[FFmpeg-devel] [PATCH v4] Mark C globals with small code model
Martin Storsjö
martin at martin.st
Wed Mar 12 09:05:59 EET 2025
On Wed, 12 Mar 2025, Andreas Rheinhardt wrote:
> Pranav Kant via ffmpeg-devel:
>> By default, all globals in C/C++ compiled by clang are allocated
>> in non-large data sections. See [1] for background on code models.
>> For PIC (Position independent code), this is fine as long as binary is
>> small but as binary size increases, users maybe want to use medium/large
>> code models (-mcmodel=medium) which moves data in to large sections.
>> As data in these large sections cannot be accessed using PIC code
>> anymore (as it may be too far away), compiler ends up using a different
>> instruction sequence when building C/C++ code -- using GOT to access
>> these globals (which can be relaxed by linker at link time if binary
>> ends up being smaller). However, assembly files continue to access these
>> globals defined in C/C++ files using older (and invalid instruction
>> sequence). So, we mark all such globals with an attribute that forces
>> them to be allocated in small sections allowing them to validly be
>> accessed from the assembly code.
>>
>> This patch should not have any affect on builds that use small code
>> model, which is the default mode.
>>
>> [1] https://eli.thegreenplace.net/2012/01/03/understanding-the-x64-code-models
>>
>> Signed-off-by: Pranav Kant <prka at google.com>
>> ---
>> libavcodec/ac3dsp.c | 2 ++
>> libavcodec/cabac.c | 2 ++
>> libavcodec/x86/constants.c | 8 ++++++++
>> libavutil/attributes.h | 6 ++++++
>> libavutil/attributes_internal.h | 16 ++++++++++++++++
>> 5 files changed, 34 insertions(+)
>>
>> diff --git a/libavcodec/ac3dsp.c b/libavcodec/ac3dsp.c
>> index 730fa70fff..d16b6c24c3 100644
>> --- a/libavcodec/ac3dsp.c
>> +++ b/libavcodec/ac3dsp.c
>> @@ -25,6 +25,7 @@
>>
>> #include "config.h"
>> #include "libavutil/attributes.h"
>> +#include "libavutil/attributes_internal.h"
>> #include "libavutil/common.h"
>> #include "libavutil/intmath.h"
>> #include "libavutil/mem_internal.h"
>> @@ -104,6 +105,7 @@ static void ac3_update_bap_counts_c(uint16_t mant_cnt[16], uint8_t *bap,
>> mant_cnt[bap[len]]++;
>> }
>>
>> +attribute_mcmodel_small
>> DECLARE_ALIGNED(16, const uint16_t, ff_ac3_bap_bits)[16] = {
>
> Shouldn't stuff like this be applied to the declaration so that C code
> can also take advantage of the knowledge that this object will be placed
> in the small code section?
>
>> 0, 0, 0, 3, 0, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16
>> };
>> diff --git a/libavcodec/cabac.c b/libavcodec/cabac.c
>> index 7d41cd2ae6..b8c6db29a2 100644
>> --- a/libavcodec/cabac.c
>> +++ b/libavcodec/cabac.c
>> @@ -24,11 +24,13 @@
>> * Context Adaptive Binary Arithmetic Coder.
>> */
>>
>> +#include "libavutil/attributes_internal.h"
>> #include "libavutil/error.h"
>> #include "libavutil/mem_internal.h"
>>
>> #include "cabac.h"
>>
>> +attribute_mcmodel_small
>> DECLARE_ASM_ALIGNED(1, const uint8_t, ff_h264_cabac_tables)[512 + 4*2*64 + 4*64 + 63] = {
>
> Your commit message ("However, assembly files continue to access")
> speaks only of assembly files, i.e. external asm. Yet this here is only
> used by inline ASM. Looking through the code the reason for this is that
> I thought that specifying the memory model is only necessary for stuff
> used by external asm, yet ff_h264_cabac_tables does not seem to be used
> by external ASM at all, only inline ASM. If I see this correctly, the
> reason for this is that LOCAL_MANGLE (and therefore MANGLE) uses rip
> addressing on x64 when configure sets the RIP define. But this means
> that the set of files needing attribute_mcmodel_small is a superset of
> the files currently using DECLARE_ASM_ALIGNED. This means that one would
> only need two macros for the variables accessed by ASM: One for only
> external ASM and one for inline ASM (and potentially external ASM)
> instead of adding attribute_mcmodel_small at various places in the codebase.
