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* [FFmpeg-devel] [PATCH] Mark C globals with small code model
@ 2025-02-25 21:37 Pranav Kant via ffmpeg-devel
  2025-02-25 23:03 ` Andreas Rheinhardt
  0 siblings, 1 reply; 2+ messages in thread
From: Pranav Kant via ffmpeg-devel @ 2025-02-25 21:37 UTC (permalink / raw)
  To: ffmpeg-devel; +Cc: Pranav Kant

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
+
 /**
  * Disable warnings about deprecated features
  * This is useful for sections of code kept for backward compatibility and
-- 
2.48.1.658.g4767266eb4-goog

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^ permalink raw reply	[flat|nested] 2+ messages in thread
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2025-02-25 21:37 [FFmpeg-devel] [PATCH] Mark C globals with small code model Pranav Kant via ffmpeg-devel
2025-02-25 23:03 ` Andreas Rheinhardt

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