flac dev - Mar 2014 - About "attempt to fix differences between x86 FPU and SSE calculations"

Olivier Tristan <o.tristan at uvi.net> ?????(?) ? ????? ?????? Fri, 21 Mar
2014 22:41:00 +0400:
> Check with -mfpmath=387 to be sure that x87 FPU code is used and not some
> SSE optim made by GCC
I added "XIPH_ADD_CFLAGS([-mfpmath=387])" into configure.ac
Still the result is different from SSE version.

---------------

MSVS adds two instructions to the generated code after the patch:

	fld	DWORD PTR [eax]
	inc	ecx
	fmul	ST(0), ST(1)
	add	eax, 4
	fstp	DWORD PTR tv2337[esp+20]    <- this: (copy from FP stack to tmp)
	fld	DWORD PTR tv2337[esp+20]    <- and this (copy from tmp to FP stack)
	fadd	DWORD PTR [ebx+ecx*4-4]
	fstp	DWORD PTR [ebx+ecx*4-4]


However GCC doesn't do this:

	lea	ecx, [eax+2]
	fld	DWORD PTR [edx+ecx*4]
	fmul	st, st(1)
	fadd	DWORD PTR [ebx+ecx*4]
	fstp	DWORD PTR [ebx+ecx*4]

Also MSVS doesn't add these instructions if Floating Point Model is set to
Fast (/fp:fast).

op 22-03-14 09:17, lvqcl schreef:
> I added "XIPH_ADD_CFLAGS([-mfpmath=387])" into configure.ac
> Still the result is different from SSE version.
Maybe it should be left alone. The difference is not really 
harmful (aside from the people wondering who might think it is a 
serious bug), it has been documented in the FAQ for years and 
'solving' this is probably going to be a wild goose chase.

I think this is not worth the added code complexity.

Martijn van Beurden wrote:
> op 22-03-14 09:17, lvqcl schreef:
> > I added "XIPH_ADD_CFLAGS([-mfpmath=387])" into configure.ac
> > Still the result is different from SSE version.
> 
> Maybe it should be left alone. The difference is not really 
> harmful (aside from the people wondering who might think it is a 
> serious bug), it has been documented in the FAQ for years and 
> 'solving' this is probably going to be a wild goose chase.
> 
> I think this is not worth the added code complexity.
I agree. Unless anyone can some up with a good reason for it
to stay, I'm going to revert this patch tomorrow.

Cheers,
Erik
-- 
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Erik de Castro Lopo
http://www.mega-nerd.com/