[Bug c/37823] New: Missed optimization - ffs() from strings.h
jjk at acm dot org
gcc-bugzilla@gcc.gnu.org
Tue Oct 14 09:14:00 GMT 2008
GCC produces some redundant addl/subl instructions for the following function,
which calls the built-in ffs():
--- snip ---
#include <strings.h>
int
foo(int i, int j)
{
return i + ffs(j) - 1;
}
--- output of gcc -O3 -m32 -S foo.c ---
foo:
pushl %ebp
xorl %edx, %edx
movl %esp, %ebp
bsfl 12(%ebp), %eax
sete %dl
negl %edx
orl %edx, %eax
addl $1, %eax
subl $1, %eax
addl 8(%ebp), %eax
popl %ebp
ret
--- output of gcc -O3 -S foo.c ---
foo:
bsfl %esi, %esi
movl $-1, %eax
cmove %eax, %esi
addl $1, %esi
leal -1(%rsi,%rdi), %eax
ret
--
Summary: Missed optimization - ffs() from strings.h
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jjk at acm dot org
GCC host triplet: x86_64-redhat-linux
GCC target triplet: x86_64-redhat-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37823
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