diff options
Diffstat (limited to 'llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp')
| -rw-r--r-- | llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp | 1124 |
1 files changed, 1124 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp new file mode 100644 index 000000000000..968a13110b16 --- /dev/null +++ b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp @@ -0,0 +1,1124 @@ +//===- DeadArgumentElimination.cpp - Eliminate dead arguments -------------===// +// +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. +// See https://llvm.org/LICENSE.txt for license information. +// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception +// +//===----------------------------------------------------------------------===// +// +// This pass deletes dead arguments from internal functions. Dead argument +// elimination removes arguments which are directly dead, as well as arguments +// only passed into function calls as dead arguments of other functions. This +// pass also deletes dead return values in a similar way. +// +// This pass is often useful as a cleanup pass to run after aggressive +// interprocedural passes, which add possibly-dead arguments or return values. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/IPO/DeadArgumentElimination.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/IR/Argument.h" +#include "llvm/IR/Attributes.h" +#include "llvm/IR/BasicBlock.h" +#include "llvm/IR/CallSite.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DerivedTypes.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/InstrTypes.h" +#include "llvm/IR/Instruction.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Intrinsics.h" +#include "llvm/IR/Module.h" +#include "llvm/IR/PassManager.h" +#include "llvm/IR/Type.h" +#include "llvm/IR/Use.h" +#include "llvm/IR/User.h" +#include "llvm/IR/Value.h" +#include "llvm/Pass.h" +#include "llvm/Support/Casting.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/IPO.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include <cassert> +#include <cstdint> +#include <utility> +#include <vector> + +using namespace llvm; + +#define DEBUG_TYPE "deadargelim" + +STATISTIC(NumArgumentsEliminated, "Number of unread args removed"); +STATISTIC(NumRetValsEliminated , "Number of unused return values removed"); +STATISTIC(NumArgumentsReplacedWithUndef, + "Number of unread args replaced with undef"); + +namespace { + + /// DAE - The dead argument elimination pass. + class DAE : public ModulePass { + protected: + // DAH uses this to specify a different ID. + explicit DAE(char &ID) : ModulePass(ID) {} + + public: + static char ID; // Pass identification, replacement for typeid + + DAE() : ModulePass(ID) { + initializeDAEPass(*PassRegistry::getPassRegistry()); + } + + bool runOnModule(Module &M) override { + if (skipModule(M)) + return false; + DeadArgumentEliminationPass DAEP(ShouldHackArguments()); + ModuleAnalysisManager DummyMAM; + PreservedAnalyses PA = DAEP.run(M, DummyMAM); + return !PA.areAllPreserved(); + } + + virtual bool ShouldHackArguments() const { return false; } + }; + +} // end anonymous namespace + +char DAE::ID = 0; + +INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false) + +namespace { + + /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but + /// deletes arguments to functions which are external. This is only for use + /// by bugpoint. + struct DAH : public DAE { + static char ID; + + DAH() : DAE(ID) {} + + bool ShouldHackArguments() const override { return true; } + }; + +} // end anonymous namespace + +char DAH::ID = 0; + +INITIALIZE_PASS(DAH, "deadarghaX0r", + "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)", + false, false) + +/// createDeadArgEliminationPass - This pass removes arguments from functions +/// which are not used by the body of the function. +ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); } + +ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); } + +/// DeleteDeadVarargs - If this is an function that takes a ... list, and if +/// llvm.vastart is never called, the varargs list is dead for the function. +bool DeadArgumentEliminationPass::DeleteDeadVarargs(Function &Fn) { + assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!"); + if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false; + + // Ensure that the function is only directly called. + if (Fn.hasAddressTaken()) + return false; + + // Don't touch naked functions. The assembly might be using an argument, or + // otherwise rely on the frame layout in a way that this analysis will not + // see. + if (Fn.hasFnAttribute(Attribute::Naked)) { + return false; + } + + // Okay, we know we can transform this function if safe. Scan its body + // looking for calls marked musttail or calls to llvm.vastart. + for (BasicBlock &BB : Fn) { + for (Instruction &I : BB) { + CallInst *CI = dyn_cast<CallInst>(&I); + if (!CI) + continue; + if (CI->isMustTailCall()) + return false; + if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) { + if (II->getIntrinsicID() == Intrinsic::vastart) + return false; + } + } + } + + // If