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

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

Any thoughts on this?

On Thu, Mar 20, 2025 at 5:30 PM Pranav Kant <prka@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 <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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