diff mbox series

[3/6] Change the symbols order when --ffuntion-sections is enabled

Message ID 20240728203001.2551083-4-xur@google.com
State New
Headers show
Series Add AutoFDO and Propeller support for Clang build | expand

Commit Message

Rong Xu July 28, 2024, 8:29 p.m. UTC 
When the -ffunction-sections compiler option is enabled, each function
is placed in a separate section named .text.function_name rather than
putting all functions in a single .text section.

However, using -function-sections can cause problems with the
linker script. The comments included in include/asm-generic/vmlinux.lds.h
note these issues.:
  “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
   code elimination is enabled, so these sections should be converted
   to use ".." first.”

It is unclear whether there is a straightforward method for converting
a suffix to "..". This patch modifies the order of subsections within the
text output section when the -ffunction-sections flag is enabled.
Specifically, it repositions sections with certain fixed patterns (for
example .text.unlikely) before TEXT_MAIN, ensuring that they are grouped
and matched together.

Note that the limitation arises because the linker script employs glob
patterns instead of regular expressions for string matching. While there
is a method to maintain the current order using complex patterns, this
significantly complicates the pattern and increases the likelihood of
errors.

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>
Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
---
 include/asm-generic/vmlinux.lds.h | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)
diff mbox series

Patch

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 5703526d6ebf..f3de66bda293 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -582,9 +582,21 @@ 
  * during second ld run in second ld pass when generating System.map
  *
  * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
- * code elimination is enabled, so these sections should be converted
- * to use ".." first.
+ * code elimination or function-section is enabled. Match these symbols
+ * first when in these builds.
  */
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
+#define TEXT_TEXT							\
+		*(.text.asan.* .text.tsan.*)				\
+		*(.text.unknown .text.unknown.*)			\
+		*(.text.unlikely .text.unlikely.*)			\
+		ALIGN_FUNCTION();					\
+		*(.text.hot .text.hot.*)				\
+		*(TEXT_MAIN .text.fixup)				\
+		NOINSTR_TEXT						\
+		*(.ref.text)						\
+	MEM_KEEP(init.text*)
+#else
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
 		*(.text.hot .text.hot.*)				\
@@ -594,7 +606,8 @@ 
 		NOINSTR_TEXT						\
 		*(.ref.text)						\
 		*(.text.asan.* .text.tsan.*)				\
-	MEM_KEEP(init.text*)						\
+	MEM_KEEP(init.text*)
+#endif
 
 
 /* sched.text is aling to function alignment to secure we have same