we get here, there are no calls to llvm.vastart in the function body, + // remove the "..." and adjust all the calls. + + // Start by computing a new prototype for the function, which is the same as + // the old function, but doesn't have isVarArg set. + FunctionType *FTy = Fn.getFunctionType(); + + std::vector<Type *> Params(FTy->param_begin(), FTy->param_end()); + FunctionType *NFTy = FunctionType::get(FTy->getReturnType(), + Params, false); + unsigned NumArgs = Params.size(); + + // Create the new function body and insert it into the module... + Function *NF = Function::Create(NFTy, Fn.getLinkage(), Fn.getAddressSpace()); + NF->copyAttributesFrom(&Fn); + NF->setComdat(Fn.getComdat()); + Fn.getParent()->getFunctionList().insert(Fn.getIterator(), NF); + NF->takeName(&Fn); + + // Loop over all of the callers of the function, transforming the call sites + // to pass in a smaller number of arguments into the new function. + // + std::vector<Value *> Args; + for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) { + CallSite CS(*I++); + if (!CS) + continue; + Instruction *Call = CS.getInstruction(); + + // Pass all the same arguments. + Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs); + + // Drop any attributes that were on the vararg arguments. + AttributeList PAL = CS.getAttributes(); + if (!PAL.isEmpty()) { + SmallVector<AttributeSet, 8> ArgAttrs; + for (unsigned ArgNo = 0; ArgNo < NumArgs; ++ArgNo) + ArgAttrs.push_back(PAL.getParamAttributes(ArgNo)); + PAL = AttributeList::get(Fn.getContext(), PAL.getFnAttributes(), + PAL.getRetAttributes(), ArgAttrs); + } + + SmallVector<OperandBundleDef, 1> OpBundles; + CS.getOperandBundlesAsDefs(OpBundles); + + CallSite NewCS; + if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { + NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), + Args, OpBundles, "", Call); + } else { + NewCS = CallInst::Create(NF, Args, OpBundles, "", Call); + cast<CallInst>(NewCS.getInstruction()) + ->setTailCallKind(cast<CallInst>(Call)->getTailCallKind()); + } + NewCS.setCallingConv(CS.getCallingConv()); + NewCS.setAttributes(PAL); + NewCS->setDebugLoc(Call->getDebugLoc()); + uint64_t W; + if (Call->extractProfTotalWeight(W)) + NewCS->setProfWeight(W); + + Args.clear(); + + if (!Call->use_empty()) + Call->replaceAllUsesWith(NewCS.getInstruction()); + + NewCS->takeName(Call); + + // Finally, remove the old call from the program, reducing the use-count of + // F. + Call->eraseFromParent(); + } + + // 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(), Fn.getBasicBlockList()); + + // Loop over the argument list, transferring uses of the old arguments over to + // the new arguments, also transferring over the names as well. While we're at + // it, remove the dead arguments from the DeadArguments list. + for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(), + I2 = NF->arg_begin(); I != E; ++I, ++I2) { + // Move the name and users over to the new version. + I->replaceAllUsesWith(&*I2); + I2->takeName(&*I); + } + + // Clone metadatas from the old function, including debug info descriptor. + SmallVector<std::pair<unsigned, MDNode *>, 1> MDs; + Fn.getAllMetadata(MDs); + for (auto MD : MDs) + NF->addMetadata(MD.first, *MD.second); + + // Fix up any BlockAddresses that refer to the function. + Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType())); + // Delete the bitcast that we just created, so that NF does not + // appear to be address-taken. + NF->removeDeadConstantUsers(); + // Finally, nuke the old function. + Fn.eraseFromParent(); + return true; +} + +/// RemoveDeadArgumentsFromCallers - Checks if the given function has any +/// arguments that are unused, and changes the caller parameters to be undefined +/// instead. +bool DeadArgumentEliminationPass::RemoveDeadArgumentsFromCallers(Function &Fn) { + // We cannot change the arguments if this TU does not define the function or + // if the linker may choose a function body from another TU, even if the + // nominal linkage indicates that other copies of the function have the same + // semantics. In the below example, the dead load from %p may not have been + // eliminated from the linker-chosen copy of f, so replacing %p with undef + // in callers may introduce undefined behavior. + // + // define linkonce_odr void @f(i32* %p) { + // %v = load i32 %p + // ret void + // } + if (!Fn.hasExactDefinition()) + return false; + + // Functions with local linkage should already have been handled, except the + // fragile (variadic) ones which we can improve here. + if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg()) + return false; + + // Don't touch naked functions. The assembly might be using an argument, or + // otherwise rely on the frame layout in a way that this analysis will not + // see. + if (Fn.hasFnAttribute(Attribute::Naked)) + return false; + + if (Fn.use_empty()) + return false; + + SmallVector<unsigned, 8> UnusedArgs; + bool Changed = false; + + for (Argument &Arg : Fn.args()) { + if (!Arg.hasSwiftErrorAttr() && Arg.use_empty() && !Arg.hasByValOrInAllocaAttr()) { + if (Arg.isUsedByMetadata()) { + Arg.replaceAllUsesWith(UndefValue::get(Arg.getType())); + Changed = true; + } + UnusedArgs.push_back(Arg.getArgNo()); + } + } + + if (UnusedArgs.empty()) + return false; + + for (Use &U : Fn.uses()) { + CallSite CS(U.getUser()); + if (!CS || !CS.isCallee(&U)) + continue; + + // Now go through all unused args and replace them with "undef". + for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) { + unsigned ArgNo = UnusedArgs[I]; + + Value *Arg = CS.getArgument(ArgNo); + CS.setArgument(ArgNo, UndefValue::get(Arg->getType())); + ++NumArgumentsReplacedWithUndef; + Changed = true; + } + } + + return Changed; +} + +/// Convenience function that returns the number of return values. It returns 0 +/// for void functions and 1 for functions not returning a struct. It returns +/// the number of struct elements for functions returning a struct. +static unsigned NumRetVals(const Function *F) { + Type *RetTy = F->getReturnType(); + if (RetTy->isVoidTy()) + return 0; + else if (StructType *STy = dyn_cast<StructType>(RetTy)) + return STy->getNumElements(); + else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy)) + return ATy->getNumElements(); + else + return 1; +} + +/// Returns the sub-type a function will return at a given Idx. Should +/// correspond to the result type of an ExtractValue instruction executed with +/// just that one Idx (i.e. only top-level structure is considered). +static Type *getRetComponentType(const Function *F, unsigned Idx) { + Type *RetTy = F->getReturnType(); + assert(!RetTy->isVoidTy() && "void type has no subtype"); + + if (StructType *STy = dyn_cast<StructType>(RetTy)) + return STy->getElementType(Idx); + else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy)) + return ATy->getElementType(); + else + return RetTy; +} + +/// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not +/// live, it adds Use to the MaybeLiveUses argument. Returns the determined +/// liveness of Use. +DeadArgumentEliminationPass::Liveness +DeadArgumentEliminationPass::MarkIfNotLive(RetOrArg Use, + UseVector &MaybeLiveUses) { + // We're live if our use or its Function is already marked as live. + if (LiveFunctions.count(Use.F) || LiveValues.count(Use)) + return Live; + + // We're maybe live otherwise, but remember that we must become live if + // Use becomes live. + MaybeLiveUses.push_back(Use); + return MaybeLive; +} + +/// SurveyUse - This looks at a single use of an argument or return value +/// and determines if it should be alive or not. Adds this use to MaybeLiveUses +/// if it causes the used value to become MaybeLive. +/// +/// RetValNum is the return value number to use when this use is used in a +/// return instruction. This is used in the recursion, you should always leave +/// it at 0. +DeadArgumentEliminationPass::Liveness +DeadArgumentEliminationPass::SurveyUse(const Use *U, UseVector &MaybeLiveUses, + unsigned RetValNum) { + const User *V = U->getUser(); + if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) { + // The value is returned from a function. It's only live when the + // function's return value is live. We use RetValNum here, for the case + // that U is really a use of an insertvalue instruction that uses the + // original Use. + const Function *F = RI->getParent()->getParent(); + if (RetValNum != -1U) { + RetOrArg Use = CreateRet(F, RetValNum); + // We might be live, depending on the liveness of Use. + return MarkIfNotLive(Use, MaybeLiveUses); + } else { + DeadArgumentEliminationPass::Liveness Result = MaybeLive; + for (unsigned i = 0; i < NumRetVals(F); ++i) { + RetOrArg Use = CreateRet(F, i); + // We might be live, depending on the liveness of Use. If any + // sub-value is live, then the entire value is considered live. This + // is a conservative choice, and better tracking is possible. + DeadArgumentEliminationPass::Liveness SubResult = + MarkIfNotLive(Use, MaybeLiveUses); + if (Result != Live) + Result = SubResult; + } + return Result; + } + } + if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) { + if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex() + && IV->hasIndices()) + // The use we are examining is inserted into an aggregate. Our liveness + // depends on all uses of that aggregate, but if it is used as a return + // value, only index at which we were inserted counts. + RetValNum = *IV->idx_begin(); + + // Note that if we are used as the aggregate operand to the insertvalue, + // we don't change RetValNum, but do survey all our uses. + + Liveness Result = MaybeLive; + for (const Use &UU : IV->uses()) { + Result = SurveyUse(&UU, MaybeLiveUses, RetValNum); + if (Result == Live) + break; + } + return Result; + } + + if (auto CS = ImmutableCallSite(V)) { + const Function *F = CS.getCalledFunction(); + if (F) { + // Used in a direct call. + + // The function argument is live if it is used as a bundle operand. + if (CS.isBundleOperand(U)) + return Live; + + // Find the argument number. We know for sure that this use is an + // argument, since if it was the function argument this would be an + // indirect