Re: [FFmpeg-devel] [PATCH 1/2] avutil/intmath: use AV_HAS_BUILTIN to detect builtin availability

[Kacper Michajlow <kasper93-at-gmail.com@ffmpeg.org>]

On Wed, 11 Jun 2025 at 20:45, Martin Storsjö <martin@martin.st> wrote:
>
> On Wed, 11 Jun 2025, Kacper Michajlow wrote:
>
> > On Mon, 31 Mar 2025 at 14:03, Kacper Michajłow <kasper93@gmail.com> wrote:
> >>
> >> Fixes use of bultins on clang x86_64-pc-windows-msvc which does not
> >> define any __GNUC__. Also on other targets __GNUC__ is defined to 4 by
> >> default, so any feature testing based on version is not really valid.
> >>
> >> Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
> >> ---
> >>  libavutil/intmath.h     | 12 +++++-------
> >>  libavutil/x86/intmath.h |  4 ++--
> >>  2 files changed, 7 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/libavutil/intmath.h b/libavutil/intmath.h
> >> index 52e11a8d5f..b177a848c6 100644
> >> --- a/libavutil/intmath.h
> >> +++ b/libavutil/intmath.h
> >> @@ -35,7 +35,7 @@
> >>  #endif
> >>
> >>  #if HAVE_FAST_CLZ
> >> -#if AV_GCC_VERSION_AT_LEAST(3,4)
> >> +#if AV_GCC_VERSION_AT_LEAST(3,4) || AV_HAS_BUILTIN(__builtin_clz)
> >>  #ifndef ff_log2
> >>  #   define ff_log2(x) (31 - __builtin_clz((x)|1))
> >>  #   ifndef ff_log2_16bit
> >> @@ -90,18 +90,16 @@ static av_always_inline av_const int ff_log2_16bit_c(unsigned int v)
> >>   */
> >>
> >>  #if HAVE_FAST_CLZ
> >> -#if AV_GCC_VERSION_AT_LEAST(3,4)
> >> -#ifndef ff_ctz
> >> +#if !defined(ff_ctz) && (AV_GCC_VERSION_AT_LEAST(3,4) || AV_HAS_BUILTIN(__builtin_ctz))
> >>  #define ff_ctz(v) __builtin_ctz(v)
> >>  #endif
> >> -#ifndef ff_ctzll
> >> +#if !defined(ff_ctzll) && (AV_GCC_VERSION_AT_LEAST(3,4) || AV_HAS_BUILTIN(__builtin_ctzll))
> >>  #define ff_ctzll(v) __builtin_ctzll(v)
> >>  #endif
> >> -#ifndef ff_clz
> >> +#if !defined(ff_clz) && (AV_GCC_VERSION_AT_LEAST(3,4) || AV_HAS_BUILTIN(__builtin_clz))
> >>  #define ff_clz(v) __builtin_clz(v)
> >>  #endif
> >>  #endif
> >> -#endif
> >>
> >>  #ifndef ff_ctz
> >>  #define ff_ctz ff_ctz_c
> >> @@ -154,7 +152,7 @@ static av_always_inline av_const unsigned ff_clz_c(unsigned x)
> >>  }
> >>  #endif
> >>
> >> -#if AV_GCC_VERSION_AT_LEAST(3,4)
> >> +#if AV_GCC_VERSION_AT_LEAST(3,4) || AV_HAS_BUILTIN(__builtin_parity)
> >>  #ifndef av_parity
> >>  #define av_parity __builtin_parity
> >>  #endif
> >> diff --git a/libavutil/x86/intmath.h b/libavutil/x86/intmath.h
> >> index 4893a1f1b4..030ded6b6f 100644
> >> --- a/libavutil/x86/intmath.h
> >> +++ b/libavutil/x86/intmath.h
> >> @@ -66,7 +66,7 @@ static av_always_inline av_const int ff_ctzll_x86(long long v)
> >>
> >>  #endif /* HAVE_FAST_CLZ */
> >>
> >> -#if defined(__GNUC__)
> >> +#if defined(__GNUC__) || defined(__clang__)
> >>
> >>  /* Our generic version of av_popcount is faster than GCC's built-in on
> >>   * CPUs that don't support the popcnt instruction.
> >> @@ -81,7 +81,7 @@ static av_always_inline av_const int ff_ctzll_x86(long long v)
> >>
> >>  #if defined(__BMI2__)
> >>
> >> -#if AV_GCC_VERSION_AT_LEAST(5,1)
> >> +#if AV_GCC_VERSION_AT_LEAST(5,1) || AV_HAS_BUILTIN(__builtin_ia32_bzhi_si)
> >>  #if defined(ASSERT_LEVEL) && ASSERT_LEVEL >= 2
> >>  #define av_zero_extend av_zero_extend_bmi2
> >>  static av_always_inline av_const unsigned av_zero_extend_bmi2(unsigned a, unsigned p)
> >> --
> >> 2.45.1
> >>
> >
> > bump
>
> Can you specify the symptoms that are fixed? Is there a specific build
> configuration that fails, or is it a case of using more optimal builtins
> where suitable? (Same goes for 2/2 probably.)

As the commit message says it "Fixes use of bultins on clang
x86_64-pc-windows-msvc". It now uses builtins where previously it
would fallback to C implementation.

- Kacper
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