http://llvm.org/bugs/show_bug.cgi?id=20900
Bug ID: 20900
Summary: optimize reciprocal square root with fast-math (x86)
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
$ ./clang -v
clang version 3.6.0 (217530)
Target: x86_64-apple-darwin13.3.0
Thread model: posix
$ cat rsqrt.c
#include <math.h>
float reciprocal_square_root(float x) {
return 1.0f / sqrtf(x);
}
$ ./clang -O2 -ffast-math -S -o - rsqrt.c
...
sqrtss %xmm0, %xmm1
movss LCPI0_0(%rip), %xmm0
divss %xmm1, %xmm0
---------------------------------------------------------------------
This should be optimized to use 'rsqrtss'.
ICC 14 does this at -O2:
rsqrtss %xmm0, %xmm2
mulss %xmm2, %xmm0
mulss %xmm2, %xmm0
movss L_2il0floatpacket.2(%rip), %xmm1
mulss %xmm1, %xmm2
subss L_2il0floatpacket.1(%rip), %xmm0
mulss %xmm2, %xmm0
ret
L_2il0floatpacket.1:
.long 0x40400000
L_2il0floatpacket.2:
.long 0xbf000000
--
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