[PATCH 12/12] Force always inline for gcc 4.5 when optimizing for size

Andi Kleen
2011-10-13T23:09:45Z

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))
--
1.7.4.4