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

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.

llvm-svn: 93145
This commit is contained in:
Chris Lattner 2010-01-11 04:05:13 +00:00
parent 12bd8992b3
commit 0a85420409
2 changed files with 49 additions and 0 deletions
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18
llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -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;

31
llvm/test/Transforms/InstCombine/cast.ll
View File

@ -572,3 +572,34 @@ define i64 @test57(i64 %A) nounwind {
; CHECK-NEXT: %E = and i64 %C, 16777215
; CHECK-NEXT: ret i64 %E
}
define i64 @test58(i64 %A) nounwind {
%B = trunc i64 %A to i32
%C = lshr i32 %B, 8
%D = or i32 %C, 128
%E = zext i32 %D to i64
ret i64 %E
; CHECK: @test58
; CHECK-NEXT: %C = lshr i64 %A, 8
; CHECK-NEXT: %D = or i64 %C, 128
; CHECK-NEXT: %E = and i64 %D, 16777215
; CHECK-NEXT: ret i64 %E
}
define i64 @test59(i8 %A, i8 %B) nounwind {
%C = zext i8 %A to i32
%D = shl i32 %C, 4
%E = and i32 %D, 48
%F = zext i8 %B to i32
%G = lshr i32 %F, 4
%H = or i32 %G, %E
%I = zext i32 %H to i64
ret i64 %I
; CHECK: @test59
; CHECK-NEXT: %C = zext i8 %A to i64
; CHECK-NOT: i32
; CHECK: %F = zext i8 %B to i64
; CHECK-NOT: i32
; CHECK: ret i64 %H
}