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

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

Hello again. Is there anything else I can do here?

On Fri, Apr 4, 2025 at 11:40 AM Pranav Kant <prka@google.com> wrote:

> 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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