"Ronald S. Bultje" <[email protected]> writes:
> From: "Ronald S. Bultje" <[email protected]>
>
> Based on work (for GCI) by Aneesh Dogra <[email protected]>, and
> inspired by patch in Chromium by Chris Evans <[email protected]>.
>
> When turned on, H264/CAVLC gets ~15% (CVPCMNL1_SVA_C.264) slower for
> ultra-high-bitrate files, or ~2.5% (CVFI1_SVA_C.264) for lower-bitrate
> files. Other codecs are affected to a lesser extent because they are
> less optimized; e.g., VC-1 slows down by less than 1% (all on x86).
> The patch generated 3 extra instructions (cmp, cmovae and mov) per
> call to get_bits().
> ---
> configure | 2 ++
> libavcodec/get_bits.h | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
> libavcodec/wmavoice.c | 2 ++
> 3 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/configure b/configure
> index b3af2e9..056f409 100755
> --- a/configure
> +++ b/configure
> @@ -113,6 +113,7 @@ Configuration options:
> --disable-dxva2 disable DXVA2 code
> --enable-runtime-cpudetect detect cpu capabilities at runtime (bigger
> binary)
> --enable-hardcoded-tables use hardcoded tables instead of runtime
> generation
> + --disable-safe-bitstream-reader disable buffer boundary checking in
> bitreaders (faster, but may crash)
> --enable-memalign-hack emulate memalign, interferes with memory debuggers
> --disable-everything disable all components listed below
> --disable-encoder=NAME disable encoder NAME
The help output should list the flag needed to change the default. As
written, it gives the impression it is on by default.
> @@ -144,7 +164,13 @@ for examples see get_bits, show_bits, skip_bits, get_vlc
> # endif
>
> // FIXME name?
> +#if UNCHECKED_BITSTREAM_READER
> # define SKIP_COUNTER(name, gb, num) name##_index += (num)
> +#else
> +# define SKIP_COUNTER(name, gb, num) name##_index = \
> + (gb)->size_in_bits_plus1 - name##_index < (num) ? \
> + (gb)->size_in_bits_plus1 : name##_index + (num)
> +#endif
Thinking more carefully about it, this addition is overflow-safe since
the skip amount must be small and we require end padding. Your old
version with FFMIN(size, index + num) is thus safe here after all,
should it result in better code.
> # define SKIP_BITS(name, gb, num) do { \
> SKIP_CACHE(name, gb, num); \
> @@ -171,7 +197,12 @@ static inline int get_bits_count(const GetBitContext *s){
> }
>
> static inline void skip_bits_long(GetBitContext *s, int n){
> +#if UNCHECKED_BITSTREAM_READER
> s->index += n;
> +#else
> + s->index = s->size_in_bits_plus1 - s->index < n ?
> + s->size_in_bits_plus1 : s->index + n;
> +#endif
> }
This one allows an arbitrary n, so it must be done safely, e.g. like this.
> #elif defined A32_BITSTREAM_READER
> @@ -196,7 +227,9 @@ static inline void skip_bits_long(GetBitContext *s, int
> n){
> const uint32_t next = av_be2ne32(*name##_buffer_ptr); \
> name##_cache0 |= NEG_USR32(next, name##_bit_count); \
> name##_cache1 |= next << name##_bit_count; \
> - name##_buffer_ptr++; \
> + if (UNCHECKED_BITSTREAM_READER || \
> + name##_buffer_ptr < (const uint32_t *)(gb)->buffer_end) \
> + name##_buffer_ptr++; \
> name##_bit_count -= 32; \
> } \
> } while (0)
> @@ -230,7 +263,12 @@ static inline int get_bits_count(const GetBitContext *s)
> {
> static inline void skip_bits_long(GetBitContext *s, int n){
> OPEN_READER(re, s);
> re_bit_count += n;
> +#if UNCHECKED_BITSTREAM_READER
> re_buffer_ptr += re_bit_count>>5;
> +#else
> + re_buffer_ptr = (re_bit_count >> 5) < (const uint32_t *) s->buffer_end -
> re_buffer_ptr ?
> + re_buffer_ptr + (re_bit_count >> 5) : (const uint32_t *)
> s->buffer_end;
> +#endif
This is again guaranteed to fall within the padding region, so whatever
variant gives better code is OK to use here.
> re_bit_count &= 31;
> re_cache0 = av_be2ne32(re_buffer_ptr[-1]) << re_bit_count;
> re_cache1 = 0;
> @@ -311,7 +349,10 @@ static inline unsigned int get_bits1(GetBitContext *s){
> result <<= index & 7;
> result >>= 8 - 1;
> #endif
> - index++;
> +#if !UNCHECKED_BITSTREAM_READER
> + if (s->index < s->size_in_bits_plus1)
> +#endif
> + index++;
> s->index = index;
Why #if rather than if (UNCHECKED_BITSTREAM_READER || ...) here?
--
Måns Rullgård
[email protected]
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