[FFmpeg-devel] [PATCH 09/13] avcodec/svq1dec: clear MMX state after MB decode loop

[Ronald S. Bultje]

Hi,

On Mon, Oct 24, 2016 at 8:47 AM, wm4 <nfxjfg at googlemail.com> wrote:

> On Mon, 24 Oct 2016 07:54:47 -0400
> "Ronald S. Bultje" <rsbultje at gmail.com> wrote:
>
> > Hi,
> >
> > On Mon, Oct 24, 2016 at 3:36 AM, wm4 <nfxjfg at googlemail.com> wrote:
> >
> > > On Sun, 23 Oct 2016 13:02:01 -0400
> > > "Ronald S. Bultje" <rsbultje at gmail.com> wrote:
> > >
> > > > Hi,
> > > >
> > > > On Sat, Oct 22, 2016 at 11:36 PM, Michael Niedermayer <
> > > > michael at niedermayer.cc> wrote:
> > > >
> > > > > On Sat, Oct 22, 2016 at 10:10:01PM -0400, Ronald S. Bultje wrote:
> > > > > > Hi,
> > > > > >
> > > > > > general comment about all other dec patches.
> > > > > >
> > > > > > On Sat, Oct 22, 2016 at 3:02 PM, Michael Niedermayer
> > > > > <michael at niedermayer.cc
> > > > > > > wrote:
> > > > > >
> > > > > > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > > > > > ---
> > > > > > >  libavcodec/svq1dec.c | 2 ++
> > > > > > >  1 file changed, 2 insertions(+)
> > > > > > >
> > > > > > > diff --git a/libavcodec/svq1dec.c b/libavcodec/svq1dec.c
> > > > > > > index 2b72e08..0fe222e 100644
> > > > > > > --- a/libavcodec/svq1dec.c
> > > > > > > +++ b/libavcodec/svq1dec.c
> > > > > > > @@ -744,6 +744,7 @@ static int svq1_decode_frame(AVCodecConte
> xt
> > > > > *avctx,
> > > > > > > void *data,
> > > > > > >              }
> > > > > > >          }
> > > > > > >      }
> > > > > > > +    emms_c();
> > > > > >
> > > > > >
> > > > > > This is hideous, you're sprinkling emms_c in various places to
> make
> > > some
> > > > > > stupid test pass. The test is morbidly stupid and there is no
> general
> > > > > > consensus on patterns to be followed as for where to place
> emms_c.
> > > > > Someone
> > > > > > who doesn't know any better will litter each new decoder with
> 10-20
> > > calls
> > > > > > to emms_c just because he found that other decoders do it in
> > > > > undocumented,
> > > > > > unexplained and unclear locations also.
> > > > > >
> > > > > > If you want this to be a "thing", you need to design and document
> > > > > carefully
> > > > > > where emms_c is necessary. Then come up with some system that
> makes
> > > this
> > > > > > work by itself.
> > > > >
> > > > > Your mail sounds quite aggressive to me, did i say something to
> anger
> > > > > you ? It was certainly not intended
> > > > >
> > > > > About this patchset
> > > > > FFmpeg is broken ATM (both in theory and practice with some libcs),
> > > > > the way MMX code is used is not correct, emms_c()
> > > > > calls are missing and required. The obvious way to fix that
> > > > > (in practice) is adding emms_c() calls where they are missing.
> > > > > I can document why each call is needed, if thats wanted?
> > > >
> > > >
> > > > Your representation of facts is strange, to say the least. Let's
> explore
> > > > two related claims:
> > > >
> > > > - (A) ffmpeg is broken in practice when calling musl malloc/free
> > > functions
> > > > after calling MMX SIMD functions on x86-32 (which crashes).
> > > > - (B) ffmpeg is broken in theory because we don't clear FPU state (as
> > > > required) at the end of MMX SIMD functions.
> > >
> > > I was under the impression that it is UB to have the FPU in MMX state
> > > at any time while in C, not just while e.g. calling the stdlib. Maybe I
> > > got that wrong (how would MMX intrinsics even work?) - can anyone shed
> > > light on the exact requirements? (Possibly again, sorry.)
> >
> >
> > I'm under the impression that it's part of the calling convention. That
> is,
> > any code anywhere (including mmx intrinsics, indeed) can - when called -
> > expect the state to be cleared by the caller, just like you'd expect
> > eax/edx to be caller-save (whereas esi/edi are callee-save).
> >
> > However, if you never call external code (including intrinsics), you can
> > ignore this, just as you can ignore / create your own calling
> > convention (remember fastcall etc.?). However, when calling any external
> > code, this could (in theory) crash; it's just that right now it only
> > crashes with musl when calling malloc/free. So basically, ffmpeg has its
> > own calling convention, and manually calling emms_c() fixes "ffmpeg"
> > calling convention to be compatible with "standard" calling
> convention...?
>
> It can't really be a calling convention unless the compiler is aware of
> it?
>
> We're doing things behind the compiler's back here. The safest
> assumption would be that leaving the FPU state invalid while in C is
> not allowed, period.
>
> The next safest assumption is that it's fine as long as we explicitly
> don't use floating point or call external functions that aren't
> MMX-aware. This would mean calling any libc functions or user callbacks
> (including indirectly through e.g. av_log) is not fine.
>

Yes.

And while I don't agree with it, I feel we should also mention Carl Eugen's
alternative proposal, which is to document that ffmpeg doesn't formally
adhere to this calling convention requirement [1].

Of course this is not as easy to verify as adding a hack-assert to
> av_malloc/av_free.


I agree.

(Which is why I insist that the design of this particular "fix" is so
important.)

Ronald

[1] https://patchwork.ffmpeg.org/patch/841/