>
>> 9,8,7,7,6,6,6,6,5,5,5,5,5,5,5,5,
>> 4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
>> diff --git a/libavcodec/x86/constants.c b/libavcodec/x86/constants.c
>> index bc7f2b17b8..347b7dd1d3 100644
>> --- a/libavcodec/x86/constants.c
>> +++ b/libavcodec/x86/constants.c
>> @@ -18,17 +18,21 @@
>> * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>> */
>>
>> +#include "libavutil/attributes_internal.h"
>> #include "libavutil/mem_internal.h"
>> #include "libavutil/x86/asm.h" // for xmm_reg
>> #include "constants.h"
>>
>> +attribute_mcmodel_small
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pw_1) = {
> 0x0001000100010001ULL, 0x0001000100010001ULL,>
> 0x0001000100010001ULL, 0x0001000100010001ULL };
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pw_2) = { 0x0002000200020002ULL, 0x0002000200020002ULL,
>> 0x0002000200020002ULL, 0x0002000200020002ULL };
>> DECLARE_ASM_ALIGNED(16, const xmm_reg, ff_pw_3) = { 0x0003000300030003ULL, 0x0003000300030003ULL };
>> +attribute_mcmodel_small
>> DECLARE_ASM_ALIGNED(32, const ymm_reg, ff_pw_4) = { 0x0004000400040004ULL, 0x0004000400040004ULL,
>> 0x0004000400040004ULL, 0x0004000400040004ULL };
>> +attribute_mcmodel_small
>> DECLARE_ASM_ALIGNED(16, const xmm_reg, ff_pw_5) = { 0x0005000500050005ULL, 0x0005000500050005ULL };
>> DECLARE_ALIGNED(16, const xmm_reg, ff_pw_8) = { 0x0008000800080008ULL, 0x0008000800080008ULL };
>> DECLARE_ASM_ALIGNED(16, const xmm_reg, ff_pw_9) = { 0x0009000900090009ULL, 0x0009000900090009ULL };
>> @@ -49,6 +53,7 @@ DECLARE_ALIGNED(32, const ymm_reg, ff_pw_256) = { 0x0100010001000100ULL, 0x010
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pw_512) = { 0x0200020002000200ULL, 0x0200020002000200ULL,
>> 0x0200020002000200ULL, 0x0200020002000200ULL };
>> DECLARE_ALIGNED(16, const xmm_reg, ff_pw_1019) = { 0x03FB03FB03FB03FBULL, 0x03FB03FB03FB03FBULL };
>> +attribute_mcmodel_small
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pw_1023) = { 0x03ff03ff03ff03ffULL, 0x03ff03ff03ff03ffULL,
>> 0x03ff03ff03ff03ffULL, 0x03ff03ff03ff03ffULL};
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pw_1024) = { 0x0400040004000400ULL, 0x0400040004000400ULL,
>> @@ -66,13 +71,16 @@ DECLARE_ALIGNED(32, const ymm_reg, ff_pw_m1) = { 0xFFFFFFFFFFFFFFFFULL, 0xFFF
>>
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pb_0) = { 0x0000000000000000ULL, 0x0000000000000000ULL,
>> 0x0000000000000000ULL, 0x0000000000000000ULL };
>> +attribute_mcmodel_small
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pb_1) = { 0x0101010101010101ULL, 0x0101010101010101ULL,
>> 0x0101010101010101ULL, 0x0101010101010101ULL };
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pb_2) = { 0x0202020202020202ULL, 0x0202020202020202ULL,
>> 0x0202020202020202ULL, 0x0202020202020202ULL };
>> +attribute_mcmodel_small
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pb_3) = { 0x0303030303030303ULL, 0x0303030303030303ULL,
>> 0x0303030303030303ULL, 0x0303030303030303ULL };
>> DECLARE_ALIGNED(32, const xmm_reg, ff_pb_15) = { 0x0F0F0F0F0F0F0F0FULL, 0x0F0F0F0F0F0F0F0FULL };
>> +attribute_mcmodel_small
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pb_80) = { 0x8080808080808080ULL, 0x8080808080808080ULL,
>> 0x8080808080808080ULL, 0x8080808080808080ULL };
>> DECLARE_ALIGNED(32, const ymm_reg, ff_pb_FE) = { 0xFEFEFEFEFEFEFEFEULL, 0xFEFEFEFEFEFEFEFEULL,
>
> Why do you add this attribute to only eight of these constants? How did
> you come up with this list? In addition to the above, git grep cextern
> shows that there are several more variables than just these one (e.g.
