Re: [patch 00/2] improve .text size on gcc 4.0 and newer compilers

Ingo Molnar
2005-12-28T21:49:06Z

* Ingo Molnar <mingo@elte.hu> wrote:

> (there's a third thing that i was also playing with, -ffunction-sections
> and -fdata-sections, but those dont seem to be reliable on the binutils
> side yet.)
>
> here are the isolated unit-at-a-time numbers as well:
>
> text data bss dec hex filename
> 3286166 869852 387260 4543278 45532e vmlinux-orig
> 3259928 833176 387748 4480852 445f54 vmlinux-units -0.8%
> 3194123 955168 387260 4536551 4538e7 vmlinux-inline -2.9%
> 3119495 884960 387748 4392203 43050b vmlinux-inline+units -5.3%
>
> so both inlining and unit-at-a-time is a win independently [although
> inlining alone does bloat .data], but applied together they bring an
> additional 1.6% of .text savings. All builds done with:
>
> gcc version 4.0.2 20051109 (Red Hat 4.0.2-6)
>
> how about giving the inlining stuff some more exposure in -mm (if it's
> fine with Andrew), to check for any regressions? I'd suggest the same
> for the unit-at-a-time thing too, in any case.

another thing: i wanted to decrease the size of -Os
(CONFIG_CC_OPTIMIZE_FOR_SIZE) kernels, which e.g. Fedora uses too (to
keep the icache footprint down).

I think gcc should arguably not be forced to inline things when doing
-Os, and it's also expected to mess up much less than when optimizing
for speed. So maybe forced inlining should be dependent on
!CONFIG_CC_OPTIMIZE_FOR_SIZE?

I.e. like the patch below?

Ingo

----------------->
Subject: allow gcc4 to control inlining

allow gcc4 compilers to decide what to inline and what not - instead
of the kernel forcing gcc to inline all the time.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@infradead.org>
----

include/linux/compiler-gcc4.h | 13 +++++++++----
1 files changed, 9 insertions(+), 4 deletions(-)

Index: linux-gcc.q/include/linux/compiler-gcc4.h
===================================================================
--- linux-gcc.q.orig/include/linux/compiler-gcc4.h
+++ linux-gcc.q/include/linux/compiler-gcc4.h
@@ -3,14 +3,19 @@
/* These definitions are for GCC v4.x. */
#include <linux/compiler-gcc.h>

-#define inline inline __attribute__((always_inline))
-#define __inline__ __inline__ __attribute__((always_inline))
-#define __inline __inline __attribute__((always_inline))
+
+#ifndef CONFIG_CC_OPTIMIZE_FOR_SIZE
+# define inline inline __attribute__((always_inline))
+# define __inline__ __inline__ __attribute__((always_inline))
+# define __inline __inline __attribute__((always_inline))
+#endif
+
#define __deprecated __attribute__((deprecated))
#define __attribute_used__ __attribute__((__used__))
#define __attribute_pure__ __attribute__((pure))
#define __attribute_const__ __attribute__((__const__))
-#define noinline __attribute__((noinline))
+#define noinline __attribute__((noinline))
+#define __always_inline inline __attribute__((always_inline))
#define __must_check __attribute__((warn_unused_result))
#define __compiler_offsetof(a,b) __builtin_offsetof(a,b)