New LLVM pass: argument promotion. This version only handles simple scalar
variables.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12193 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/IPO/ArgumentPromotion.cpp b/lib/Transforms/IPO/ArgumentPromotion.cpp
new file mode 100644
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--- /dev/null
+++ b/lib/Transforms/IPO/ArgumentPromotion.cpp
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+//===-- ArgumentPromotion.cpp - Promote 'by reference' arguments ----------===//
+//
+// 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 promotes "by reference" arguments to be "by value" arguments. In
+// practice, this means looking for internal functions that have pointer
+// arguments. If we can prove, through the use of alias analysis, that that an
+// argument is *only* loaded, then we can pass the value into the function
+// instead of the address of the value. This can cause recursive simplification
+// of code, and lead to the elimination of allocas, especially in C++ template
+// code like the STL.
+//
+// Note that this transformation could also be done for arguments that are only
+// stored to (returning the value instead), but we do not currently handle that
+// case.
+//
+// Note that we should be able to promote pointers to structures that are only
+// loaded from as well. The danger is creating way to many arguments, so this
+// transformation should be limited to 3 element structs or something.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Support/CallSite.h"
+#include "llvm/Support/CFG.h"
+#include "Support/Debug.h"
+#include "Support/DepthFirstIterator.h"
+#include "Support/Statistic.h"
+#include <set>
+using namespace llvm;
+
+namespace {
+ Statistic<> NumArgumentsPromoted("argpromotion",
+ "Number of pointer arguments promoted");
+ Statistic<> NumArgumentsDead("argpromotion",
+ "Number of dead pointer args eliminated");
+
+ /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
+ ///
+ class ArgPromotion : public Pass {
+ // WorkList - The set of internal functions that we have yet to process. As
+ // we eliminate arguments from a function, we push all callers into this set
+ // so that the by reference argument can be bubbled out as far as possible.
+ // This set contains only internal functions.
+ std::set<Function*> WorkList;
+ public:
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired<AliasAnalysis>();
+ AU.addRequired<TargetData>();
+ }
+
+ virtual bool run(Module &M);
+ private:
+ bool PromoteArguments(Function *F);
+ bool isSafeToPromoteArgument(Argument *Arg) const;
+ void DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
+ };
+
+ RegisterOpt<ArgPromotion> X("argpromotion",
+ "Promote 'by reference' arguments to scalars");
+}
+
+Pass *llvm::createArgumentPromotionPass() {
+ return new ArgPromotion();
+}
+
+bool ArgPromotion::run(Module &M) {
+ bool Changed = false;
+ for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
+ if (I->hasInternalLinkage()) {
+ WorkList.insert(I);
+
+ // If there are any constant pointer refs pointing to this function,
+ // eliminate them now if possible.
+ ConstantPointerRef *CPR = 0;
+ for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+ ++UI)
+ if ((CPR = dyn_cast<ConstantPointerRef>(*UI)))
+ break; // Found one!
+ if (CPR) {
+ // See if we can transform all users to use the function directly.
+ while (!CPR->use_empty()) {
+ User *TheUser = CPR->use_back();
+ if (!isa<Constant>(TheUser)) {
+ Changed = true;
+ TheUser->replaceUsesOfWith(CPR, I);
+ } else {
+ // We won't be able to eliminate all users. :(
+ WorkList.erase(I); // Minor efficiency win.
+ break;
+ }
+ }
+
+ // If we nuked all users of the CPR, kill the CPR now!
+ if (CPR->use_empty()) {
+ CPR->destroyConstant();
+ Changed = true;
+ }
+ }
+ }
+
+ while (!WorkList.empty()) {
+ Function *F = *WorkList.begin();
+ WorkList.erase(WorkList.begin());
+
+ if (PromoteArguments(F)) // Attempt to promote an argument.
+ Changed = true; // Remember that we changed something.
+ }
+
+ return Changed;
+}
+
+
+bool ArgPromotion::PromoteArguments(Function *F) {
+ assert(F->hasInternalLinkage() && "We can only process internal functions!");
+
+ // First check: see if there are any pointer arguments! If not, quick exit.
+ std::vector<Argument*> PointerArgs;
+ for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
+ if (isa<PointerType>(I->getType()))
+ PointerArgs.push_back(I);
+ if (PointerArgs.empty()) return false;
+
+ // Second check: make sure that all callers are direct callers. We can't
+ // transform functions that have indirect callers.
+ for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
+ UI != E; ++UI)
+ // What about CPRs?
+ if (!CallSite::get(*UI).getInstruction())
+ return false; // Cannot promote an indirect call!
+
+ // Check to see which arguments are promotable. If an argument is not
+ // promotable, remove it from the PointerArgs vector.
+ for (unsigned i = 0; i != PointerArgs.size(); ++i)
+ if (!isSafeToPromoteArgument(PointerArgs[i])) {
+ std::swap(PointerArgs[i--], PointerArgs.back());
+ PointerArgs.pop_back();
+ }
+
+ // No promotable pointer arguments.
+ if (PointerArgs.empty()) return false;
+
+ // Okay, promote all of the arguments are rewrite the callees!
+ DoPromotion(F, PointerArgs);
+ return true;
+}
+
+bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
+ // We can only promote this argument if all of the uses are loads...
