Re: [FFmpeg-devel] [PATCH 2/4] avcodec/get_bits: Avoid 2nd bitstream read in GET_VLC() if bits are known at build and small

From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH 2/4] avcodec/get_bits: Avoid 2nd bitstream read in GET_VLC() if bits are known at build and small
Date: Fri, 27 Oct 2023 05:10:32 +0200
Message-ID: <AS8P250MB0744FA16EEE8F6C73B5E94B28FDCA@AS8P250MB0744.EURP250.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <20231024150443.7438-2-michael@niedermayer.cc>

Michael Niedermayer:
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavcodec/get_bits.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/get_bits.h b/libavcodec/get_bits.h
> index cfcf97c021c..86cea00494a 100644
> --- a/libavcodec/get_bits.h
> +++ b/libavcodec/get_bits.h
> @@ -581,8 +581,12 @@ static inline const uint8_t *align_get_bits(GetBitContext *s)
>          n     = table[index].len;                               \
>                                                                  \
>          if (max_depth > 1 && n < 0) {                           \
> -            LAST_SKIP_BITS(name, gb, bits);                     \
> -            UPDATE_CACHE(name, gb);                             \
> +            if (av_builtin_constant_p(bits <= MIN_CACHE_BITS/2) && bits <= MIN_CACHE_BITS/2) { \
> +                SKIP_BITS(name, gb, bits);                      \
> +            } else {                                            \
> +                LAST_SKIP_BITS(name, gb, bits);                 \
> +                UPDATE_CACHE(name, gb);                         \
> +            }                                                   \
>                                                                  \
>              nb_bits = -n;                                       \
>                                                                  \

This is problematic: The GET_VLC macro does not presume that
MIN_CACHE_BITS are available; there is code that directly uses GET_VLC
instead of get_vlc2().

I had the same idea when I made my VLC patchset, yet I wanted to first
apply it (which I forgot). While investigating the above issue, I found
out that all users of GET_VLC always call UPDATE_CACHE immediately
before GET_VLC, so UPDATE_CACHE should be moved into GET_VLC;
furthermore, no user of GET_VLC relies on the reloads inside of GET_VLC.
The patches for this are here:
https://github.com/mkver/FFmpeg/commits/vlc Shall I send them?

Notice that making GET_VLC more standalone enables improvements over the
current approach; yet it will not lead to optimal code: E.g. the VLCs in
decode_alpha_block() in speedhqdec.c are so short that one could read
both VLCs with only one UPDATE_CACHE(); another example is mjpegdec.c
which currently does this:

        GET_VLC(code, re, &s->gb, s->vlcs[1][ac_index].table, 9, 2);

        i += ((unsigned)code) >> 4;
            code &= 0xf;
        if (code) {
            if (code > MIN_CACHE_BITS - 16)
                UPDATE_CACHE(re, &s->gb);

            {
                int cache = GET_CACHE(re, &s->gb);
                int sign  = (~cache) >> 31;
                level     = (NEG_USR32(sign ^ cache,code) ^ sign) - sign;
            }

            LAST_SKIP_BITS(re, &s->gb, code);

Because of the reloads in GET_VLC, there will always be at least
MIN_CACHE_BITS - 9 (= 16) bits available after GET_VLC, so one can read
code (<= 15) bits without updating the cache at all (16 in
MIN_CACHE_BITS - 16 is the maximum length of a VLC code used here); this
will no longer be possible with this optimization.
Btw: I am surprised that there is a branch before UPDATE_CACHE instead
of an unconditional UPDATE_CACHE. I also do not really see why this uses
these macros directly and not the functions.

Given my objection to your patch #1, magicyuv will not benefit from
this; a different approach (see
https://github.com/mkver/FFmpeg/commit/9b5a977957968c0718dea55a5b15f060ef6201dc)
is to add a get_vlc() that uses the nb of bits used to create the VLC
and a compile-time upper bound for the maximum length of a VLC code as
parameters instead of the maximum depth of the VLC.

Reading VLCs for the cached bitstream reader can btw also be improved:
https://github.com/mkver/FFmpeg/commit/fba57506a9cf6be2f4aa5eeee7b10d54729fd92a

- Andreas

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