forked from OSchip/llvm-project
New LLVM pass: argument promotion. This version only handles simple scalar
variables. llvm-svn: 12193
This commit is contained in:
Chris Lattner
parent
f2b8692b73
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483ae01c9c
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//===-- ArgumentPromotion.cpp - Promote 'by reference' arguments ----------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass promotes "by reference" arguments to be "by value" arguments. In
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// practice, this means looking for internal functions that have pointer
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// arguments. If we can prove, through the use of alias analysis, that that an
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// argument is *only* loaded, then we can pass the value into the function
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// instead of the address of the value. This can cause recursive simplification
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// of code, and lead to the elimination of allocas, especially in C++ template
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// code like the STL.
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//
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// Note that this transformation could also be done for arguments that are only
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// stored to (returning the value instead), but we do not currently handle that
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// case.
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//
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// Note that we should be able to promote pointers to structures that are only
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// loaded from as well. The danger is creating way to many arguments, so this
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// transformation should be limited to 3 element structs or something.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/IPO.h"
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#include "llvm/Constants.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Support/CallSite.h"
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#include "llvm/Support/CFG.h"
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#include "Support/Debug.h"
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#include "Support/DepthFirstIterator.h"
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#include "Support/Statistic.h"
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#include <set>
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using namespace llvm;
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namespace {
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Statistic<> NumArgumentsPromoted("argpromotion",
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"Number of pointer arguments promoted");
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Statistic<> NumArgumentsDead("argpromotion",
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"Number of dead pointer args eliminated");
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/// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
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///
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class ArgPromotion : public Pass {
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// WorkList - The set of internal functions that we have yet to process. As
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// we eliminate arguments from a function, we push all callers into this set
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// so that the by reference argument can be bubbled out as far as possible.
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// This set contains only internal functions.
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std::set<Function*> WorkList;
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public:
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<AliasAnalysis>();
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AU.addRequired<TargetData>();
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}
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virtual bool run(Module &M);
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private:
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bool PromoteArguments(Function *F);
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bool isSafeToPromoteArgument(Argument *Arg) const;
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void DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
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};
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RegisterOpt<ArgPromotion> X("argpromotion",
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"Promote 'by reference' arguments to scalars");
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}
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Pass *llvm::createArgumentPromotionPass() {
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return new ArgPromotion();
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}
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bool ArgPromotion::run(Module &M) {
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bool Changed = false;
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for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
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if (I->hasInternalLinkage()) {
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WorkList.insert(I);
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// If there are any constant pointer refs pointing to this function,
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// eliminate them now if possible.
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ConstantPointerRef *CPR = 0;
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for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
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++UI)
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if ((CPR = dyn_cast<ConstantPointerRef>(*UI)))
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break; // Found one!
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if (CPR) {
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// See if we can transform all users to use the function directly.
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while (!CPR->use_empty()) {
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User *TheUser = CPR->use_back();
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if (!isa<Constant>(TheUser)) {
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Changed = true;
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TheUser->replaceUsesOfWith(CPR, I);
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} else {
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// We won't be able to eliminate all users. :(
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WorkList.erase(I); // Minor efficiency win.
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break;
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}
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}
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// If we nuked all users of the CPR, kill the CPR now!
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if (CPR->use_empty()) {
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CPR->destroyConstant();
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Changed = true;
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}
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}
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}
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while (!WorkList.empty()) {
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Function *F = *WorkList.begin();
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WorkList.erase(WorkList.begin());
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if (PromoteArguments(F)) // Attempt to promote an argument.
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Changed = true; // Remember that we changed something.
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}
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return Changed;
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}
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bool ArgPromotion::PromoteArguments(Function *F) {
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assert(F->hasInternalLinkage() && "We can only process internal functions!");
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// First check: see if there are any pointer arguments! If not, quick exit.
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std::vector<Argument*> PointerArgs;
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for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
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if (isa<PointerType>(I->getType()))
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PointerArgs.push_back(I);
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if (PointerArgs.empty()) return false;
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// Second check: make sure that all callers are direct callers. We can't
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// transform functions that have indirect callers.
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for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
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UI != E; ++UI)
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// What about CPRs?
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if (!CallSite::get(*UI).getInstruction())
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return false; // Cannot promote an indirect call!
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// Check to see which arguments are promotable. If an argument is not
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// promotable, remove it from the PointerArgs vector.
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for (unsigned i = 0; i != PointerArgs.size(); ++i)
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if (!isSafeToPromoteArgument(PointerArgs[i])) {
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std::swap(PointerArgs[i--], PointerArgs.back());
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PointerArgs.pop_back();
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}
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// No promotable pointer arguments.
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if (PointerArgs.empty()) return false;
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// Okay, promote all of the arguments are rewrite the callees!
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DoPromotion(F, PointerArgs);
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return true;
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}
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bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
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// We can only promote this argument if all of the uses are loads...
