[FFmpeg-devel] [PATCH] split-radix FFT

[Michael Niedermayer]

On Tue, Jul 29, 2008 at 05:20:15PM +0100, M?ns Rullg?rd wrote:
> 
> Michael Niedermayer wrote:
> > On Tue, Jul 29, 2008 at 06:26:49PM +0300, Uoti Urpala wrote:
> >> On Tue, 2008-07-29 at 17:10 +0200, Michael Niedermayer wrote:
> >> > And just to clarify, yes what i considered a good argument was the
> >> sentance
> >> > above where my reply is. That is to use MANGLE in speed critical code.
> >> > That way most textrels are avoided while minimizing the speed impact.
> >> >
> >> > I do not think you ever argued for that.
> >>
> >> IIRC I did mention the possibility of omitting -fPIC for a subset of
> >> files.
> >>
> >> >  I remember you strongly arguing
> >> > toward replacing all MANGLE by "m" knowing that it would break gcc 2.95
> >> > and not really caring that it would slow down code compiled with -fPIC.
> >>
> >> Of course the code would be slower on x86. If you want it to be as fast
> >> as possible then compile it with -fPIC on x86. I don't think it's
> >> worthwhile to pick only the globals used inside asm for such special
> >> treatment.
> >
> > x86-64 shared libs require -fPIC, unless that has been fixed.
> 
> The x86-64 instruction set hasn't been "fixed", and I doubt it ever
> will be.  You simply can't fit a 64-bit offset in a 32-bit immediate
> operand.

Thats not what i meant


> 
> > so the user does not always have the option to omit -fPIC
> 
> But in these cases, forcing a textrel will break the build.

MANGLE forces rip relative addressing on x86-64 and thus avoids the
occasional GOT indirection gcc adds.

Heres a example:
long globivar;

void func(){
    asm(
        "mov globivar(%rip), %rax\n\t"
    );
    asm(
        "mov %0, %%rax\n\t"
        :: "m"(globivar)
    );
}

results in:
0000000000000554 <func>:
 554:	55                   	push   %rbp
 555:	48 89 e5             	mov    %rsp,%rbp
 558:	48 8b 05 d1 02 20 00 	mov    0x2002d1(%rip),%rax        # 200830 <globivar>
 55f:	48 8b 05 8a 02 20 00 	mov    0x20028a(%rip),%rax        # 2007f0 <_DYNAMIC+0x1b8>
 566:	48 8b 00             	mov    (%rax),%rax
 569:	c9                   	leaveq 
 56a:	c3                   	retq   


you can see the second needs 2 instructions, the first just 1.


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Dictatorship naturally arises out of democracy, and the most aggravated
form of tyranny and slavery out of the most extreme liberty. -- Plato
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: Digital signature
URL: <http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/attachments/20080729/ef1e8495/attachment.pgp>