[PATCH RFC v3 8/8] lib/Kconfig.debug: introduce CONFIG_NO_AUTO_INLINE

[Chen Linxuan via B4 Relay <devnull+chenlinxuan.uniontech.com@kernel.org>]

From: Chen Linxuan <chenlinxuan@uniontech.com>

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@gmail.com/

This patch is modified from commit 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

Cc: Changbin Du <changbin.du@gmail.com>
Co-developed-by: Winston Wen <wentao@uniontech.com>
Signed-off-by: Winston Wen <wentao@uniontech.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
---
 Makefile          | 16 ++++++++++++++++
 lib/Kconfig.debug | 21 +++++++++++++++++++++
 lib/Makefile      |  3 +++
 3 files changed, 40 insertions(+)

diff --git a/Makefile b/Makefile
index 5aa9ee52a765b7aed27f44028cdcc34a90979acb..60dec6c123543150a3332a9a819fa6933e94db4f 100644
--- a/Makefile
+++ b/Makefile
@@ -1073,6 +1073,22 @@ endif
 # Ensure compilers do not transform certain loops into calls to wcslen()
 KBUILD_CFLAGS += -fno-builtin-wcslen
 
+ifdef CONFIG_NO_AUTO_INLINE
+# -fno-inline-functions behaves differently between gcc and clang.
+# With gcc, it prevents auto-inlining of functions but still considers functions
+# explicitly marked with "inline" for inlining. However, with clang, the flag
+# prevents inlining of all functions, including those explicitly marked with
+# inline. Clang provides the "-finline-hint-functions" option, which
+# specifically allows inlining of functions marked with "inline".
+#
+# In summary, to achieve equivalent behavior across compilers:
+# -fno-inline-functions (gcc) = -fno-inline-functions + -finline-hint-functions (clang)
+KBUILD_CFLAGS   += -fno-inline-functions \
+		   $(call cc-option, -finline-hint-functions) \
+		   $(call cc-option, -fno-inline-small-functions) \
+		   $(call cc-option, -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 f9051ab610d54358b21d61c141b737bb345b4cee..56530f0145c885e9846dae1d2f8c6125c610d25b 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -436,8 +436,29 @@ 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
+	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.
+
+	  Note that Clang with -O2 optimization does not fully support
+	  disabling all inline-related optimizations,
+	  as Clang does not support options like
+	  -fno-inline-small-functions and -fno-inline-functions-called-once
+	  that gcc does.
+	  Some functions without the inline keyword may still be inlined.
+
+	  If unsure, select N.
+
 config FRAME_WARN
 	int "Warn for stack frames larger than"
 	range 0 8192
diff --git a/lib/Makefile b/lib/Makefile
index f07b24ce1b3f8db28796e461db1324d97133fdd5..2ac97f0856a12f66e6c3825af6aabafa61869262 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -87,6 +87,9 @@ obj-$(CONFIG_TEST_BITMAP) += test_bitmap.o
 ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_KASAN),yy)
 # FIXME: Clang breaks test_bitmap_const_eval when KASAN and GCOV are enabled
 GCOV_PROFILE_test_bitmap.o := n
+# FIXME:
+# Clang breaks test_bitmap_const_eval when NO_AUTO_INLINE and KASAN are enabled
+CFLAGS_test_bitmap.o += -finline-functions
 endif
 
 obj-$(CONFIG_TEST_UUID) += test_uuid.o

-- 
2.43.0