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Implement rdar://6480391, extending of equality icmp's to avoid a truncation.

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I noticed this in the code compiled for a routine using std::map, which produced
this code:
	%25 = tail call i32 @memcmp(i8* %24, i8* %23, i32 6) nounwind readonly
	%.lobit.i = lshr i32 %25, 31		; <i32> [#uses=1]
	%tmp.i = trunc i32 %.lobit.i to i8		; <i8> [#uses=1]
	%toBool = icmp eq i8 %tmp.i, 0		; <i1> [#uses=1]
	br i1 %toBool, label %bb3, label %bb4
which compiled to:

	call	L_memcmp$stub
	shrl	$31, %eax
	testb	%al, %al
	jne	LBB1_11	## 

with this change, we compile it to:

	call	L_memcmp$stub
	testl	%eax, %eax
	js	LBB1_11

This triggers all the time in common code, with patters like this:

	%169 = and i32 %ply, 1		; <i32> [#uses=1]
	%170 = trunc i32 %169 to i8		; <i8> [#uses=1]
	%toBool = icmp ne i8 %170, 0		; <i1> [#uses=1]

 	%7 = lshr i32 %6, 24		; <i32> [#uses=1]
	%9 = trunc i32 %7 to i8		; <i8> [#uses=1]
	%10 = icmp ne i8 %9, 0		; <i1> [#uses=1]

etc



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61985 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-01-09 07:47:06 +00:00
parent 6d6b410698
commit a80d668215
3 changed files with 41 additions and 4 deletions
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22
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -6343,6 +6343,28 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
const APInt &RHSV = RHS->getValue();
switch (LHSI->getOpcode()) {
case Instruction::Trunc:
if (ICI.isEquality() && LHSI->hasOneUse()) {
// Simplify icmp eq (trunc x to i8), 42 -> icmp eq x, 42|highbits if all
// of the high bits truncated out of x are known.
unsigned DstBits = LHSI->getType()->getPrimitiveSizeInBits(),
SrcBits = LHSI->getOperand(0)->getType()->getPrimitiveSizeInBits();
APInt Mask(APInt::getHighBitsSet(SrcBits, SrcBits-DstBits));
APInt KnownZero(SrcBits, 0), KnownOne(SrcBits, 0);
ComputeMaskedBits(LHSI->getOperand(0), Mask, KnownZero, KnownOne);
// If all the high bits are known, we can do this xform.
if ((KnownZero|KnownOne).countLeadingOnes() >= SrcBits-DstBits) {
// Pull in the high bits from known-ones set.
APInt NewRHS(RHS->getValue());
NewRHS.zext(SrcBits);
NewRHS |= KnownOne;
return new ICmpInst(ICI.getPredicate(), LHSI->getOperand(0),
ConstantInt::get(NewRHS));
}
}
break;
case Instruction::Xor: // (icmp pred (xor X, XorCST), CI)
if (ConstantInt *XorCST = dyn_cast<ConstantInt>(LHSI->getOperand(1))) {
// If this is a comparison that tests the signbit (X < 0) or (x > -1),

22
test/Transforms/InstCombine/cast.ll
View File

@ -1,7 +1,5 @@
; Tests to make sure elimination of casts is working correctly
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | \
; RUN: grep %c | notcast
; END.
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep %c | notcast
@inbuf = external global [32832 x i8] ; <[32832 x i8]*> [#uses=1]
@ -238,3 +236,21 @@ define i16 @test35(i16 %a) {
ret i16 %c2
}
; icmp sgt i32 %a, -1
; rdar://6480391
define i1 @test36(i32 %a) {
%b = lshr i32 %a, 31
%c = trunc i32 %b to i8
%d = icmp eq i8 %c, 0
ret i1 %d
}
; ret i1 false
define i1 @test37(i32 %a) {
%b = lshr i32 %a, 31
%c = or i32 %b, 512
%d = trunc i32 %c to i8
%e = icmp eq i8 %d, 11
ret i1 %e
}

1
test/Transforms/InstCombine/icmp.ll
View File

@ -1,5 +1,4 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | not grep icmp
; END.
define i32 @test1(i32 %X) {
entry: