AutoFDO: Enable machine function split optimization for AutoFDO

Enable the machine function split optimization for AutoFDO in Clang.

Machine function split (MFS) is a pass in the Clang compiler that
splits a function into hot and cold parts. The linker groups all
cold blocks across functions together. This decreases hot code
fragmentation and improves iCache and iTLB utilization.

MFS requires a profile so this is enabled only for the AutoFDO builds.

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

include/asm-generic/vmlinux.lds.h

···
 		__cpuidle_text_end = .;					\
 		__noinstr_text_end = .;
 
+#define TEXT_SPLIT							\
+		__split_text_start = .;					\
+		*(.text.split .text.split.[0-9a-zA-Z_]*)		\
+		__split_text_end = .;
+
 #define TEXT_UNLIKELY							\
 		__unlikely_text_start = .;				\
 		*(.text.unlikely .text.unlikely.*)			\
···
 		ALIGN_FUNCTION();					\
 		*(.text.asan.* .text.tsan.*)				\
 		*(.text.unknown .text.unknown.*)			\
+		TEXT_SPLIT						\
 		TEXT_UNLIKELY						\
 		. = ALIGN(PAGE_SIZE);					\
 		TEXT_HOT						\
 		*(TEXT_MAIN .text.fixup)				\
 		NOINSTR_TEXT						\
 		*(.ref.text)
-
 
 /* sched.text is aling to function alignment to secure we have same
  * address even at second ld pass when generating System.map */

scripts/Makefile.autofdo

···
 
 ifdef CLANG_AUTOFDO_PROFILE
   CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
+  CFLAGS_AUTOFDO_CLANG += -fsplit-machine-functions
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN
···
     KBUILD_LDFLAGS += --lto-sample-profile=$(CLANG_AUTOFDO_PROFILE)
   endif
   KBUILD_LDFLAGS += --mllvm=-enable-fs-discriminator=true --mllvm=-improved-fs-discriminator=true -plugin-opt=thinlto
+  KBUILD_LDFLAGS += -plugin-opt=-split-machine-functions
 endif
 
 export CFLAGS_AUTOFDO_CLANG