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 <shenhan@google.com>
Signed-off-by: Han Shen <shenhan@google.com>
Signed-off-by: Rong Xu <xur@google.com>
Suggested-by: Sriraman Tallam <tmsriram@google.com>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Change-Id: I6dc9150c46983a576f4d4f1415dbdcab78a48021
---
 include/asm-generic/vmlinux.lds.h | 11 +++++++++--
 scripts/Makefile.autofdo          |  2 +-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e02973f3b4189..bd64fdedabd2f 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/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
diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index ff96a63fea7cd..6155d6fc4ca7f 100644
--- a/scripts/Makefile.autofdo
+++ b/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
-- 
2.47.0.163.g1226f6d8fa-goog

On Fri, Oct 25, 2024 at 10:14:07PM -0700, Rong Xu wrote:
> 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 <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook