AutoFDO: Enable -ffunction-sections for the AutoFDO build

Enable -ffunction-sections by default for the AutoFDO build.

With -ffunction-sections, the compiler places each function in its own
section named .text.function_name instead of placing all functions in
the .text section. In the AutoFDO build, this allows the linker to
utilize profile information to reorganize functions for improved
utilization of iCache and iTLB.

Co-developed-by: Han Shen <[email protected]>
Signed-off-by: Han Shen <[email protected]>
Signed-off-by: Rong Xu <[email protected]>
Suggested-by: Sriraman Tallam <[email protected]>
Tested-by: Yonghong Song <[email protected]>
Tested-by: Yabin Cui <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

Author: xur-llvm

include/asm-generic/vmlinux.lds.h

@@ -95,18 +95,25 @@
  * With LTO_CLANG, the linker also splits sections by default, so we need
  * these macros to combine the sections during the final link.
  *
+ * With AUTOFDO_CLANG, by default, the linker splits text sections and
+ * regroups functions into subsections.
+ *
  * RODATA_MAIN is not used because existing code already defines .rodata.x
  * sections to be brought in with rodata.
  */
-#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
+defined(CONFIG_AUTOFDO_CLANG)
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
+#else
+#define TEXT_MAIN .text
+#endif
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
 #define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L*
 #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
 #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
 #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..L* .bss..compoundliteral*
 #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
 #else
-#define TEXT_MAIN .text
 #define DATA_MAIN .data
 #define SDATA_MAIN .sdata
 #define RODATA_MAIN .rodata

scripts/Makefile.autofdo

@@ -9,7 +9,7 @@ ifndef CONFIG_DEBUG_INFO
 endif
 
 ifdef CLANG_AUTOFDO_PROFILE
-  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE)
+  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN