From: Andi Kleen ak@linux.intel.com
I found that gcc 4.5 didn't inline a lot of inlines with CONFIG_OPTIMIZE_INLINING and CONFIG_CC_OPTIMIZE_FOR_SIZE. It was quite common to have very small inlines to be out of line, or worse inline statics in include files to be out of line with a copy for every file using it too.
This is handily visible in a function graph trace for might_fault:
10) | might_fault() { 10) | _cond_resched() { 10) | should_resched() { 10) | need_resched() { 10) 0.063 us | test_ti_thread_flag(); 10) 0.643 us | } 10) 1.238 us | } 10) 1.845 us | } 10) 2.438 us | }
Note all of these functions are very small and should be definitely inlined in each other. In many cases even copy_from_user ends up out of line now which is really bad!
If I switch to -O2 it is also not quite as bad, but since a lot of people use -Os I was trying to fix it up.
So this patch forces inlining with gcc 4.5 with -Os.
Unfortunately it costs some code size with just this patch.
text data bss dec hex filename 11507035 1940276 1191936 14639247 df608f vmlinux-O2 10189858 1908124 1187840 13285822 cab9be vmlinux-Os-force 9808525 1940204 1187840 12936569 c56579 vmlinux-Os-orig
It turned out most of this was because of unnecessary inlines. With a lot of inlines removed I now get:
11175824 1977200 1191936 14344960 dae300 vmlinux-inlines-removed-no-optimize 11642530 2018416 1191936 14852882 e2a312 vmlinux-master-no-optimize 11530439 2001264 1191936 14723639 e0aa37 vmlinux-master-optimize
which is significantly smaller.
I haven't tested earlier gcc 4.x versions, but they may need the same treatment.
Signed-off-by: Andi Kleen ak@linux.intel.com --- include/linux/compiler-gcc.h | 5 ++++- 1 files changed, 4 insertions(+), 1 deletions(-)
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index 59e4028..e477a7c 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -44,9 +44,12 @@ /* * Force always-inline if the user requests it so via the .config, * or if gcc is too old: + * When optimizing for size on gcc 4.5 always force inlining too. */ #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) || \ - !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4) + !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4) || \ + (defined(CONFIG_CC_OPTIMIZE_FOR_SIZE) && \ + (__GNUC__ == 4 && __GNUC_MINOR__ == 5)) # define inline inline __attribute__((always_inline)) # define __inline__ __inline__ __attribute__((always_inline)) # define __inline __inline __attribute__((always_inline))