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new 1764683668 swscale/ops_backend: disable FP contraction where possible
1764683668 is described below
commit 17646836686369ea5bea1c71c488b94f7c7c775a
Author: Niklas Haas <[email protected]>
AuthorDate: Sun Apr 12 16:14:38 2026 +0200
Commit: Niklas Haas <[email protected]>
CommitDate: Thu Apr 16 17:19:51 2026 +0000
swscale/ops_backend: disable FP contraction where possible
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 <[email protected]>
---
libswscale/ops_backend.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/libswscale/ops_backend.c b/libswscale/ops_backend.c
index e7aaeda9d4..62b6262a1f 100644
--- a/libswscale/ops_backend.c
+++ b/libswscale/ops_backend.c
@@ -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
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