On Sun, 3 Dec 2023, Timo Rothenpieler wrote:
FFmpeg has instances of DECLARE_ALIGNED(32, ...) in a lot of structs,
which then end up heap-allocated.
By declaring any variable in a struct, or tree of structs, to be 32 byte
aligned, it allows the compiler to safely assume the entire struct
itself is also 32 byte aligned.
This might make the compiler emit code which straight up crashes or
misbehaves in other ways, and at least in one instances is now
documented to actually do (see ticket 10549 on trac).
The issue there is that an unrelated variable in SingleChannelElement is
declared to have an alignment of 32 bytes. So if the compiler does a copy
in decode_cpe() with avx instructions, but ffmpeg is built with
--disable-avx, this results in a crash, since the memory is only 16 byte
aligned.
Mind you, even if the compiler does not emit avx instructions, the code
is still invalid and could misbehave. It just happens not to. Declaring
any variable in a struct with a 32 byte alignment promises 32 byte
alignment of the whole struct to the compiler.
Instead of now going through all instances of variables in structs
being declared as 32 byte aligned, this patch bumps the minimum alignment
to 32 bytes.
---
libavutil/mem.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libavutil/mem.c b/libavutil/mem.c
index 36b8940a0c..26a9b9753b 100644
--- a/libavutil/mem.c
+++ b/libavutil/mem.c
@@ -62,7 +62,7 @@ void free(void *ptr);
#endif /* MALLOC_PREFIX */
-#define ALIGN (HAVE_AVX512 ? 64 : (HAVE_AVX ? 32 : 16))
+#define ALIGN (HAVE_AVX512 ? 64 : 32)
LGTM
It could be good to add a comment here, to indicate how this value relates
to the alignemnts used in structs.
For others who commented in this thread, it all boils down to something
like this:
struct MyData {
uint8_t __attribute__((aligned(32))) aligned_data[1024];
};
void func(void) {
struct MyData *obj = malloc(sizeof(*obj)); // Uusally only aligned to 8
bytes
// operate on obj->aligned_data[]
}
Due to how aligned_data is declared, we promise to the compiler that it is
aligned to 32 bytes, and that the compiler can assume this wherever.
Depending on -march or whatever, this can be to access it with
instructions that assume 32 byte alignment.
// Martin
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