!Call || !CS.isCallee(&U))
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continue;
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// Call result not used?
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if (Call->use_empty())
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continue;
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MadeChange = true;
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if (!STy) {
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Value* New = RetVals[0];
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if (Argument *A = dyn_cast<Argument>(New))
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// Was an argument returned? Then find the corresponding argument in
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// the call instruction and use that.
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New = CS.getArgument(A->getArgNo());
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Call->replaceAllUsesWith(New);
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continue;
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}
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for (auto I = Call->user_begin(), E = Call->user_end(); I != E;) {
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Instruction *Ins = cast<Instruction>(*I);
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// Increment now, so we can remove the use
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++I;
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// Find the index of the retval to replace with
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int index = -1;
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if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins))
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if (EV->hasIndices())
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index = *EV->idx_begin();
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// If this use uses a specific return value, and we have a replacement,
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// replace it.
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if (index != -1) {
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Value *New = RetVals[index];
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if (New) {
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if (Argument *A = dyn_cast<Argument>(New))
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// Was an argument returned? Then find the corresponding argument in
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// the call instruction and use that.
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New = CS.getArgument(A->getArgNo());
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Ins->replaceAllUsesWith(New);
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Ins->eraseFromParent();
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}
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}
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}
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}
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if (MadeChange) ++NumReturnValProped;
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return MadeChange;
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}
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char IPCP::ID = 0;
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INITIALIZE_PASS(IPCP, "ipconstprop",
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"Interprocedural constant propagation", false, false)
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ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
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bool IPCP::runOnModule(Module &M) {
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if (skipModule(M))
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return false;
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bool Changed = false;
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bool LocalChange = true;
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// FIXME: instead of using smart algorithms, we just iterate until we stop
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// making changes.
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while (LocalChange) {
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LocalChange = false;
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for (Function &F : M)
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if (!F.isDeclaration()) {
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// Delete any klingons.
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F.removeDeadConstantUsers();
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if (F.hasLocalLinkage())
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LocalChange |= PropagateConstantsIntoArguments(F);
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Changed |= PropagateConstantReturn(F);
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}
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Changed |= LocalChange;
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}
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return Changed;
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}
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