Joachim B Haga <[EMAIL PROTECTED]> wrote on 9 Dec 2007 1:23:
> It looks like it could be significantly faster in the general case, but
> possibly not if most of the compared times differ in the tv_sec field.
>
> A non-macro variant could be something like
>...
Good idea. You may help current compilers/processors with an explicit branch
prediction. But
your code is not bad, because it creates the same output ;-)
#include <sys/cdefs.h>
#include <sys/time.h>
int
timeval_before(struct timeval *t1, struct timeval *t2)
{
return (t1->tv_sec != t2->tv_sec ?
t1->tv_sec < t2->tv_sec :
t1->tv_usec < t2->tv_usec);
}
int
timeval_before2(struct timeval *t1, struct timeval *t2)
{
if ( __predict_true(t1->tv_sec != t2->tv_sec))
return (t1->tv_sec < t2->tv_sec);
else
return (t1->tv_usec < t2->tv_usec);
}
#gcc -O2 -march=pentium4 -Wall -fomit-frame-pointer -c -S tst.c
#cat tst.s
.file "tst.c"
.text
.globl timeval_before
.type timeval_before, @function
timeval_before:
movl 4(%esp), %edx
movl 8(%esp), %ecx
movl (%ecx), %eax
cmpl %eax, (%edx)
je .L2
setl %al
movzbl %al, %eax
ret
.L2:
movl 4(%edx), %eax
cmpl 4(%ecx), %eax
setl %al
movzbl %al, %eax
ret
.size timeval_before, .-timeval_before
.globl timeval_before2
.type timeval_before2, @function
timeval_before2:
movl 4(%esp), %edx
movl 8(%esp), %ecx
movl (%ecx), %eax
cmpl %eax, (%edx)
je .L8
setl %al
movzbl %al, %eax
ret
.L8:
movl 4(%edx), %eax
cmpl 4(%ecx), %eax
setl %al
movzbl %al, %eax
ret
.size timeval_before2, .-timeval_before2
.ident "GCC: (GNU) 4.2.1 20070719 [FreeBSD]"
The prediction macros in cdefs.h are defined as
#define __predict_true(exp) __builtin_expect((exp), 1)
#define __predict_false(exp) __builtin_expect((exp), 0)
--
Frank Behrens, Osterwieck, Germany
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