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InstCombine: Turn icmp + sext into bitwise/integer ops when the input has only one unknown bit.

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int test1(unsigned x) { return (x&8) ? 0 : -1; }
int test3(unsigned x) { return (x&8) ? -1 : 0; }

before (x86_64):
_test1:
	andl	$8, %edi
	cmpl	$1, %edi
	sbbl	%eax, %eax
	ret
_test3:
	andl	$8, %edi
	cmpl	$1, %edi
	sbbl	%eax, %eax
	notl	%eax
	ret

after:
_test1:
	shrl	$3, %edi
	andl	$1, %edi
	leal	-1(%rdi), %eax
	ret
_test3:
	shll	$28, %edi
	movl	%edi, %eax
	sarl	$31, %eax
	ret

llvm-svn: 128732
This commit is contained in:
Benjamin Kramer 2011-04-01 20:09:10 +00:00
parent d74739be04
commit 7c0178b9ec
2 changed files with 98 additions and 0 deletions
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50
lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -882,6 +882,10 @@ Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
Value *Op0 = ICI->getOperand(0), *Op1 = ICI->getOperand(1);
ICmpInst::Predicate Pred = ICI->getPredicate();
// Transforming icmps with more than one use is not profitable.
if (!ICI->hasOneUse())
return 0;
if (ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
// (x <s 0) ? -1 : 0 -> ashr x, 31 -> all ones if signed
// (x >s -1) ? -1 : 0 -> ashr x, 31 -> all ones if not signed
@ -898,6 +902,52 @@ Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
In = Builder->CreateNot(In, In->getName()+".not");
return ReplaceInstUsesWith(CI, In);
}
// If we know that only one bit of the LHS of the icmp can be set and we
// have an equality comparison with zero or a power of 2, we can transform
// the icmp and sext into bitwise/integer operations.
if (ICI->isEquality() && (Op1C->isZero() || Op1C->getValue().isPowerOf2())){
unsigned BitWidth = Op1C->getType()->getBitWidth();
APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
APInt TypeMask(APInt::getAllOnesValue(BitWidth));
ComputeMaskedBits(Op0, TypeMask, KnownZero, KnownOne);
if ((~KnownZero).isPowerOf2()) {
Value *In = ICI->getOperand(0);
if (!Op1C->isZero() == (Pred == ICmpInst::ICMP_NE)) {
// sext ((x & 2^n) == 0) -> (x >> n) - 1
// sext ((x & 2^n) != 2^n) -> (x >> n) - 1
unsigned ShiftAmt = KnownZeroMask.countTrailingZeros();
// Perform a right shift to place the desired bit in the LSB.
if (ShiftAmt)
In = Builder->CreateLShr(In,
ConstantInt::get(In->getType(), ShiftAmt));
// At this point "In" is either 1 or 0. Subtract 1 to turn
// {1, 0} -> {0, -1}.
In = Builder->CreateAdd(In,
ConstantInt::getAllOnesValue(In->getType()),
"sext");
} else {
// sext ((x & 2^n) != 0) -> (x << bitwidth-n) a>> bitwidth-1
// sext ((x & 2^n) != 2^n) -> (x << bitwidth-n) a>> bitwidth-1
unsigned ShiftAmt = KnownZeroMask.countLeadingZeros();
// Perform a left shift to place the desired bit in the MSB.
if (ShiftAmt)
In = Builder->CreateShl(In,
ConstantInt::get(In->getType(), ShiftAmt));
// Distribute the bit over the whole bit width.
In = Builder->CreateAShr(In, ConstantInt::get(In->getType(),
BitWidth - 1), "sext");
}
if (CI.getType() == In->getType())
return ReplaceInstUsesWith(CI, In);
return CastInst::CreateIntegerCast(In, CI.getType(), true/*SExt*/);
}
}
}
// vector (x <s 0) ? -1 : 0 -> ashr x, 31 -> all ones if signed.

48
test/Transforms/InstCombine/sext.ll
View File

@ -136,3 +136,51 @@ define i64 @test12(i32 %x) nounwind {
; CHECK: sext
; CHECK: ret
}
define i32 @test13(i32 %x) nounwind {
%and = and i32 %x, 8
%cmp = icmp eq i32 %and, 0
%ext = sext i1 %cmp to i32
ret i32 %ext
; CHECK: @test13
; CHECK-NEXT: %and = lshr i32 %x, 3
; CHECK-NEXT: %1 = and i32 %and, 1
; CHECK-NEXT: %sext = add i32 %1, -1
; CHECK-NEXT: ret i32 %sext
}
define i32 @test14(i16 %x) nounwind {
%and = and i16 %x, 16
%cmp = icmp ne i16 %and, 16
%ext = sext i1 %cmp to i32
ret i32 %ext
; CHECK: @test14
; CHECK-NEXT: %and = lshr i16 %x, 4
; CHECK-NEXT: %1 = and i16 %and, 1
; CHECK-NEXT: %sext = add i16 %1, -1
; CHECK-NEXT: %ext = sext i16 %sext to i32
; CHECK-NEXT: ret i32 %ext
}
define i32 @test15(i32 %x) nounwind {
%and = and i32 %x, 16
%cmp = icmp ne i32 %and, 0
%ext = sext i1 %cmp to i32
ret i32 %ext
; CHECK: @test15
; CHECK-NEXT: %1 = shl i32 %x, 27
; CHECK-NEXT: %sext = ashr i32 %1, 31
; CHECK-NEXT: ret i32 %sext
}
define i32 @test16(i16 %x) nounwind {
%and = and i16 %x, 8
%cmp = icmp eq i16 %and, 8
%ext = sext i1 %cmp to i32
ret i32 %ext
; CHECK: @test16
; CHECK-NEXT: %1 = shl i16 %x, 12
; CHECK-NEXT: %sext = ashr i16 %1, 15
; CHECK-NEXT: %ext = sext i16 %sext to i32
; CHECK-NEXT: ret i32 %ext
}