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

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Pranav Kant via ffmpeg-devel:
> 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, assembly files continue to access these
> globals defined in C/C++ files using older (and invalid instruction
> sequence). So, we mark all such globals with an attribute that forces
> them to be allocated in small sections allowing them to validly be
> accessed from the 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
> ---
>  libavcodec/ac3dsp.c        | 2 +-
>  libavcodec/cabac.c         | 3 ++-
>  libavcodec/x86/constants.c | 8 ++++++++
>  libavutil/attributes.h     | 6 ++++++
>  4 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/ac3dsp.c b/libavcodec/ac3dsp.c
> index 730fa70fff..43b4fcbda9 100644
> --- a/libavcodec/ac3dsp.c
> +++ b/libavcodec/ac3dsp.c
> @@ -104,7 +104,7 @@ static void ac3_update_bap_counts_c(uint16_t mant_cnt[16], uint8_t *bap,
>          mant_cnt[bap[len]]++;
>  }
>  
> -DECLARE_ALIGNED(16, const uint16_t, ff_ac3_bap_bits)[16] = {
> +av_mcmodel_small DECLARE_ALIGNED(16, const uint16_t, ff_ac3_bap_bits)[16] = {
>      0,  0,  0,  3,  0,  4,  5,  6,  7,  8,  9, 10, 11, 12, 14, 16
>  };
>  
> diff --git a/libavcodec/cabac.c b/libavcodec/cabac.c
> index 7d41cd2ae6..dfc3ba135a 100644
> --- a/libavcodec/cabac.c
> +++ b/libavcodec/cabac.c
> @@ -24,12 +24,13 @@
>   * Context Adaptive Binary Arithmetic Coder.
>   */
>  
> +#include "libavutil/attributes.h"
>  #include "libavutil/error.h"
>  #include "libavutil/mem_internal.h"
>  
>  #include "cabac.h"
>  
> -DECLARE_ASM_ALIGNED(1, const uint8_t, ff_h264_cabac_tables)[512 + 4*2*64 + 4*64 + 63] = {
> +av_mcmodel_small DECLARE_ASM_ALIGNED(1, const uint8_t, ff_h264_cabac_tables)[512 + 4*2*64 + 4*64 + 63] = {
>      9,8,7,7,6,6,6,6,5,5,5,5,5,5,5,5,
>      4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
>      3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,
> diff --git a/libavcodec/x86/constants.c b/libavcodec/x86/constants.c
> index bc7f2b17b8..9a5af2871c 100644
> --- a/libavcodec/x86/constants.c
> +++ b/libavcodec/x86/constants.c
> @@ -18,17 +18,21 @@
>   * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>   */
>  
> +#include "libavutil/attributes.h"
>  #include "libavutil/mem_internal.h"
>  #include "libavutil/x86/asm.h" // for xmm_reg
>  #include "constants.h"
>  
> +av_mcmodel_small
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_1)    = { 0x0001000100010001ULL, 0x0001000100010001ULL,
>                                                      0x0001000100010001ULL, 0x0001000100010001ULL };
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_2)    = { 0x0002000200020002ULL, 0x0002000200020002ULL,
>                                                      0x0002000200020002ULL, 0x0002000200020002ULL };
>  DECLARE_ASM_ALIGNED(16, const xmm_reg,  ff_pw_3)    = { 0x0003000300030003ULL, 0x0003000300030003ULL };
> +av_mcmodel_small
>  DECLARE_ASM_ALIGNED(32, const ymm_reg,  ff_pw_4)    = { 0x0004000400040004ULL, 0x0004000400040004ULL,
>                                                      0x0004000400040004ULL, 0x0004000400040004ULL };
> +av_mcmodel_small
>  DECLARE_ASM_ALIGNED(16, const xmm_reg,  ff_pw_5)    = { 0x0005000500050005ULL, 0x0005000500050005ULL };
>  DECLARE_ALIGNED(16, const xmm_reg,  ff_pw_8)    = { 0x0008000800080008ULL, 0x0008000800080008ULL };
>  DECLARE_ASM_ALIGNED(16, const xmm_reg,  ff_pw_9)    = { 0x0009000900090009ULL, 0x0009000900090009ULL };
> @@ -49,6 +53,7 @@ DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_256)  = { 0x0100010001000100ULL, 0x010
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_512)  = { 0x0200020002000200ULL, 0x0200020002000200ULL,
>                                                      0x0200020002000200ULL, 0x0200020002000200ULL };
>  DECLARE_ALIGNED(16, const xmm_reg,  ff_pw_1019) = { 0x03FB03FB03FB03FBULL, 0x03FB03FB03FB03FBULL };
> +av_mcmodel_small
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_1023) = { 0x03ff03ff03ff03ffULL, 0x03ff03ff03ff03ffULL,
>                                                      0x03ff03ff03ff03ffULL, 0x03ff03ff03ff03ffULL};
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_1024) = { 0x0400040004000400ULL, 0x0400040004000400ULL,
> @@ -66,13 +71,16 @@ DECLARE_ALIGNED(32, const ymm_reg,  ff_pw_m1)   = { 0xFFFFFFFFFFFFFFFFULL, 0xFFF
>  
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pb_0)    = { 0x0000000000000000ULL, 0x0000000000000000ULL,
>                                                      0x0000000000000000ULL, 0x0000000000000000ULL };
> +av_mcmodel_small
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pb_1)    = { 0x0101010101010101ULL, 0x0101010101010101ULL,
>                                                      0x0101010101010101ULL, 0x0101010101010101ULL };
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pb_2)    = { 0x0202020202020202ULL, 0x0202020202020202ULL,
>                                                      0x0202020202020202ULL, 0x0202020202020202ULL };
> +av_mcmodel_small
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pb_3)    = { 0x0303030303030303ULL, 0x0303030303030303ULL,
>                                                      0x0303030303030303ULL, 0x0303030303030303ULL };
>  DECLARE_ALIGNED(32, const xmm_reg,  ff_pb_15)   = { 0x0F0F0F0F0F0F0F0FULL, 0x0F0F0F0F0F0F0F0FULL };
> +av_mcmodel_small
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pb_80)   = { 0x8080808080808080ULL, 0x8080808080808080ULL,
>                                                      0x8080808080808080ULL, 0x8080808080808080ULL };
>  DECLARE_ALIGNED(32, const ymm_reg,  ff_pb_FE)   = { 0xFEFEFEFEFEFEFEFEULL, 0xFEFEFEFEFEFEFEFEULL,
> diff --git a/libavutil/attributes.h b/libavutil/attributes.h
> index 04c615c952..704a7070db 100644
> --- a/libavutil/attributes.h
> +++ b/libavutil/attributes.h
> @@ -104,6 +104,12 @@
>  #    define attribute_deprecated
>  #endif
>  
> +#if defined(__clang__) && __has_attribute(model)
> +#    define av_mcmodel_small __attribute__((model("small")))
> +#else
> +#    define av_mcmodel_small
> +#endif

This should be in attributes_internal.h. Or maybe we should add new
macros in mem_internal.h for the case of variables with external linkage
and static storage duration (one macro for the const case, one for the
non-const case) that would automatically include your proposed attribute?

> +
>  /**
>   * Disable warnings about deprecated features
>   * This is useful for sections of code kept for backward compatibility and

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