> ff_h263_loop_filter_strength, ff_sbr_noise_table).
>
>> diff --git a/libavutil/attributes.h b/libavutil/attributes.h
>> index 04c615c952..dfc35fa31e 100644
>> --- a/libavutil/attributes.h
>> +++ b/libavutil/attributes.h
>> @@ -40,6 +40,12 @@
>> # define AV_HAS_BUILTIN(x) 0
>> #endif
>>
>> +#ifdef __has_attribute
>> +# define AV_HAS_ATTRIBUTE(x) __has_attribute(x)
>> +#else
>> +# define AV_HAS_ATTRIBUTE(x) 0
>> +#endif
>> +
>> #ifndef av_always_inline
>> #if AV_GCC_VERSION_AT_LEAST(3,1)
>> # define av_always_inline __attribute__((always_inline)) inline
>> diff --git a/libavutil/attributes_internal.h b/libavutil/attributes_internal.h
>> index bc85ce77ff..c557fa0af0 100644
>> --- a/libavutil/attributes_internal.h
>> +++ b/libavutil/attributes_internal.h
>> @@ -19,6 +19,7 @@
>> #ifndef AVUTIL_ATTRIBUTES_INTERNAL_H
>> #define AVUTIL_ATTRIBUTES_INTERNAL_H
>>
>> +#include "config.h"
>> #include "attributes.h"
>>
>> #if (AV_GCC_VERSION_AT_LEAST(4,0) || defined(__clang__)) && (defined(__ELF__) || defined(__MACH__))
>> @@ -33,4 +34,19 @@
>>
>> #define EXTERN extern attribute_visibility_hidden
>>
>> +/**
>> + * Some globals defined in C files are used from hardcoded asm that assumes small
>> + * code model (that is, accessing these globals without GOT). This is a problem
>> + * when FFMpeg is built with medium code model (-mcmodel=medium) which allocates
>
> s/FFMpeg/FFmpeg/
>
>> + * all globals in a data section that's unreachable with PC relative instructions
>> + * (small code model instruction sequence). We mark all such globals with this
>> + * attribute_mcmodel_small to ensure assembly accessible globals continue to be
>> + * allocated in sections reachable from PC relative instructions.
>> + */
>> +#if ARCH_X86_64 && defined(__ELF__) && AV_HAS_ATTRIBUTE(model)
>
> You make this ARCH_X86_64 only, yet the concept of code models also
> exists for other arches, e.g. AArch64. I presume that assembly for other
> arches presumes that we are not using the large code model for accesses,
> so that the same issue can happen with them. Then we have two options:
FWIW, in e4ac156b7c47725327dff78bb83f5eecbaee3add we added
attribute_visibility_hidden to a bunch of symbols that are accessed from
aarch64 assembly, in a similar fashion. While the effects are different, I
wonder if we should abstract these two down to a more generic attribute
for any symbol accessed directly from any assembly.
// Martin
More information about the ffmpeg-devel
mailing list