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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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24
test/Transforms/InstCombine/select-select.ll Normal file
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@@ -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 }