+ std::vector<LoadInst*> Loads;
+ for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
+ UI != E; ++UI)
+ if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+ if (LI->isVolatile()) return false; // Don't hack volatile loads
+ Loads.push_back(LI);
+ } else
+ return false;
+
+ if (Loads.empty()) return true; // No users, dead argument.
+
+ const Type *LoadTy = cast<PointerType>(Arg->getType())->getElementType();
+ unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LoadTy);
+
+ // Okay, now we know that the argument is only used by load instructions.
+ // Check to see if the pointer is guaranteed to not be modified from entry of
+ // the function to each of the load instructions.
+ Function &F = *Arg->getParent();
+
+ // Because there could be several/many load instructions, remember which
+ // blocks we know to be transparent to the load.
+ std::set<BasicBlock*> TranspBlocks;
+
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+ for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
+ // Check to see if the load is invalidated from the start of the block to
+ // the load itself.
+ LoadInst *Load = Loads[i];
+ BasicBlock *BB = Load->getParent();
+ if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
+ return false; // Pointer is invalidated!
+
+ // Now check every path from the entry block to the load for transparency.
+ // To do this, we perform a depth first search on the inverse CFG from the
+ // loading block.
+ for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
+ for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
+ E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
+ if (AA.canBasicBlockModify(**I, Arg, LoadSize))
+ return false;
+ }
+
+ // If the path from the entry of the function to each load is free of
+ // instructions that potentially invalidate the load, we can make the
+ // transformation!
+ return true;
+}
+
+
+void ArgPromotion::DoPromotion(Function *F, std::vector<Argument*> &Args2Prom) {
+ std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
+
+ // Start by computing a new prototype for the function, which is the same as
+ // the old function, but has modified arguments.
+ const FunctionType *FTy = F->getFunctionType();
+ std::vector<const Type*> Params;
+
+ for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
+ if (!ArgsToPromote.count(I)) {
+ Params.push_back(I->getType());
+ } else if (!I->use_empty()) {
+ Params.push_back(cast<PointerType>(I->getType())->getElementType());
+ ++NumArgumentsPromoted;
+ } else {
+ ++NumArgumentsDead;
+ }
+
+ const Type *RetTy = FTy->getReturnType();
+
+ // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
+ // have zero fixed arguments.
+ bool ExtraArgHack = false;
+ if (Params.empty() && FTy->isVarArg()) {
+ ExtraArgHack = true;
+ Params.push_back(Type::IntTy);
+ }
+ FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
+
+ // Create the new function body and insert it into the module...
+ Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
+ F->getParent()->getFunctionList().insert(F, NF);
+
+ // Loop over all of the callers of the function, transforming the call sites
+ // to pass in the loaded pointers.
+ //
+ std::vector<Value*> Args;
+ while (!F->use_empty()) {
+ CallSite CS = CallSite::get(F->use_back());
+ Instruction *Call = CS.getInstruction();
+
+ // Make sure the caller of this function is revisited.
+ if (Call->getParent()->getParent()->hasInternalLinkage())
+ WorkList.insert(Call->getParent()->getParent());
+
+ // Loop over the operands, deleting dead ones...
+ CallSite::arg_iterator AI = CS.arg_begin();
+ for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
+ if (!ArgsToPromote.count(I))
+ Args.push_back(*AI); // Unmodified argument
+ else if (!I->use_empty()) {
+ // Non-dead instruction
+ Args.push_back(new LoadInst(*AI, (*AI)->getName()+".val", Call));
+ }
+
+ if (ExtraArgHack)
+ Args.push_back(Constant::getNullValue(Type::IntTy));
+
+ // Push any varargs arguments on the list
+ for (; AI != CS.arg_end(); ++AI)
+ Args.push_back(*AI);
+
+ Instruction *New;
+ if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
+ New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
+ Args, "", Call);
+ } else {
+ New = new CallInst(NF, Args, "", Call);
+ }
+ Args.clear();
+
+ if (!Call->use_empty()) {
+ Call->replaceAllUsesWith(New);
+ std::string Name = Call->getName();
+ Call->setName("");
+ New->setName(Name);
+ }
+
+ // Finally, remove the old call from the program, reducing the use-count of
+ // F.
+ Call->getParent()->getInstList().erase(Call);
+ }
+
+ // Since we have now created the new function, splice the body of the old
+ // function right into the new function, leaving the old rotting hulk of the
+ // function empty.
+ NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
+
+ // Loop over the argument list, transfering uses of the old arguments over to
+ // the new arguments, also transfering over the names as well.
+ //
+ for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
+ I != E; ++I)
+ if (!ArgsToPromote.count(I)) {
+ // If this is an unmodified argument, move the name and users over to the
+ // new version.
+ I->replaceAllUsesWith(I2);
+ I2->setName(I->getName());
+ ++I2;
+ } else if (!I->use_empty()) {
+ // Otherwise, if we promoted this argument, then all users are load
+ // instructions, and all loads should be using the new argument that we
+ // added.
+ /*DEBUG*/(std::cerr << "*** Promoted argument '" << I->getName()
+ << "' of function '" << F->getName() << "'\n");
+ I2->setName(I->getName()+".val");
+ while (!I->use_empty()) {
+ LoadInst *LI = cast<LoadInst>(I->use_back());
+ LI->replaceAllUsesWith(I2);
+ LI->getParent()->getInstList().erase(LI);
+ }
+ ++I2;
+ }
+
+ // Now that the old function is dead, delete it.
+ F->getParent()->getFunctionList().erase(F);
+}