[RFC PATCH 7/7] lib/Kconfig.debug: introduce CONFIG_NO_AUTO_INLINE

[Chen Linxuan]

From: Winston Wen <wentao@xxxxxxxxxxxxx>

Add a new kernel hacking option CONFIG_NO_AUTO_INLINE that prevents the
compiler from auto-inlining functions not explicitly marked with the
'inline' keyword.

This enhancement improves function tracer capabilities as it can only
trace functions that haven't been inlined by the compiler.

Previous discussions:
Link: https://lore.kernel.org/all/20181028130945.23581-3-changbin.du@xxxxxxxxx/

This patch is modified from 917fad29febd ("kernel hacking: add a config
option to disable compiler auto-inlining") which can be founded in
linux-next-history:
Link: https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next-history.git/commit/?id=917fad29febd

Unlike the original implementation, this patch:

1. Make it depends on CC_IS_GCC,
   as Clang 18.1.3 break test_bitmap_const_eval() in lib/test_bitmap.c

2. Make it depends on X86_64,
   as I haven't test other architectures

3. Removes unnecessary cc-option checks per 7d73c3e9c514 ("Makefile:
   remove stale cc-option checks").

4. Update help information.

Cc: Changbin Du <changbin.du@xxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Signed-off-by: Winston Wen <wentao@xxxxxxxxxxxxx>
Co-Developed-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx>
Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx>
---
 Makefile          |  6 ++++++
 lib/Kconfig.debug | 15 +++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/Makefile b/Makefile
index f42418556507..0a9bf33ce75f 100644
--- a/Makefile
+++ b/Makefile
@@ -1071,6 +1071,12 @@ endif
 # Ensure compilers do not transform certain loops into calls to wcslen()
 KBUILD_CFLAGS += -fno-builtin-wcslen
 
+ifdef CONFIG_NO_AUTO_INLINE
+KBUILD_CFLAGS   += -fno-inline-functions \
+		   -fno-inline-small-functions \
+		   -fno-inline-functions-called-once
+endif
+
 # change __FILE__ to the relative path to the source directory
 ifdef building_out_of_srctree
 KBUILD_CPPFLAGS += $(call cc-option,-ffile-prefix-map=$(srcroot)/=)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9fe4d8dfe578..2ebb4802886a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -436,8 +436,23 @@ config GDB_SCRIPTS
 	  instance. See Documentation/process/debugging/gdb-kernel-debugging.rst
 	  for further details.
 
+
 endif # DEBUG_INFO
 
+config NO_AUTO_INLINE
+	bool "Disable compiler auto-inline optimizations (EXPERIMENTAL)"
+	default n
+	depends on CC_IS_GCC && (X86 || LOONGARCH)
+	help
+	  This will prevent the compiler from optimizing the kernel by
+	  auto-inlining functions not marked with the inline keyword.
+	  With this option, only functions explicitly marked with
+	  "inline" will be inlined. This will allow the function tracer
+	  to trace more functions because it only traces functions that
+	  the compiler has not inlined.
+
+	  If unsure, select N.
+
 config FRAME_WARN
 	int "Warn for stack frames larger than"
 	range 0 8192
-- 
2.48.1