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std::vector<LoadInst*> Loads;
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for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
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UI != E; ++UI)
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if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
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if (LI->isVolatile()) return false; // Don't hack volatile loads
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Loads.push_back(LI);
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} else
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return false;
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if (Loads.empty()) return true; // No users, dead argument.
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const Type *LoadTy = cast<PointerType>(Arg->getType())->getElementType();
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unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LoadTy);
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// Okay, now we know that the argument is only used by load instructions.
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// Check to see if the pointer is guaranteed to not be modified from entry of
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// the function to each of the load instructions.
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Function &F = *Arg->getParent();
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// Because there could be several/many load instructions, remember which
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// blocks we know to be transparent to the load.
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std::set<BasicBlock*> TranspBlocks;
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
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// Check to see if the load is invalidated from the start of the block to
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// the load itself.
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LoadInst *Load = Loads[i];
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BasicBlock *BB = Load->getParent();
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if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
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return false; // Pointer is invalidated!
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// Now check every path from the entry block to the load for transparency.
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// To do this, we perform a depth first search on the inverse CFG from the
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// loading block.
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
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for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
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E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
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if (AA.canBasicBlockModify(**I, Arg, LoadSize))
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return false;
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}
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// If the path from the entry of the function to each load is free of
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// instructions that potentially invalidate the load, we can make the
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// transformation!
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return true;
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}
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void ArgPromotion::DoPromotion(Function *F, std::vector<Argument*> &Args2Prom) {
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std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
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// Start by computing a new prototype for the function, which is the same as
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// the old function, but has modified arguments.
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const FunctionType *FTy = F->getFunctionType();
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std::vector<const Type*> Params;
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for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
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if (!ArgsToPromote.count(I)) {
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Params.push_back(I->getType());
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} else if (!I->use_empty()) {
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Params.push_back(cast<PointerType>(I->getType())->getElementType());
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++NumArgumentsPromoted;
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} else {
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++NumArgumentsDead;
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}
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const Type *RetTy = FTy->getReturnType();
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// Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
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// have zero fixed arguments.
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bool ExtraArgHack = false;
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if (Params.empty() && FTy->isVarArg()) {
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ExtraArgHack = true;
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Params.push_back(Type::IntTy);
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}
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FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
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// Create the new function body and insert it into the module...
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Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
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F->getParent()->getFunctionList().insert(F, NF);
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// Loop over all of the callers of the function, transforming the call sites
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// to pass in the loaded pointers.
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//
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std::vector<Value*> Args;
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while (!F->use_empty()) {
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CallSite CS = CallSite::get(F->use_back());
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Instruction *Call = CS.getInstruction();
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// Make sure the caller of this function is revisited.
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if (Call->getParent()->getParent()->hasInternalLinkage())
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WorkList.insert(Call->getParent()->getParent());
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// Loop over the operands, deleting dead ones...
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CallSite::arg_iterator AI = CS.arg_begin();
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for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
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if (!ArgsToPromote.count(I))
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Args.push_back(*AI); // Unmodified argument
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else if (!I->use_empty()) {
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// Non-dead instruction
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Args.push_back(new LoadInst(*AI, (*AI)->getName()+".val", Call));
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}
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if (ExtraArgHack)
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Args.push_back(Constant::getNullValue(Type::IntTy));
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// Push any varargs arguments on the list
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for (; AI != CS.arg_end(); ++AI)
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Args.push_back(*AI);
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Instruction *New;
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if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
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New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
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Args, "", Call);
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} else {
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New = new CallInst(NF, Args, "", Call);
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}
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Args.clear();
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if (!Call->use_empty()) {
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Call->replaceAllUsesWith(New);
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std::string Name = Call->getName();
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Call->setName("");
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New->setName(Name);
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}
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// Finally, remove the old call from the program, reducing the use-count of
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// F.
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Call->getParent()->getInstList().erase(Call);
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}
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// Since we have now created the new function, splice the body of the old
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// function right into the new function, leaving the old rotting hulk of the
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// function empty.
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NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
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// Loop over the argument list, transfering uses of the old arguments over to
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// the new arguments, also transfering over the names as well.
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//
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for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
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I != E; ++I)
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if (!ArgsToPromote.count(I)) {
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// If this is an unmodified argument, move the name and users over to the
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// new version.
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I->replaceAllUsesWith(I2);
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I2->setName(I->getName());
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++I2;
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} else if (!I->use_empty()) {
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// Otherwise, if we promoted this argument, then all users are load
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// instructions, and all loads should be using the new argument that we
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// added.
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/*DEBUG*/(std::cerr << "*** Promoted argument '" << I->getName()
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<< "' of function '" << F->getName() << "'\n");
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I2->setName(I->getName()+".val");
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while (!I->use_empty()) {
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LoadInst *LI = cast<LoadInst>(I->use_back());
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LI->replaceAllUsesWith(I2);
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LI->getParent()->getInstList().erase(LI);
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}
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++I2;
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}
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// Now that the old function is dead, delete it.
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F->getParent()->getFunctionList().erase(F);
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}
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