call and the we know can't be looking at a value of the + // label type (for the invoke instruction). + unsigned ArgNo = CS.getArgumentNo(U); + + if (ArgNo >= F->getFunctionType()->getNumParams()) + // The value is passed in through a vararg! Must be live. + return Live; + + assert(CS.getArgument(ArgNo) + == CS->getOperand(U->getOperandNo()) + && "Argument is not where we expected it"); + + // Value passed to a normal call. It's only live when the corresponding + // argument to the called function turns out live. + RetOrArg Use = CreateArg(F, ArgNo); + return MarkIfNotLive(Use, MaybeLiveUses); + } + } + // Used in any other way? Value must be live. + return Live; +} + +/// SurveyUses - This looks at all the uses of the given value +/// Returns the Liveness deduced from the uses of this value. +/// +/// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If +/// the result is Live, MaybeLiveUses might be modified but its content should +/// be ignored (since it might not be complete). +DeadArgumentEliminationPass::Liveness +DeadArgumentEliminationPass::SurveyUses(const Value *V, + UseVector &MaybeLiveUses) { + // Assume it's dead (which will only hold if there are no uses at all..). + Liveness Result = MaybeLive; + // Check each use. + for (const Use &U : V->uses()) { + Result = SurveyUse(&U, MaybeLiveUses); + if (Result == Live) + break; + } + return Result; +} + +// SurveyFunction - This performs the initial survey of the specified function, +// checking out whether or not it uses any of its incoming arguments or whether +// any callers use the return value. This fills in the LiveValues set and Uses +// map. +// +// We consider arguments of non-internal functions to be intrinsically alive as +// well as arguments to functions which have their "address taken". +void DeadArgumentEliminationPass::SurveyFunction(const Function &F) { + // Functions with inalloca parameters are expecting args in a particular + // register and memory layout. + if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) { + MarkLive(F); + return; + } + + // Don't touch naked functions. The assembly might be using an argument, or + // otherwise rely on the frame layout in a way that this analysis will not + // see. + if (F.hasFnAttribute(Attribute::Naked)) { + MarkLive(F); + return; + } + + unsigned RetCount = NumRetVals(&F); + + // Assume all return values are dead + using RetVals = SmallVector<Liveness, 5>; + + RetVals RetValLiveness(RetCount, MaybeLive); + + using RetUses = SmallVector<UseVector, 5>; + + // These vectors map each return value to the uses that make it MaybeLive, so + // we can add those to the Uses map if the return value really turns out to be + // MaybeLive. Initialized to a list of RetCount empty lists. + RetUses MaybeLiveRetUses(RetCount); + + bool HasMustTailCalls = false; + + for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { + if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { + if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType() + != F.getFunctionType()->getReturnType()) { + // We don't support old style multiple return values. + MarkLive(F); + return; + } + } + + // If we have any returns of `musttail` results - the signature can't + // change + if (BB->getTerminatingMustTailCall() != nullptr) + HasMustTailCalls = true; + } + + if (HasMustTailCalls) { + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName() + << " has musttail calls\n"); + } + + if (!F.hasLocalLinkage() && (!ShouldHackArguments || F.isIntrinsic())) { + MarkLive(F); + return; + } + + LLVM_DEBUG( + dbgs() << "DeadArgumentEliminationPass - Inspecting callers for fn: " + << F.getName() << "\n"); + // Keep track of the number of live retvals, so we can skip checks once all + // of them turn out to be live. + unsigned NumLiveRetVals = 0; + + bool HasMustTailCallers = false; + + // Loop all uses of the function. + for (const Use &U : F.uses()) { + // If the function is PASSED IN as an argument, its address has been + // taken. + ImmutableCallSite CS(U.getUser()); + if (!CS || !CS.isCallee(&U)) { + MarkLive(F); + return; + } + + // The number of arguments for `musttail` call must match the number of + // arguments of the caller + if (CS.isMustTailCall()) + HasMustTailCallers = true; + + // If this use is anything other than a call site, the function is alive. + const Instruction *TheCall = CS.getInstruction(); + if (!TheCall) { // Not a direct call site? + MarkLive(F); + return; + } + + // If we end up here, we are looking at a direct call to our function. + + // Now, check how our return value(s) is/are used in this caller. Don't + // bother checking return values if all of them are live already. + if (NumLiveRetVals == RetCount) + continue; + + // Check all uses of the return value. + for (const Use &U : TheCall->uses()) { + if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) { + // This use uses a part of our return value, survey the uses of + // that part and store the results for this index only. + unsigned Idx = *Ext->idx_begin(); + if (RetValLiveness[Idx] != Live) { + RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]); + if (RetValLiveness[Idx] == Live) + NumLiveRetVals++; + } + } else { + // Used by something else than extractvalue. Survey, but assume that the + // result applies to all sub-values. + UseVector MaybeLiveAggregateUses; + if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) { + NumLiveRetVals = RetCount; + RetValLiveness.assign(RetCount, Live); + break; + } else { + for (unsigned i = 0; i != RetCount; ++i) { + if (RetValLiveness[i] != Live) + MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(), + MaybeLiveAggregateUses.end()); + } + } + } + } + } + + if (HasMustTailCallers) { + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName() + << " has musttail callers\n"); + } + + // Now we've inspected all callers, record the liveness of our return values. + for (unsigned i = 0; i != RetCount; ++i) + MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]); + + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Inspecting args for fn: " + << F.getName() << "\n"); + + // Now, check all of our arguments. + unsigned i = 0; + UseVector MaybeLiveArgUses; + for (Function::const_arg_iterator AI = F.arg_begin(), + E = F.arg_end(); AI != E; ++AI, ++i) { + Liveness Result; + if (F.getFunctionType()->isVarArg() || HasMustTailCallers || + HasMustTailCalls) { + // Variadic functions will already have a va_arg function expanded inside + // them, making them potentially very sensitive to ABI changes resulting + // from removing arguments entirely, so don't. For example AArch64 handles + // register and stack HFAs very differently, and this is reflected in the + // IR which has already been generated. + // + // `musttail` calls to this function restrict argument removal attempts. + // The signature of the caller must match the signature of the function. + // + // `musttail` calls in this function prevents us from changing its + // signature + Result = Live; + } else { + // See what the effect of this use is (recording any uses that cause + // MaybeLive in MaybeLiveArgUses). + Result = SurveyUses(&*AI, MaybeLiveArgUses); + } + + // Mark the result. + MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses); + // Clear the vector again for the next iteration. + MaybeLiveArgUses.clear(); + } +} + +/// MarkValue - This function marks the liveness of RA depending on L. If L is +/// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses, +/// such that RA will be marked live if any use in MaybeLiveUses gets marked +/// live later on. +void DeadArgumentEliminationPass::MarkValue(const RetOrArg &RA, Liveness L, + const UseVector &MaybeLiveUses) { + switch (L) { + case Live: + MarkLive(RA); + break; + case MaybeLive: + // Note any uses of this value, so this return value can be + // marked live whenever one of the uses becomes live. + for (const auto &MaybeLiveUse : MaybeLiveUses) + Uses.insert(std::make_pair(MaybeLiveUse, RA)); + break; + } +} + +/// MarkLive - Mark the given Function as alive, meaning that it cannot be +/// changed in any way. Additionally, +/// mark any values that are used as this function's parameters or by its return +/// values (according to Uses) live as well. +void DeadArgumentEliminationPass::MarkLive(const Function &F) { + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Intrinsically live fn: " + << F.getName() << "\n"); + // Mark the function as live. + LiveFunctions.insert(&F); + // Mark all arguments as live. + for (unsigned i = 0, e = F.arg_size(); i != e; ++i) + PropagateLiveness(CreateArg(&F, i)); + // Mark all return values as live. + for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i) + PropagateLiveness(CreateRet(&F, i)); +} + +/// MarkLive - Mark the given return value or argument as live. Additionally, +/// mark any values that are used by this value (according to Uses) live as +/// well. +void DeadArgumentEliminationPass::MarkLive(const RetOrArg &RA) { + if (LiveFunctions.count(RA.F)) + return; // Function was already marked Live. + + if (!LiveValues.insert(RA).second) + return; // We were already marked Live. + + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Marking " + << RA.getDescription() << " live\n"); + PropagateLiveness(RA); +} + +/// PropagateLiveness - Given that RA is a live value, propagate it's liveness +/// to any other values it uses (according to Uses). +void DeadArgumentEliminationPass::PropagateLiveness(const RetOrArg &RA) { + // We don't use upper_bound (or equal_range) here, because our recursive call + // to ourselves is likely to cause the upper_bound (which is the first value + // not belonging to RA) to become erased and the iterator invalidated. + UseMap::iterator Begin = Uses.lower_bound(RA); + UseMap::iterator E = Uses.end(); + UseMap::iterator I; + for (I = Begin; I != E && I->first == RA; ++I) + MarkLive(I->second); + + // Erase RA from the Uses map (from the lower bound to wherever we ended up + // after the loop). + Uses.erase(Begin, I); +} + +// RemoveDeadStuffFromFunction - Remove any arguments and return values from F +// that are not in LiveValues. Transform the function