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For x86, canonicalize max

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(x > y) ? x : y
=>
(x >= y) ? x : y

So for something like
(x - y) > 0 : (x - y) ? 0
It will be
(x - y) >= 0 : (x - y) ? 0

This makes is possible to test sign-bit and eliminate a comparison against
zero. e.g.
subl   %esi, %edi
testl  %edi, %edi
movl   $0, %eax
cmovgl %edi, %eax
=>
xorl   %eax, %eax
subl   %esi, $edi
cmovsl %eax, %edi

rdar://10633221

llvm-svn: 147512
This commit is contained in:
Evan Cheng 2012-01-04 01:41:39 +00:00
parent c69557e758
commit caba5d2fc2
2 changed files with 46 additions and 1 deletions
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31
lib/Target/X86/X86ISelLowering.cpp
View File

@ -13139,6 +13139,37 @@ static SDValue PerformSELECTCombine(SDNode *N, SelectionDAG &DAG,
}
}
// Canonicalize max and min:
// (x > y) ? x : y -> (x >= y) ? x : y
// (x < y) ? x : y -> (x <= y) ? x : y
// This allows use of COND_S / COND_NS (see TranslateX86CC) which eliminates
// the need for an extra compare
// against zero. e.g.
// (x - y) > 0 : (x - y) ? 0 -> (x - y) >= 0 : (x - y) ? 0
// subl %esi, %edi
// testl %edi, %edi
// movl $0, %eax
// cmovgl %edi, %eax
// =>
// xorl %eax, %eax
// subl %esi, $edi
// cmovsl %eax, %edi
if (N->getOpcode() == ISD::SELECT && Cond.getOpcode() == ISD::SETCC &&
DAG.isEqualTo(LHS, Cond.getOperand(0)) &&
DAG.isEqualTo(RHS, Cond.getOperand(1))) {
ISD::CondCode CC = cast<CondCodeSDNode>(Cond.getOperand(2))->get();
switch (CC) {
default: break;
case ISD::SETLT:
case ISD::SETGT: {
ISD::CondCode NewCC = (CC == ISD::SETLT) ? ISD::SETLE : ISD::SETGE;
Cond = DAG.getSetCC(Cond.getDebugLoc(), Cond.getValueType(),
Cond.getOperand(0), Cond.getOperand(1), NewCC);
return DAG.getNode(ISD::SELECT, DL, VT, Cond, LHS, RHS);
}
}
}
return SDValue();
}

16
test/CodeGen/X86/jump_sign.ll
View File

@ -1,7 +1,9 @@
; RUN: llc < %s -march=x86 | grep jns
; RUN: llc < %s -march=x86 | FileCheck %s
define i32 @f(i32 %X) {
entry:
; CHECK: f:
; CHECK: jns
%tmp1 = add i32 %X, 1 ; <i32> [#uses=1]
%tmp = icmp slt i32 %tmp1, 0 ; <i1> [#uses=1]
br i1 %tmp, label %cond_true, label %cond_next
@ -18,3 +20,15 @@ cond_next: ; preds = %cond_true, %entry
declare i32 @bar(...)
declare i32 @baz(...)
; rdar://10633221
define i32 @g(i32 %a, i32 %b) nounwind {
entry:
; CHECK: g:
; CHECK-NOT: test
; CHECK: cmovs
%sub = sub nsw i32 %a, %b
%cmp = icmp sgt i32 %sub, 0
%cond = select i1 %cmp, i32 %sub, i32 0
ret i32 %cond
}