Any thoughts on this? On Thu, Mar 20, 2025 at 5:30 PM Pranav Kant <p...@google.com> wrote:
> Patch version v5: > - Uses two new macros DECLARE_ASM_VAR (used for both external and inline > asm) and DECLARE_EXTERNAL_ASM_VAR (used only for external asm) > - I intend to remove explicit existing use of attribute_visibility_hidden > in follow-up patch and instead use DECLARE_EXTERNAL_ASM_VAR for those > variables > - Other variables will be marked with these two new macros in a follow-up > patch. I want to settle down on the infrastructure first with a handful of > variables. > > Let me know if you have any questions. > > On Thu, Mar 20, 2025 at 5:28 PM Pranav Kant <p...@google.com> wrote: > >> 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, external assembly and inline assembly >> continue to access these globals using older PC-relative addressing >> which may not work because globals may be placed too far away. >> >> Introduce new macros for such variables that mark them with small code >> model attribute. This ensures that these variables are never allocated >> in large data sections, and continue to be validly accessed from 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 <p...@google.com> >> --- >> libavcodec/ac3dsp.h | 4 +++- >> libavcodec/cabac.h | 4 +++- >> libavcodec/x86/constants.h | 12 +++++++----- >> libavutil/attributes.h | 6 ++++++ >> libavutil/attributes_internal.h | 16 ++++++++++++++++ >> libavutil/mem_internal.h | 13 +++++++++++++ >> 6 files changed, 48 insertions(+), 7 deletions(-) >> >> diff --git a/libavcodec/ac3dsp.h b/libavcodec/ac3dsp.h >> index b1b2bced8f..a3c55a833b 100644 >> --- a/libavcodec/ac3dsp.h >> +++ b/libavcodec/ac3dsp.h >> @@ -25,11 +25,13 @@ >> #include <stddef.h> >> #include <stdint.h> >> >> +#include "libavutil/mem_internal.h" >> + >> /** >> * Number of mantissa bits written for each bap value. >> * bap values with fractional bits are set to 0 and are calculated >> separately. >> */ >> -extern const uint16_t ff_ac3_bap_bits[16]; >> +extern DECLARE_EXTERNAL_ASM_VAR(16, const uint16_t, ff_ac3_bap_bits)[16]; >> >> typedef struct AC3DSPContext { >> /** >> diff --git a/libavcodec/cabac.h b/libavcodec/cabac.h >> index 38d06b2842..df352258c6 100644 >> --- a/libavcodec/cabac.h >> +++ b/libavcodec/cabac.h >> @@ -29,7 +29,9 @@ >> >> #include <stdint.h> >> >> -extern const uint8_t ff_h264_cabac_tables[512 + 4*2*64 + 4*64 + 63]; >> +#include "libavutil/mem_internal.h" >> + >> +extern DECLARE_ASM_VAR(1, const uint8_t, ff_h264_cabac_tables)[512 + >> 4*2*64 + 4*64 + 63]; >> #define H264_NORM_SHIFT_OFFSET 0 >> #define H264_LPS_RANGE_OFFSET 512 >> #define H264_MLPS_STATE_OFFSET 1024 >> diff --git a/libavcodec/x86/constants.h b/libavcodec/x86/constants.h >> index 0c6bf41fa0..2561302604 100644 >> --- a/libavcodec/x86/constants.h >> +++ b/libavcodec/x86/constants.h >> @@ -23,13 +23,14 @@ >> >> #include <stdint.h> >> >> +#include "libavutil/mem_internal.h" >> #include "libavutil/x86/asm.h" >> >> -extern const ymm_reg ff_pw_1; >> +extern DECLARE_EXTERNAL_ASM_VAR(32, const ymm_reg, ff_pw_1); >> extern const ymm_reg ff_pw_2; >> extern const xmm_reg ff_pw_3; >> -extern const ymm_reg ff_pw_4; >> -extern const xmm_reg ff_pw_5; >> +extern DECLARE_ASM_VAR(32, const ymm_reg, ff_pw_4); >> +extern DECLARE_ASM_VAR(16, const xmm_reg, ff_pw_5); >> extern const xmm_reg ff_pw_8; >> extern const xmm_reg ff_pw_9; >> extern const uint64_t ff_pw_15; >> @@ -43,7 +44,7 @@ extern const uint64_t ff_pw_128; >> extern const ymm_reg ff_pw_255; >> extern const ymm_reg ff_pw_256; >> extern const ymm_reg ff_pw_512; >> -extern const ymm_reg ff_pw_1023; >> +extern DECLARE_EXTERNAL_ASM_VAR(32, const ymm_reg, ff_pw_1023); >> extern const ymm_reg ff_pw_1024; >> extern const ymm_reg ff_pw_2048; >> extern const ymm_reg ff_pw_4095; >> @@ -52,9 +53,10 @@ extern const ymm_reg ff_pw_8192; >> extern const ymm_reg ff_pw_m1; >> >> extern const ymm_reg ff_pb_0; >> -extern const ymm_reg ff_pb_1; >> +extern DECLARE_EXTERNAL_ASM_VAR(32, const ymm_reg, ff_pb_1); >> extern const ymm_reg ff_pb_2; >> extern const ymm_reg ff_pb_3; >> +extern DECLARE_ASM_VAR(32, const xmm_reg, ff_pb_15); >> extern const ymm_reg ff_pb_80; >> extern const ymm_reg ff_pb_FE; >> extern const uint64_t ff_pb_FC; >> 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 >> + * 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) >> +# define attribute_mcmodel_small __attribute__((model("small"))) >> +#else >> +# define attribute_mcmodel_small >> +#endif >> + >> #endif /* AVUTIL_ATTRIBUTES_INTERNAL_H */ >> diff --git a/libavutil/mem_internal.h b/libavutil/mem_internal.h >> index c027fa51c3..efb4c89c39 100644 >> --- a/libavutil/mem_internal.h >> +++ b/libavutil/mem_internal.h >> @@ -29,6 +29,7 @@ >> #endif >> >> #include "attributes.h" >> +#include "attributes_internal.h" >> #include "macros.h" >> >> /** >> @@ -113,6 +114,18 @@ >> #define DECLARE_ALIGNED_32(t,v) DECLARE_ALIGNED_T(ALIGN_32, t, v) >> #define DECLARE_ALIGNED_64(t,v) DECLARE_ALIGNED_T(ALIGN_64, t, v) >> >> +// DECLARE_ASM_VAR used for variables accessed by inline asm >> +// and external assembly >> +#define DECLARE_ASM_VAR(n,t,v) \ >> + attribute_mcmodel_small \ >> + alignas(n) t v >> +// DECLARE_EXTERNAL_ASM_VAR used for variables exclusively >> +// accessed by external assembly >> +#define DECLARE_EXTERNAL_ASM_VAR(n,t,v) \ >> + attribute_visibility_hidden \ >> + attribute_mcmodel_small \ >> + alignas(n) t v >> + >> // Some broken preprocessors need a second expansion >> // to be forced to tokenize __VA_ARGS__ >> #define E1(x) x >> -- >> 2.49.0.395.g12beb8f557-goog >> >> _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
