Message ID | 20240726163219.348645-1-remi@remlab.net |
---|---|
State | New |
Headers | show |
Series | [FFmpeg-devel] lavu/common: use fabs(f) for FFABS/FFNABS | expand |
Context | Check | Description |
---|---|---|
andriy/make_x86 | success | Make finished |
andriy/make_fate_x86 | success | Make fate finished |
Quoting Rémi Denis-Courmont (2024-07-26 18:32:19) > 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 Is __STDC_VERSION__ defined for C++?
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.