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

Ingo Molnar
2005-12-28T11:47:08Z

this patchset (for the 2.6.16 tree) consists of two patches:

gcc-no-forced-inlining.patch
gcc-unit-at-a-time.patch

the purpose of these patches is to reduce the kernel's .text size, in
particular if CONFIG_CC_OPTIMIZE_FOR_SIZE is specified. The effect of
the patches on x86 is:

text data bss dec hex filename
3286166 869852 387260 4543278 45532e vmlinux-orig
3194123 955168 387260 4536551 4538e7 vmlinux-inline
3119495 884960 387748 4392203 43050b vmlinux-inline+units
437271 77646 32192 547109 85925 vmlinux-tiny-orig
452694 77646 32192 562532 89564 vmlinux-tiny-inline
431891 77422 32128 541441 84301 vmlinux-tiny-inline+units

i.e. a 5.3% .text reduction (!) with a larger .config, and a 1.2% .text
reduction with a smaller .config.

i've also done test-builds with CC_OPTIMIZE_FOR_SIZE disabled:

text data bss dec hex filename
4080998 870384 387260 5338642 517612 vmlinux-speed-orig
4084421 872024 387260 5343705 5189d9 vmlinux-speed-inline
4010957 834048 387748 5232753 4fd871 vmlinux-speed-inline+units

so the more flexible inlining did not result in many changes [which is
good, we want gcc to inline those in the optimized-for-speed case], but
unit-at-a-time optimization resulted in smaller code - very likely
meaning speed advantages as well.

unit-at-a-time still increases the kernel stack footprint somewhat (by
about 5% in the CC_OPTIMIZE_FOR_SIZE case), but not by the insane degree
gcc3 used to, which prompted the original -fno-unit-at-a-time addition.

so i think the combination of the two patches is a win both for small
and for large systems. In fact the 5.3% .text reduction for embedded
kernels is very significant.

the patches are against -git, and were test-built and test-booted on
x86, using gcc 4.0.2.

Ingo