This fixes building with Clang in MSVC mode, for x86, which was
broken in 6e49b86996 (in Nov 2024);
previously it failed with undefined symbols for the constants
defined with DECLARE_ASM_CONST, accessed via inline assembly.
Before 57861911a3, there was an
#elif defined(__GNUC__) || defined(__clang__)
case before the
#elif defined(_MSC_VER)
case for defining DECLARE_ASM_CONST, which included av_used.
(This case included the explicit "defined(__clang__)" since
f637046d3134a331e4b5a7243ac3dfb92735b8a5.)
After 57861911a3, it used the
generic definition of DECLARE_ASM_CONST that also included
av_used - which also worked for Clang in MSVC mode. But after
6e49b86996, Clang in MSVC mode
ended up using the MSVC specific variant which lacked the
av_used declaration, causing linker errors due to undefined
symbols.
Signed-off-by: Martin Storsjö <martin@martin.st>
It's currently actually not used in MSVC builds, since
6e49b86996.
Older versions of MSVC (or, in particular, older versions of UCRT)
don't have stdalign.h; it's available since WinSDK 10.0.20348.0;
such a new enough version has been installed by default only since
MSVC 2022 17.4 and newer.
With this change, ffmpeg can still be built with MSVC 2019 16.8
(v19.28).
Signed-off-by: Martin Storsjö <martin@martin.st>
alignas is portable than compiler's specific __attribute__. It do
have a limitation, that alignas don't support specify aligment on
the declarations of struct (it works for specify alignment on struct
fields), which only used by avcodec/cavs, and is removed now.
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
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.
This patch limits the maximum alignment to the maximum possible simd
alignment according to configure.
While not perfect, it at the very least gets rid of a lot of UB, by
matching up the maximum DECLARE_ALIGNED value with the alignment of heap
allocations done by lavu.
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.
This patch limits the maximum alignment to the maximum possible simd
alignment according to configure.
While not perfect, it at the very least gets rid of a lot of UB, by
matching up the maximum DECLARE_ALIGNED value with the alignment of heap
allocations done by lavu.
internal.h is difficult to use due to circular dependancies
mem.h is a public header ff_* is not public
Alternative solutions probably are possible too
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
