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DeadArgumentElimination: keep return value on functions that have a live argument with the 'returned' attribute (rather than generate invalid IR); however, if both can be eliminated, both will be
llvm-svn: 185290
This commit is contained in:
Stephen Lin
parent
dc3c6d36e0
commit
d9ddb58848
@ -716,10 +716,42 @@ bool DAE::RemoveDeadStuffFromFunction(Function *F) {
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FunctionType *FTy = F->getFunctionType();
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std::vector<Type*> Params;
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// Keep track of if we have a live 'returned' argument
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bool HasLiveReturnedArg = false;
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// Set up to build a new list of parameter attributes.
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SmallVector<AttributeSet, 8> AttributesVec;
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const AttributeSet &PAL = F->getAttributes();
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// Remember which arguments are still alive.
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SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
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// Construct the new parameter list from non-dead arguments. Also construct
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// a new set of parameter attributes to correspond. Skip the first parameter
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// attribute, since that belongs to the return value.
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unsigned i = 0;
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for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
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I != E; ++I, ++i) {
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RetOrArg Arg = CreateArg(F, i);
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if (LiveValues.erase(Arg)) {
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Params.push_back(I->getType());
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ArgAlive[i] = true;
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// Get the original parameter attributes (skipping the first one, that is
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// for the return value.
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if (PAL.hasAttributes(i + 1)) {
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AttrBuilder B(PAL, i + 1);
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if (B.contains(Attribute::Returned))
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HasLiveReturnedArg = true;
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AttributesVec.
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push_back(AttributeSet::get(F->getContext(), Params.size(), B));
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}
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} else {
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++NumArgumentsEliminated;
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DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
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<< ") from " << F->getName() << "\n");
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}
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}
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// Find out the new return value.
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Type *RetTy = FTy->getReturnType();
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Type *NRetTy = NULL;
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@ -728,7 +760,27 @@ bool DAE::RemoveDeadStuffFromFunction(Function *F) {
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// -1 means unused, other numbers are the new index
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SmallVector<int, 5> NewRetIdxs(RetCount, -1);
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std::vector<Type*> RetTypes;
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if (RetTy->isVoidTy()) {
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// If there is a function with a live 'returned' argument but a dead return
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// value, then there are two possible actions:
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// 1) Eliminate the return value and take off the 'returned' attribute on the
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// argument.
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// 2) Retain the 'returned' attribute and treat the return value (but not the
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// entire function) as live so that it is not eliminated.
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//
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// It's not clear in the general case which option is more profitable because,
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// even in the absence of explicit uses of the return value, code generation
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// is free to use the 'returned' attribute to do things like eliding
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// save/restores of registers across calls. Whether or not this happens is
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// target and ABI-specific as well as depending on the amount of register
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// pressure, so there's no good way for an IR-level pass to figure this out.
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//
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// Fortunately, the only places where 'returned' is currently generated by
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// the FE are places where 'returned' is basically free and almost always a
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// performance win, so the second option can just be used always for now.
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//
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// This should be revisited if 'returned' is ever applied more liberally.
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if (RetTy->isVoidTy() || HasLiveReturnedArg) {
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NRetTy = RetTy;
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} else {
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StructType *STy = dyn_cast<StructType>(RetTy);
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@ -798,33 +850,6 @@ bool DAE::RemoveDeadStuffFromFunction(Function *F) {
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if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
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AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
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// Remember which arguments are still alive.
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SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
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// Construct the new parameter list from non-dead arguments. Also construct
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// a new set of parameter attributes to correspond. Skip the first parameter
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// attribute, since that belongs to the return value.
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unsigned i = 0;
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for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
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I != E; ++I, ++i) {
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RetOrArg Arg = CreateArg(F, i);
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if (LiveValues.erase(Arg)) {
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Params.push_back(I->getType());
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ArgAlive[i] = true;
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// Get the original parameter attributes (skipping the first one, that is
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// for the return value.
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if (PAL.hasAttributes(i + 1)) {
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AttrBuilder B(PAL, i + 1);
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AttributesVec.
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push_back(AttributeSet::get(F->getContext(), Params.size(), B));
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}
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} else {
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++NumArgumentsEliminated;
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DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
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<< ") from " << F->getName() << "\n");
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}
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}
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if (PAL.hasAttributes(AttributeSet::FunctionIndex))
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AttributesVec.push_back(AttributeSet::get(F->getContext(),
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PAL.getFnAttributes()));
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@ -885,6 +910,13 @@ bool DAE::RemoveDeadStuffFromFunction(Function *F) {
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// Get original parameter attributes, but skip return attributes.
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if (CallPAL.hasAttributes(i + 1)) {
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AttrBuilder B(CallPAL, i + 1);
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// If the return type has changed, then get rid of 'returned' on the
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// call site. The alternative is to make all 'returned' attributes on
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// call sites keep the return value alive just like 'returned'
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// attributes on function declaration but it's less clearly a win
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// and this is not an expected case anyway
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if (NRetTy != RetTy && B.contains(Attribute::Returned))
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B.removeAttribute(Attribute::Returned);
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AttributesVec.
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push_back(AttributeSet::get(F->getContext(), Args.size(), B));
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}
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55
test/Transforms/DeadArgElim/returned.ll
Normal file
55
test/Transforms/DeadArgElim/returned.ll
Normal file
@ -0,0 +1,55 @@
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; RUN: opt < %s -deadargelim -S | FileCheck %s
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%Ty = type { i32, i32 }
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; sanity check that the argument and return value are both dead
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; CHECK: define internal void @test1()
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define internal %Ty* @test1(%Ty* %this) {
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ret %Ty* %this
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}
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; do not keep alive the return value of a function with a dead 'returned' argument
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; CHECK: define internal void @test2()
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define internal %Ty* @test2(%Ty* returned %this) {
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ret %Ty* %this
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}
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; dummy to keep 'this' alive
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@dummy = global %Ty* null
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; sanity check that return value is dead
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; CHECK: define internal void @test3(%Ty* %this)
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define internal %Ty* @test3(%Ty* %this) {
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store volatile %Ty* %this, %Ty** @dummy
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ret %Ty* %this
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}
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; keep alive return value of a function if the 'returned' argument is live
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; CHECK: define internal %Ty* @test4(%Ty* returned %this)
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define internal %Ty* @test4(%Ty* returned %this) {
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store volatile %Ty* %this, %Ty** @dummy
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ret %Ty* %this
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}
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; don't do this if 'returned' is on the call site...
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; CHECK: define internal void @test5(%Ty* %this)
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define internal %Ty* @test5(%Ty* %this) {
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store volatile %Ty* %this, %Ty** @dummy
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ret %Ty* %this
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}
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define %Ty* @caller(%Ty* %this) {
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%1 = call %Ty* @test1(%Ty* %this)
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%2 = call %Ty* @test2(%Ty* %this)
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%3 = call %Ty* @test3(%Ty* %this)
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%4 = call %Ty* @test4(%Ty* %this)
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; ...instead, drop 'returned' form the call site
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; CHECK: call void @test5(%Ty* %this)
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%5 = call %Ty* @test5(%Ty* returned %this)
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ret %Ty* %this
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}
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