[PATCH 4/5] kernel hacking: new config DEBUG_EXPERIENCE to apply GCC -Og optimization

[changbin.du@intel.com]

From: Changbin Du <changbin.du@intel.com>

This will apply GCC '-Og' optimization level get supported from
GCC 4.8. This optimization level offers a reasonable level of
optimization while maintaining fast compilation and a good
debugging experience. It is similar to '-O1' while perfer keeping
debug ability over runtime speed.

If enabling this option break your kernel, you should either
disable this or find a fix (mostly in the arch code). Currently
this option has only be tested in qemu x86_64 guest.

This option can satisfy people who was searching for a method
to disable compiler optimizations so to achieve better kernel
debugging experience with kgdb or qemu.

The main problem of '-Og' is we must not use __attribute__((error(smg))).
The compiler will report error though the call to error function
still can be optimize out. So we must fallback to array tricky.

Comparison of vmlinux size: a bit smaller.

w/o CONFIG_DEBUG_EXPERIENCE
$ size vmlinux
   text    data     bss     dec     hex filename
22665554   9709674  2920908 35296136        21a9388 vmlinux

w/ CONFIG_DEBUG_EXPERIENCE
$ size vmlinux
   text    data     bss     dec     hex filename
21499032   10102758 2920908 34522698        20ec64a vmlinux

Comparison of system performance: a bit drop.

w/o CONFIG_DEBUG_EXPERIENCE
$ time make -j4
real    6m43.619s
user    19m5.160s
sys     2m20.287s

w/ CONFIG_DEBUG_EXPERIENCE
$ time make -j4
real    6m55.054s
user    19m11.129s
sys     2m36.345s

Signed-off-by: Changbin Du <changbin.du@intel.com>
---
 Makefile                     |  4 ++++
 include/linux/compiler-gcc.h |  2 +-
 include/linux/compiler.h     |  2 +-
 lib/Kconfig.debug            | 21 +++++++++++++++++++++
 4 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index eb694f6..6a10469 100644
--- a/Makefile
+++ b/Makefile
@@ -639,6 +639,9 @@ KBUILD_CFLAGS	+= $(call cc-disable-warning, format-truncation)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, format-overflow)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, int-in-bool-context)
 
+ifdef CONFIG_DEBUG_EXPERIENCE
+KBUILD_CFLAGS	+= $(call cc-option, -Og)
+else
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS	+= $(call cc-option,-Oz,-Os)
 KBUILD_CFLAGS	+= $(call cc-disable-warning,maybe-uninitialized,)
@@ -649,6 +652,7 @@ else
 KBUILD_CFLAGS   += -O2
 endif
 endif
+endif
 
 KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \
 			$(call cc-disable-warning,maybe-uninitialized,))
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index b4bf73f..b8b3832 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -192,7 +192,7 @@
 
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
 
-#ifndef __CHECKER__
+#if !defined(__CHECKER__) && !defined(CONFIG_DEBUG_EXPERIENCE)
 # define __compiletime_warning(message) __attribute__((warning(message)))
 # define __compiletime_error(message) __attribute__((error(message)))
 #endif /* __CHECKER__ */
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ab4711c..952cc7f 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -301,7 +301,7 @@ unsigned long read_word_at_a_time(const void *addr)
  * sparse see a constant array size without breaking compiletime_assert on old
  * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether.
  */
-# ifndef __CHECKER__
+# if !defined(__CHECKER__) && !defined(CONFIG_DEBUG_EXPERIENCE)
 #  define __compiletime_error_fallback(condition) \
 	do { ((void)sizeof(char[1 - 2 * condition])); } while (0)
 # endif
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 90f35ad..2432e77d 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -211,6 +211,27 @@ config NO_AUTO_INLINE
 
 	  Use only if you want to debug the kernel.
 
+config DEBUG_EXPERIENCE
+	bool "Optimize for better debugging experience (-Og)"
+	default n
+	select NO_AUTO_INLINE
+	depends on !CC_OPTIMIZE_FOR_SIZE
+	help
+	  This will apply GCC '-Og' optimization level get supported from
+	  GCC 4.8. This optimization level offers a reasonable level of
+	  optimization while maintaining fast compilation and a good
+	  debugging experience. It is similar to '-O1' while perfer keeping
+	  debug ability over runtime speed. The overall performance will
+	  drop a bit.
+
+	  If enabling this option break your kernel, you should either
+	  disable this or find a fix (mostly in the arch code). Currently
+	  this option has only be tested in qemu x86_64 guest.
+
+	  Use only if you want to debug the kernel, especially if you want
+	  to have better kernel debugging experience with gdb facilities
+	  like kgdb and qemu.
+
 config ENABLE_WARN_DEPRECATED
 	bool "Enable __deprecated logic"
 	default y
-- 
2.7.4