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[CGP] Be less conservative about tail-duplicating a ret to allow tail calls

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CGP tail-duplicates rets into blocks that end with a call that feed the ret.
This puts the call in tail position, potentially allowing the DAG builder to
lower it as a tail call. To avoid tail duplication in cases where we won't
form the tail call, CGP tried to predict whether this is going to be possible,
and avoids doing it when lowering as a tail call will definitely fail.
However, it was being too conservative by always throwing away calls to
functions with a signext/zeroext attribute on the return type.

Instead, we can use the same logic the builder uses to determine whether the
attributes work out.

Differential Revision: https://reviews.llvm.org/D24315


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280894 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Michael Kuperstein 2016-09-08 00:48:37 +00:00
parent 6d2157da81
commit 2a93bda6e9
4 changed files with 81 additions and 43 deletions
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14
include/llvm/CodeGen/Analysis.h
View File

@ -105,11 +105,21 @@ ISD::CondCode getICmpCondCode(ICmpInst::Predicate Pred);
/// This function only tests target-independent requirements.
bool isInTailCallPosition(ImmutableCallSite CS, const TargetMachine &TM);
/// Test if given that the input instruction is in the tail call position, if
/// there is an attribute mismatch between the caller and the callee that will
/// inhibit tail call optimizations.
/// \p AllowDifferingSizes is an output parameter which, if forming a tail call
/// is permitted, determines whether it's permitted only if the size of the
/// caller's and callee's return types match exactly.
bool attributesPermitTailCall(const Function *F, const Instruction *I,
const ReturnInst *Ret,
const TargetLoweringBase &TLI,
bool *AllowDifferingSizes = nullptr);
/// Test if given that the input instruction is in the tail call position if the
/// return type or any attributes of the function will inhibit tail call
/// optimization.
bool returnTypeIsEligibleForTailCall(const Function *F,
const Instruction *I,
bool returnTypeIsEligibleForTailCall(const Function *F, const Instruction *I,
const ReturnInst *Ret,
const TargetLoweringBase &TLI);

74
lib/CodeGen/Analysis.cpp
View File

@ -525,6 +525,47 @@ bool llvm::isInTailCallPosition(ImmutableCallSite CS, const TargetMachine &TM) {
F, I, Ret, *TM.getSubtargetImpl(*F)->getTargetLowering());
}
bool llvm::attributesPermitTailCall(const Function *F, const Instruction *I,
const ReturnInst *Ret,
const TargetLoweringBase &TLI,
bool *AllowDifferingSizes) {
// ADS may be null, so don't write to it directly.
bool DummyADS;
bool &ADS = AllowDifferingSizes ? *AllowDifferingSizes : DummyADS;
ADS = true;
AttrBuilder CallerAttrs(F->getAttributes(),
AttributeSet::ReturnIndex);
AttrBuilder CalleeAttrs(cast<CallInst>(I)->getAttributes(),
AttributeSet::ReturnIndex);
// Noalias is completely benign as far as calling convention goes, it
// shouldn't affect whether the call is a tail call.
CallerAttrs = CallerAttrs.removeAttribute(Attribute::NoAlias);
CalleeAttrs = CalleeAttrs.removeAttribute(Attribute::NoAlias);
if (CallerAttrs.contains(Attribute::ZExt)) {
if (!CalleeAttrs.contains(Attribute::ZExt))
return false;
ADS = false;
CallerAttrs.removeAttribute(Attribute::ZExt);
CalleeAttrs.removeAttribute(Attribute::ZExt);
} else if (CallerAttrs.contains(Attribute::SExt)) {
if (!CalleeAttrs.contains(Attribute::SExt))
return false;
ADS = false;
CallerAttrs.removeAttribute(Attribute::SExt);
CalleeAttrs.removeAttribute(Attribute::SExt);
}
// If they're still different, there's some facet we don't understand
// (currently only "inreg", but in future who knows). It may be OK but the
// only safe option is to reject the tail call.
return CallerAttrs == CalleeAttrs;
}
bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
const Instruction *I,
const ReturnInst *Ret,
@ -538,37 +579,8 @@ bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
if (isa<UndefValue>(Ret->getOperand(0))) return true;
// Make sure the attributes attached to each return are compatible.
AttrBuilder CallerAttrs(F->getAttributes(),
AttributeSet::ReturnIndex);
AttrBuilder CalleeAttrs(cast<CallInst>(I)->getAttributes(),
AttributeSet::ReturnIndex);
// Noalias is completely benign as far as calling convention goes, it
// shouldn't affect whether the call is a tail call.
CallerAttrs = CallerAttrs.removeAttribute(Attribute::NoAlias);
CalleeAttrs = CalleeAttrs.removeAttribute(Attribute::NoAlias);
bool AllowDifferingSizes = true;
if (CallerAttrs.contains(Attribute::ZExt)) {
if (!CalleeAttrs.contains(Attribute::ZExt))
return false;
AllowDifferingSizes = false;
CallerAttrs.removeAttribute(Attribute::ZExt);
CalleeAttrs.removeAttribute(Attribute::ZExt);
} else if (CallerAttrs.contains(Attribute::SExt)) {
if (!CalleeAttrs.contains(Attribute::SExt))
return false;
AllowDifferingSizes = false;
CallerAttrs.removeAttribute(Attribute::SExt);
CalleeAttrs.removeAttribute(Attribute::SExt);
}
// If they're still different, there's some facet we don't understand
// (currently only "inreg", but in future who knows). It may be OK but the
// only safe option is to reject the tail call.
if (CallerAttrs != CalleeAttrs)
bool AllowDifferingSizes;
if (!attributesPermitTailCall(F, I, Ret, TLI, &AllowDifferingSizes))
return false;
const Value *RetVal = Ret->getOperand(0), *CallVal = I;