and all of the callees of +// the function to not have these arguments and return values. +// +bool DeadArgumentEliminationPass::RemoveDeadStuffFromFunction(Function *F) { + // Don't modify fully live functions + if (LiveFunctions.count(F)) + return false; + + // Start by computing a new prototype for the function, which is the same as + // the old function, but has fewer arguments and a different return type. + FunctionType *FTy = F->getFunctionType(); + std::vector<Type*> Params; + + // Keep track of if we have a live 'returned' argument + bool HasLiveReturnedArg = false; + + // Set up to build a new list of parameter attributes. + SmallVector<AttributeSet, 8> ArgAttrVec; + const AttributeList &PAL = F->getAttributes(); + + // Remember which arguments are still alive. + SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false); + // Construct the new parameter list from non-dead arguments. Also construct + // a new set of parameter attributes to correspond. Skip the first parameter + // attribute, since that belongs to the return value. + unsigned i = 0; + for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); + I != E; ++I, ++i) { + RetOrArg Arg = CreateArg(F, i); + if (LiveValues.erase(Arg)) { + Params.push_back(I->getType()); + ArgAlive[i] = true; + ArgAttrVec.push_back(PAL.getParamAttributes(i)); + HasLiveReturnedArg |= PAL.hasParamAttribute(i, Attribute::Returned); + } else { + ++NumArgumentsEliminated; + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Removing argument " + << i << " (" << I->getName() << ") from " + << F->getName() << "\n"); + } + } + + // Find out the new return value. + Type *RetTy = FTy->getReturnType(); + Type *NRetTy = nullptr; + unsigned RetCount = NumRetVals(F); + + // -1 means unused, other numbers are the new index + SmallVector<int, 5> NewRetIdxs(RetCount, -1); + std::vector<Type*> RetTypes; + + // If there is a function with a live 'returned' argument but a dead return + // value, then there are two possible actions: + // 1) Eliminate the return value and take off the 'returned' attribute on the + // argument. + // 2) Retain the 'returned' attribute and treat the return value (but not the + // entire function) as live so that it is not eliminated. + // + // It's not clear in the general case which option is more profitable because, + // even in the absence of explicit uses of the return value, code generation + // is free to use the 'returned' attribute to do things like eliding + // save/restores of registers across calls. Whether or not this happens is + // target and ABI-specific as well as depending on the amount of register + // pressure, so there's no good way for an IR-level pass to figure this out. + // + // Fortunately, the only places where 'returned' is currently generated by + // the FE are places where 'returned' is basically free and almost always a + // performance win, so the second option can just be used always for now. + // + // This should be revisited if 'returned' is ever applied more liberally. + if (RetTy->isVoidTy() || HasLiveReturnedArg) { + NRetTy = RetTy; + } else { + // Look at each of the original return values individually. + for (unsigned i = 0; i != RetCount; ++i) { + RetOrArg Ret = CreateRet(F, i); + if (LiveValues.erase(Ret)) { + RetTypes.push_back(getRetComponentType(F, i)); + NewRetIdxs[i] = RetTypes.size() - 1; + } else { + ++NumRetValsEliminated; + LLVM_DEBUG( + dbgs() << "DeadArgumentEliminationPass - Removing return value " + << i << " from " << F->getName() << "\n"); + } + } + if (RetTypes.size() > 1) { + // More than one return type? Reduce it down to size. + if (StructType *STy = dyn_cast<StructType>(RetTy)) { + // Make the new struct packed if we used to return a packed struct + // already. + NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked()); + } else { + assert(isa<ArrayType>(RetTy) && "unexpected multi-value return"); + NRetTy = ArrayType::get(RetTypes[0], RetTypes.size()); + } + } else if (RetTypes.size() == 1) + // One return type? Just a simple value then, but only if we didn't use to + // return a struct with that simple value before. + NRetTy = RetTypes.front(); + else if (RetTypes.empty()) + // No return types? Make it void, but only if we didn't use to return {}. + NRetTy = Type::getVoidTy(F->getContext()); + } + + assert(NRetTy && "No new return type found?"); + + // The existing function return attributes. + AttrBuilder RAttrs(PAL.getRetAttributes()); + + // Remove any incompatible attributes, but only if we removed all return + // values. Otherwise, ensure that we don't have any conflicting attributes + // here. Currently, this should not be possible, but special handling might be + // required when new return value attributes are added. + if (NRetTy->isVoidTy()) + RAttrs.remove(AttributeFuncs::typeIncompatible(NRetTy)); + else + assert(!RAttrs.overlaps(AttributeFuncs::typeIncompatible(NRetTy)) && + "Return attributes no longer compatible?"); + + AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs); + + // Strip allocsize attributes. They might refer to the deleted