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Extend CanEvaluateZExtd to handle and/or/xor more aggressively in the

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BitsToClear case.  This allows it to promote expressions which have an
and/or/xor after the lshr, promoting cases like test2 (from PR4216) 
and test3 (random extample extracted from a spec benchmark).

clang now compiles the code in PR4216 into:

_test_bitfield:                                             ## @test_bitfield
	movl	%edi, %eax
	orl	$194, %eax
	movl	$4294902010, %ecx
	andq	%rax, %rcx
	orl	$32768, %edi
	andq	$39936, %rdi
	movq	%rdi, %rax
	orq	%rcx, %rax
	ret

instead of:

_test_bitfield:                                             ## @test_bitfield
	movl	%edi, %eax
	orl	$194, %eax
	movl	$4294902010, %ecx
	andq	%rax, %rcx
	shrl	$8, %edi
	orl	$128, %edi
	shlq	$8, %rdi
	andq	$39936, %rdi
	movq	%rdi, %rax
	orq	%rcx, %rax
	ret

which is still not great, but is progress.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93145 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
2010-01-11 04:05:13 +00:00
parent 789162a309
commit 7acc4b1281
2 changed files with 49 additions and 0 deletions
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18
lib/Transforms/InstCombine/InstCombineCasts.cpp
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@@ -634,6 +634,20 @@ static bool CanEvaluateZExtd(Value *V, const Type *Ty, unsigned &BitsToClear) {
if (BitsToClear == 0 && Tmp == 0)
return true;
// If the operation is an AND/OR/XOR and the bits to clear are zero in the
// other side, BitsToClear is ok.
if (Tmp == 0 &&
(Opc == Instruction::And || Opc == Instruction::Or ||
Opc == Instruction::Xor)) {
// We use MaskedValueIsZero here for generality, but the case we care
// about the most is constant RHS.
unsigned VSize = V->getType()->getScalarSizeInBits();
if (MaskedValueIsZero(I->getOperand(1),
APInt::getHighBitsSet(VSize, BitsToClear)))
return true;
}
// Otherwise, we don't know how to analyze this BitsToClear case yet.
return false;
case Instruction::LShr:
@@ -652,6 +666,8 @@ static bool CanEvaluateZExtd(Value *V, const Type *Ty, unsigned &BitsToClear) {
case Instruction::Select:
if (!CanEvaluateZExtd(I->getOperand(1), Ty, Tmp) ||
!CanEvaluateZExtd(I->getOperand(2), Ty, BitsToClear) ||
// TODO: If important, we could handle the case when the BitsToClear are
// known zero in the disagreeing side.
Tmp != BitsToClear)
return false;
return true;
@@ -665,6 +681,8 @@ static bool CanEvaluateZExtd(Value *V, const Type *Ty, unsigned &BitsToClear) {
return false;
for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
if (!CanEvaluateZExtd(PN->getIncomingValue(i), Ty, Tmp) ||
// TODO: If important, we could handle the case when the BitsToClear
// are known zero in the disagreeing input.
Tmp != BitsToClear)
return false;
return true;