16
lib/CodeGen/CodeGenPrepare.cpp
View File

@ -23,6 +23,7 @@
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
@ -1983,14 +1984,6 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB) {
if (PN && PN->getParent() != BB)
return false;
// It's not safe to eliminate the sign / zero extension of the return value.
// See llvm::isInTailCallPosition().
const Function *F = BB->getParent();
AttributeSet CallerAttrs = F->getAttributes();
if (CallerAttrs.hasAttribute(AttributeSet::ReturnIndex, Attribute::ZExt) ||
CallerAttrs.hasAttribute(AttributeSet::ReturnIndex, Attribute::SExt))
return false;
// Make sure there are no instructions between the PHI and return, or that the
// return is the first instruction in the block.
if (PN) {
@ -2010,13 +2003,15 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB) {
/// Only dup the ReturnInst if the CallInst is likely to be emitted as a tail
/// call.
const Function *F = BB->getParent();
SmallVector<CallInst*, 4> TailCalls;
if (PN) {
for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I) {
CallInst *CI = dyn_cast<CallInst>(PN->getIncomingValue(I));
// Make sure the phi value is indeed produced by the tail call.
if (CI && CI->hasOneUse() && CI->getParent() == PN->getIncomingBlock(I) &&
TLI->mayBeEmittedAsTailCall(CI))
TLI->mayBeEmittedAsTailCall(CI) &&
attributesPermitTailCall(F, CI, RetI, *TLI))
TailCalls.push_back(CI);
}
} else {
@ -2033,7 +2028,8 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB) {
continue;
CallInst *CI = dyn_cast<CallInst>(&*RI);
if (CI && CI->use_empty() && TLI->mayBeEmittedAsTailCall(CI))
if (CI && CI->use_empty() && TLI->mayBeEmittedAsTailCall(CI) &&
attributesPermitTailCall(F, CI, RetI, *TLI))
TailCalls.push_back(CI);
}
}

20
test/CodeGen/X86/tailcall-cgp-dup.ll
View File

@ -85,3 +85,23 @@ someThingWithValue.exit: ; preds = %if.else.i, %if.then
%retval.0.i = bitcast i8* %retval.0.in.i to %0*
ret %0* %retval.0.i
}
; Correctly handle zext returns.
declare zeroext i1 @foo_i1()
; CHECK-LABEL: zext_i1
; CHECK: jmp _foo_i1
define zeroext i1 @zext_i1(i1 %k) {
entry:
br i1 %k, label %land.end, label %land.rhs
land.rhs: ; preds = %entry
%call1 = tail call zeroext i1 @foo_i1()
br label %land.end
land.end: ; preds = %entry, %land.rhs
%0 = phi i1 [ false, %entry ], [ %call1, %land.rhs ]
ret i1 %0
}