arguments. + AttributeSet FnAttrs = PAL.getFnAttributes().removeAttribute( + F->getContext(), Attribute::AllocSize); + + // Reconstruct the AttributesList based on the vector we constructed. + assert(ArgAttrVec.size() == Params.size()); + AttributeList NewPAL = + AttributeList::get(F->getContext(), FnAttrs, RetAttrs, ArgAttrVec); + + // Create the new function type based on the recomputed parameters. + FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg()); + + // No change? + if (NFTy == FTy) + return false; + + // Create the new function body and insert it into the module... + Function *NF = Function::Create(NFTy, F->getLinkage(), F->getAddressSpace()); + NF->copyAttributesFrom(F); + NF->setComdat(F->getComdat()); + NF->setAttributes(NewPAL); + // Insert the new function before the old function, so we won't be processing + // it again. + F->getParent()->getFunctionList().insert(F->getIterator(), NF); + NF->takeName(F); + + // Loop over all of the callers of the function, transforming the call sites + // to pass in a smaller number of arguments into the new function. + std::vector<Value*> Args; + while (!F->use_empty()) { + CallSite CS(F->user_back()); + Instruction *Call = CS.getInstruction(); + + ArgAttrVec.clear(); + const AttributeList &CallPAL = CS.getAttributes(); + + // Adjust the call return attributes in case the function was changed to + // return void. + AttrBuilder RAttrs(CallPAL.getRetAttributes()); + RAttrs.remove(AttributeFuncs::typeIncompatible(NRetTy)); + AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs); + + // Declare these outside of the loops, so we can reuse them for the second + // loop, which loops the varargs. + CallSite::arg_iterator I = CS.arg_begin(); + unsigned i = 0; + // Loop over those operands, corresponding to the normal arguments to the + // original function, and add those that are still alive. + for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i) + if (ArgAlive[i]) { + Args.push_back(*I); + // Get original parameter attributes, but skip return attributes. + AttributeSet Attrs = CallPAL.getParamAttributes(i); + if (NRetTy != RetTy && Attrs.hasAttribute(Attribute::Returned)) { + // If the return type has changed, then get rid of 'returned' on the + // call site. The alternative is to make all 'returned' attributes on + // call sites keep the return value alive just like 'returned' + // attributes on function declaration but it's less clearly a win and + // this is not an expected case anyway + ArgAttrVec.push_back(AttributeSet::get( + F->getContext(), + AttrBuilder(Attrs).removeAttribute(Attribute::Returned))); + } else { + // Otherwise, use the original attributes. + ArgAttrVec.push_back(Attrs); + } + } + + // Push any varargs arguments on the list. Don't forget their attributes. + for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) { + Args.push_back(*I); + ArgAttrVec.push_back(CallPAL.getParamAttributes(i)); + } + + // Reconstruct the AttributesList based on the vector we constructed. + assert(ArgAttrVec.size() == Args.size()); + + // Again, be sure to remove any allocsize attributes, since their indices + // may now be incorrect. + AttributeSet FnAttrs = CallPAL.getFnAttributes().removeAttribute( + F->getContext(), Attribute::AllocSize); + + AttributeList NewCallPAL = AttributeList::get( + F->getContext(), FnAttrs, RetAttrs, ArgAttrVec); + + SmallVector<OperandBundleDef, 1> OpBundles; + CS.getOperandBundlesAsDefs(OpBundles); + + CallSite NewCS; + if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { + NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), + Args, OpBundles, "", Call->getParent()); + } else { + NewCS = CallInst::Create(NFTy, NF, Args, OpBundles, "", Call); + cast<CallInst>(NewCS.getInstruction()) + ->setTailCallKind(cast<CallInst>(Call)->getTailCallKind()); + } + NewCS.setCallingConv(CS.getCallingConv()); + NewCS.setAttributes(NewCallPAL); + NewCS->setDebugLoc(Call->getDebugLoc()); + uint64_t W; + if (Call->extractProfTotalWeight(W)) + NewCS->setProfWeight(W); + Args.clear(); + ArgAttrVec.clear(); + + Instruction *New = NewCS.getInstruction(); + if (!Call->use_empty() || Call->isUsedByMetadata()) { + if (New->getType() == Call->getType()) { + // Return type not changed? Just replace users then. + Call->replaceAllUsesWith(New); + New->takeName(Call); + } else if (New->getType()->isVoidTy()) { + // If the return value is dead, replace any uses of it with undef + // (any non-debug value uses will get removed later on). + if (!Call->getType()->isX86_MMXTy()) + Call->replaceAllUsesWith(UndefValue::get(Call->getType())); + } else { + assert((RetTy->isStructTy() || RetTy->isArrayTy()) && + "Return type changed, but not into a void. The old return type" + " must have been a struct or an array!"); + Instruction *InsertPt = Call; + if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { + BasicBlock *NewEdge = SplitEdge(New->getParent(), II->getNormalDest()); + InsertPt = &*NewEdge->getFirstInsertionPt(); + } + + // We used to return a struct or array. Instead of doing smart stuff + // with all the uses, we will