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swscale/ops_backend: disable FP contraction where possible

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In particular, Clang defaults to FP contraction enabled. GCC defaults to
off in standard C mode (-std=c11), but the C standard does not actually
require any particular default.

The #pragma STDC pragma, despite its name, warns on anything except Clang.

Fixes: https://code.ffmpeg.org/FFmpeg/FFmpeg/issues/22796
See-also: https://discourse.llvm.org/t/fp-contraction-fma-on-by-default/64975
Signed-off-by: Niklas Haas <git@haasn.dev>
This commit is contained in:
Niklas Haas
2026-04-12 16:14:38 +02:00
committed by Niklas Haas
parent 3b19a61837
commit 1764683668
1 changed files with 12 additions and 0 deletions
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12
libswscale/ops_backend.c
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@@ -20,6 +20,18 @@
#include "ops_backend.h"
/**
* We want to disable FP contraction because this is a reference backend that
* establishes a bit-exact reference result.
*/
#ifdef __clang__
#pragma STDC FP_CONTRACT OFF
#elif AV_GCC_VERSION_AT_LEAST(4, 8)
#pragma GCC optimize ("fp-contract=off")
#elif defined(_MSC_VER)
#pragma fp_contract (off)
#endif
#if AV_GCC_VERSION_AT_LEAST(4, 4)
#pragma GCC optimize ("finite-math-only")
#endif