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".
