The absolute value of a floating point value is easier to calculate than that of an integer: it is obtained by either clearing the sign or xoring it with itself. Accordingly, Arm, LoongArch, RISC-V have dedicated instructions to manipulate float sign bits. x86-64 has ANDPS.
As per C11 appendix F, FFABS() is not quite the same as fabs() however, due to differing NaN handling. GCC is able to optimise the current FFABS definition to match fabs() anyway with just -fno-signed-zeros. But Clang is evidently not doing so and inserts a comparison with 0 and a conditional select or branch. Now that we have C11, this is easy to fix properly. (Another option is to manually audit which of the ~880 FFABS/FFNABS use sites involve floating point values.) Note that this still preserves the old definitions for C++ and pre-C11 compilers since the 2 macros are public (even though they start with FF). --- libavutil/common.h | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/libavutil/common.h b/libavutil/common.h index 3b830daf30..48761885fb 100644 --- a/libavutil/common.h +++ b/libavutil/common.h @@ -71,7 +71,14 @@ * as with *abs() * @see FFNABS() */ +#if !defined (__cplusplus) && __STDC_VERSION__ >= 201112L +#define FFABS(a) _Generic(a, \ + float: fabsf(a), \ + double: fabs(a), \ + default: ((a) >= 0) ? (a) : (-(a))) +#else #define FFABS(a) ((a) >= 0 ? (a) : (-(a))) +#endif #define FFSIGN(a) ((a) > 0 ? 1 : -1) /** @@ -80,7 +87,14 @@ * As with many macros, this evaluates its argument twice, it thus must not have * a sideeffect, that is FFNABS(x++) has undefined behavior. */ +#if !defined (__cplusplus) && __STDC_VERSION__ >= 201112L +#define FFNABS(a) _Generic(a, \ + float: -fabsf(a), \ + double: -fabs(a), \ + default: ((a) <= 0) ? (a) : (-(a))) +#else #define FFNABS(a) ((a) <= 0 ? (a) : (-(a))) +#endif /** * Unsigned Absolute value. -- 2.45.2 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
