[FFmpeg-devel] [PATCH v5] Mark C globals with small code model
Pranav Kant
prka at google.com
Fri Mar 21 02:28:20 EET 2025
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 at 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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