Re: [FFmpeg-devel] [PATCH v5] Mark C globals with small code model

[Pranav Kant via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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 <prka@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 <prka@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
>
>
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