just rebuild it using extract/insertvalue + // chaining and let instcombine clean that up. + // + // Start out building up our return value from undef + Value *RetVal = UndefValue::get(RetTy); + for (unsigned i = 0; i != RetCount; ++i) + if (NewRetIdxs[i] != -1) { + Value *V; + if (RetTypes.size() > 1) + // We are still returning a struct, so extract the value from our + // return value + V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret", + InsertPt); + else + // We are now returning a single element, so just insert that + V = New; + // Insert the value at the old position + RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt); + } + // Now, replace all uses of the old call instruction with the return + // struct we built + Call->replaceAllUsesWith(RetVal); + New->takeName(Call); + } + } + + // Finally, remove the old call from the program, reducing the use-count of + // F. + Call->eraseFromParent(); + } + + // 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, transferring uses of the old arguments over to + // the new arguments, also transferring over the names as well. + i = 0; + for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), + I2 = NF->arg_begin(); I != E; ++I, ++i) + if (ArgAlive[i]) { + // If this is a live argument, move the name and users over to the new + // version. + I->replaceAllUsesWith(&*I2); + I2->takeName(&*I); + ++I2; + } else { + // If this argument is dead, replace any uses of it with undef + // (any non-debug value uses will get removed later on). + if (!I->getType()->isX86_MMXTy()) + I->replaceAllUsesWith(UndefValue::get(I->getType())); + } + + // If we change the return value of the function we must rewrite any return + // instructions. Check this now. + if (F->getReturnType() != NF->getReturnType()) + for (BasicBlock &BB : *NF) + if (ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator())) { + Value *RetVal; + + if (NFTy->getReturnType()->isVoidTy()) { + RetVal = nullptr; + } else { + assert(RetTy->isStructTy() || RetTy->isArrayTy()); + // The original return value was a struct or array, insert + // extractvalue/insertvalue chains to extract only the values we need + // to return and insert them into our new result. + // This does generate messy code, but we'll let it to instcombine to + // clean that up. + Value *OldRet = RI->getOperand(0); + // Start out building up our return value from undef + RetVal = UndefValue::get(NRetTy); + for (unsigned i = 0; i != RetCount; ++i) + if (NewRetIdxs[i] != -1) { + ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i, + "oldret", RI); + if (RetTypes.size() > 1) { + // We're still returning a struct, so reinsert the value into + // our new return value at the new index + + RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i], + "newret", RI); + } else { + // We are now only returning a simple value, so just return the + // extracted value. + RetVal = EV; + } + } + } + // Replace the return instruction with one returning the new return + // value (possibly 0 if we became void). + ReturnInst::Create(F->getContext(), RetVal, RI); + BB.getInstList().erase(RI); + } + + // Clone metadatas from the old function, including debug info descriptor. + SmallVector<std::pair<unsigned, MDNode *>, 1> MDs; + F->getAllMetadata(MDs); + for (auto MD : MDs) + NF->addMetadata(MD.first, *MD.second); + + // Now that the old function is dead, delete it. + F->eraseFromParent(); + + return true; +} + +PreservedAnalyses DeadArgumentEliminationPass::run(Module &M, + ModuleAnalysisManager &) { + bool Changed = false; + + // First pass: Do a simple check to see if any functions can have their "..." + // removed. We can do this if they never call va_start. This loop cannot be + // fused with the next loop, because deleting a function invalidates + // information computed while surveying other functions. + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Deleting dead varargs\n"); + for (Module::iterator I = M.begin(), E = M.end(); I != E; ) { + Function &F = *I++; + if (F.getFunctionType()->isVarArg()) + Changed |= DeleteDeadVarargs(F); + } + + // Second phase:loop through the module, determining which arguments are live. + // We assume all arguments are dead unless proven otherwise (allowing us to + // determine that dead arguments passed into recursive functions are dead). + // + LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Determining liveness\n"); + for (auto &F : M) + SurveyFunction(F); + + // Now, remove all dead arguments and return values from each function in + // turn. + for (Module::iterator I = M.begin(), E = M.end(); I != E; ) { + // Increment now, because the function will probably get removed (ie. + // replaced by a new one). + Function *F = &*I++; + Changed |= RemoveDeadStuffFromFunction(F); + } + + // Finally, look for any unused parameters in functions with non-local + // linkage and replace the passed in parameters with undef. + for (auto &F : M) + Changed |= RemoveDeadArgumentsFromCallers(F); + + if (!Changed) + return PreservedAnalyses::all(); + return PreservedAnalyses::none(); +} |