[FFmpeg-devel] [PATCH] split-radix FFT
Tue Jul 29 19:33:16 CEST 2008
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
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:
"mov globivar(%rip), %rax\n\t"
"mov %0, %%rax\n\t"
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
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