[cairo] Re: Speeding up _cairo_fixed_from_double

[Nicholas Miell]

On Mon, 30 Oct 2006 15:58:12 -0500, Behdad Esfahbod wrote:

> On Mon, 2006-10-30 at 01:42 -0500, Jamey Sharp wrote:
>> On Sun, Oct 29, 2006 at 08:12:48PM -0500, Jim Gettys wrote:
>> > It should be.  The man page refers to dead code elimination (dce) being
>> > the default at -O or higher.
>> 
>> Sadly, GCC isn't recognizing that as dead code, so it can't eliminate
>> it. With -O, the version of GCC I tested [1] does some remarkable tricks
>> to eliminate branches, but still checks endianness at runtime. I tested
>> this C:
>> 
>> 	int endian = 1;
>> 	if (*(char *) &endian)
>> 		return '\154'; /* 'l' */
>> 	else
>> 		return '\102'; /* 'B' */
>> 
>> GCC produced this x86 assembly:
>> 
>> 	movl    $1, %eax
>> 	movl    %eax, 12(%esp)
>> 	cmpb    $1, 12(%esp)
>> 	sbbl    %eax, %eax
>> 	andl    $-42, %eax
>> 	addl    $108, %eax
>> 
>> Impressive, but not what was desired.
>> 
>> --Jamey
>> 
>> [1] GCC: (GNU) 4.1.2 20060901 (prerelease) (Debian 4.1.1-13)
> 
> Cannot reproduce.
> 
> This code:
> 
> void
> main (void)
> {
>   int endian = 1;
>   if (*(char *) &endian)
>     return '\154'; /* 'l' */
>   else
>     return '\102'; /* 'B' */
> }
> 

Try "int main(void)" instead of "void main(void)".

Compiling with -Wall helps, too. :)

And for little one-off tests like this, you shouldn't use main (on x86,
anyway -- gcc inserts extra stack alignment ops which confuses the issue).