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//===- DeadArgumentElimination.cpp - Eliminate dead arguments -------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// 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/Constant.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");
"Number of unread args replaced with undef");
namespace {
/// DAE - The dead argument elimination pass.
class DAE : public ModulePass {
// DAH uses this to specify a different ID.
explicit DAE(char &ID) : ModulePass(ID) {}
static char ID; // Pass identification, replacement for typeid
DAE() : ModulePass(ID) {
bool runOnModule(Module &M) override {
if (skipModule(M))
return false;
DeadArgumentEliminationPass DAEP(ShouldHackArguments());
ModuleAnalysisManager DummyMAM;
PreservedAnalyses PA =, 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)
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.getParent()->getFunctionList().insert(Fn.getIterator(), NF);
// 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)
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)
PAL = AttributeList::get(Fn.getContext(), PAL.getFnAttributes(),
PAL.getRetAttributes(), ArgAttrs);
SmallVector<OperandBundleDef, 1> 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);
uint64_t W;
if (Call->extractProfTotalWeight(W))
if (!Call->use_empty())
// Finally, remove the old call from the program, reducing the use-count of
// F.
// 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.
// Patch the pointer to LLVM function in debug info descriptor.
// 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.
// Finally, nuke the old function.
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;
for (Argument &Arg : Fn.args()) {
if (!Arg.hasSwiftErrorAttr() && Arg.use_empty() && !Arg.hasByValOrInAllocaAttr())
if (UnusedArgs.empty())
return false;
bool Changed = false;
for (Use &U : Fn.uses()) {
CallSite CS(U.getUser());
if (!CS || !CS.isCallee(&U))
// 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()));
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();
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();
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::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.
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::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)
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;
== 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::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)
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)) {
// 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)) {
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.
// If we have any returns of `musttail` results - the signature can't
// change
if (BB->getTerminatingMustTailCall() != nullptr)
HasMustTailCalls = true;
if (HasMustTailCalls) {
DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName()
<< " has musttail calls\n");
if (!F.hasLocalLinkage() && (!ShouldHackArguments || F.isIntrinsic())) {
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)) {
// 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?
// 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)
// 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)
} 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);
} else {
for (unsigned i = 0; i != RetCount; ++i) {
if (RetValLiveness[i] != Live)
if (HasMustTailCallers) {
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]);
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.
/// 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:
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));
/// 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) {
DEBUG(dbgs() << "DeadArgumentEliminationPass - Intrinsically live fn: "
<< F.getName() << "\n");
// Mark the function as live.
// 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.
DEBUG(dbgs() << "DeadArgumentEliminationPass - Marking "
<< RA.getDescription() << " live\n");
/// 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)
// 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)) {
ArgAlive[i] = true;
HasLiveReturnedArg |= PAL.hasParamAttribute(i, Attribute::Returned);
} else {
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 {
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())
assert(!RAttrs.overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
"Return attributes no longer compatible?");
AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs);
// Reconstruct the AttributesList based on the vector we constructed.
assert(ArgAttrVec.size() == Params.size());
AttributeList NewPAL = AttributeList::get(
F->getContext(), PAL.getFnAttributes(), 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());
// Insert the new function before the old function, so we won't be processing
// it again.
F->getParent()->getFunctionList().insert(F->getIterator(), NF);
// 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();
const AttributeList &CallPAL = CS.getAttributes();
// Adjust the call return attributes in case the function was changed to
// return void.
AttrBuilder RAttrs(CallPAL.getRetAttributes());
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]) {
// 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
} else {
// Otherwise, use the original attributes.
// Push any varargs arguments on the list. Don't forget their attributes.
for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
// Reconstruct the AttributesList based on the vector we constructed.
assert(ArgAttrVec.size() == Args.size());
AttributeList NewCallPAL = AttributeList::get(
F->getContext(), CallPAL.getFnAttributes(), RetAttrs, ArgAttrVec);
SmallVector<OperandBundleDef, 1> 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(NF, Args, OpBundles, "", Call);
uint64_t W;
if (Call->extractProfTotalWeight(W))
Instruction *New = NewCS.getInstruction();
if (!Call->use_empty()) {
if (New->getType() == Call->getType()) {
// Return type not changed? Just replace users then.
} else if (New->getType()->isVoidTy()) {
// Our return value has uses, but they will get removed later on.
// Replace by null for now.
if (!Call->getType()->isX86_MMXTy())
} 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",
// 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
// Finally, remove the old call from the program, reducing the use-count of
// F.
// 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.
} else {
// If this argument is dead, replace any uses of it with null constants
// (these are guaranteed to become unused later on).
if (!I->getType()->isX86_MMXTy())
// 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);
// Patch the pointer to LLVM function in debug info descriptor.
// Now that the old function is dead, delete it.
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.
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).
DEBUG(dbgs() << "DeadArgumentEliminationPass - Determining liveness\n");
for (auto &F : M)
// 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();