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Remove a very old instcombine where we would turn sequences of selects into

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logical operations on the i1's driving them.  This is a bad idea for every
target I can think of (confirmed with micro tests on all of: x86-64, ARM,
AArch64, Mips, and PowerPC) because it forces the i1 to be materialized into
a general purpose register, whereas consuming it directly into a select generally
allows it to exist only transiently in a predicate or flags register.

Chandler ran a set of performance tests with this change, and reported no
measurable change on x86-64.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201275 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Owen Anderson 2014-02-12 23:54:07 +00:00
parent 029a76b0a2
commit 3042a65e5f
2 changed files with 24 additions and 25 deletions
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25
lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@ -3313,31 +3313,6 @@ Instruction *InstCombiner::visitFCmpInst(FCmpInst &I) {
if (Instruction *NV = FoldFCmp_IntToFP_Cst(I, LHSI, RHSC))
return NV;
break;
case Instruction::Select: {
// If either operand of the select is a constant, we can fold the
// comparison into the select arms, which will cause one to be
// constant folded and the select turned into a bitwise or.
Value *Op1 = 0, *Op2 = 0;
if (LHSI->hasOneUse()) {
if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1))) {
// Fold the known value into the constant operand.
Op1 = ConstantExpr::getCompare(I.getPredicate(), C, RHSC);
// Insert a new FCmp of the other select operand.
Op2 = Builder->CreateFCmp(I.getPredicate(),
LHSI->getOperand(2), RHSC, I.getName());
} else if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2))) {
// Fold the known value into the constant operand.
Op2 = ConstantExpr::getCompare(I.getPredicate(), C, RHSC);
// Insert a new FCmp of the other select operand.
Op1 = Builder->CreateFCmp(I.getPredicate(), LHSI->getOperand(1),
RHSC, I.getName());
}
}
if (Op1)
return SelectInst::Create(LHSI->getOperand(0), Op1, Op2);
break;
}
case Instruction::FSub: {
// fcmp pred (fneg x), C -> fcmp swap(pred) x, -C
Value *Op;

24
test/Transforms/InstCombine/select-select.ll Normal file
View File

@ -0,0 +1,24 @@
; RUN: opt -instcombine -S < %s | FileCheck %s
; CHECK: @foo1
define float @foo1(float %a) #0 {
; CHECK-NOT: xor
%b = fcmp ogt float %a, 0.000000e+00
%c = select i1 %b, float %a, float 0.000000e+00
%d = fcmp olt float %c, 1.000000e+00
%f = select i1 %d, float %c, float 1.000000e+00
ret float %f
}
; CHECK: @foo2
define float @foo2(float %a) #0 {
; CHECK-NOT: xor
%b = fcmp ogt float %a, 0.000000e+00
%c = select i1 %b, float %a, float 0.000000e+00
%d = fcmp olt float %c, 1.000000e+00
%e = select i1 %b, float %a, float 0.000000e+00
%f = select i1 %d, float %e, float 1.000000e+00
ret float %f
}
attributes #0 = { nounwind readnone ssp uwtable }