[FFmpeg-devel] [RFC] clobbers for XMM registers

[Ramiro Polla]

On Sun, Oct 3, 2010 at 11:40 PM, Ramiro Polla <ramiro.polla at gmail.com> wrote:
> On Sun, Oct 3, 2010 at 4:43 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>> On Sun, Oct 3, 2010 at 3:20 PM, Ramiro Polla <ramiro.polla at gmail.com> wrote:
>>> On Thu, Sep 30, 2010 at 10:50 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>>>> On Thu, Sep 30, 2010 at 08:49:04PM -0400, Alexander Strange wrote:
>>>>> On Sep 30, 2010, at 8:31 PM, Michael Niedermayer wrote:
>>>>> > On Fri, Oct 01, 2010 at 12:42:33AM +0100, M?ns Rullg?rd wrote:
>>>>> >> Michael Niedermayer <michaelni at gmx.at> writes:
>>>>> >>> On Thu, Sep 30, 2010 at 04:02:00PM -0300, Ramiro Polla wrote:
>>>>> >>>> 2010/9/30 M?ns Rullg?rd <mans at mansr.com>:
>>>>> >>>>> Ramiro Polla <ramiro.polla at gmail.com> writes:
>>>>> >>>>>> 2010/9/30 M?ns Rullg?rd <mans at mansr.com>:
>>>>> >>>>>>> Ramiro Polla <ramiro.polla at gmail.com> writes:
>>>>> >>>>>>>> What about
>>>>> >>>>>>>> #if HAVE_XMM_CLOBBERS
>>>>> >>>>>>>> # ? ?define XMM_CLOBBERS(a, ...) __VA_ARGS__
>>>>> >>>>>>>> #else
>>>>> >>>>>>>> # ? ?define XMM_CLOBBERS(a, ...) a
>>>>> >>>>>>>> #endif
>>>>> >>>>>>>>
>>>>> >>>>>>>> to be used as in lavc/x86/fft_sse.c:
>>>>> >>>>>>>> ? ? ? ? :"+r"(j), "+r"(k)
>>>>> >>>>>>>> ? ? ? ? :"r"(output+n4), "r"(output+n4*3),
>>>>> >>>>>>>> ? ? ? ? ?"m"(*m1m1m1m1)
>>>>> >>>>>>>> ? ? ? ? XMM_CLOBBERS(, : "%xmm0", "%xmm1", "%xmm7")
>>>>> >>>>>>>> ? ? );
>>>>> >>>>>>>
>>>>> >>>>>>> That falls over if any other clobbers are needed.
>>>>> >>>>>>
>>>>> >>>>>> If any other clobbers are needed they could be written before the macro.
>>>>> >>>>>
>>>>> >>>>> That won't work.
>>>>> >>>>
>>>>> >>>> : "eax" XMM_CLOBBERS(,, "%xmm0", "%xmm1", "%xmm7")
>>>>> >>>>
>>>>> >>>> But I agree having "cc" as dummy is simpler.
>>>>> >>>
>>>>> >>> it may be simpler but your variant is more correct
>>>>> >>
>>>>> >> If "cc" is a no-op neither variant is more correct. ?Mine is a hell of
>>>>> >> a lot prettier though.
>>>>> >
>>>>> > unless "cc" is documented to be a no-op on x86 we should not assume it is
>>>>>
>>>>> svn blame on gcc says it hasn't changed since x86 was rewritten in 1999.
>>>>> (the code in question being ix86_md_asm_clobbers(), in a file too big to display in viewcvs)
>>>>>
>>>>> I hardly see how it could change now since it would break all users of asm.
>>>>
>>>> i agree with you still i dont think the risk with cc byting us is 0 and theres
>>>> a perfectly working solution, namely ramiros macro.
>>>> and it wont cost us anything to use ramiros macro now. Changing it later will
>>>> cost us man hours.
>>>
>>> So to keep this thread going, 3 options have been suggested:
>>> 1- using "cc" as a dummy clobber
>>> 2- using 2 macros
>>> 3- using 1 macro that has both clobber lists (with and without xmm_support)
>>>
>>> 3 will inevitably lead someone to write XMM_CLOBBERS("xmm6", "xmm7")
>>> without the first leading comma which is not intuitive so I consider
>>> it the worst option. 2 seems nice to me but apparently just me. 1 is
>>> more elegant at the cost of depending on "cc" being a no-op on i386 (I
>>> couldn't find any occurrence in gcc's source code where it did matter
>>> though).
>>>
>>> Ronald, what's your choice?
>>
>> 2 is fine with me also. It's not prettiest, but (imo) our asm isn't
>> supposed to be pretty, it's supposed to be rock-solid (and fast). If
>> you want something pretty, you use C or some fancy
>> scripting-language-of-the-day. Can you re-post your patch plus add a
>> few lines of doxy that include an example (yes, I want the docs to
>> include an example) of which to use when and how, plus your patch that
>> makes this used in fft? I'll start working on fixing the remaining
>> dsputil/enc functions then (that'll take some time since I need to
>> merge various asm blocks).
>
> Attached. I'm not happy with the documentation though, feel free to improve it.

Both applied. Flame (or better yet, fix directly) on -cvslog.