|From:||Ingo Molnar <email@example.com>|
|Subject:||[patch 00/2] improve .text size on gcc 4.0 and newer compilers|
|Date:||Wed, 28 Dec 2005 12:46:37 +0100|
|Cc:||Linus Torvalds <firstname.lastname@example.org>, Andrew Morton <email@example.com>, Arjan van de Ven <firstname.lastname@example.org>, Matt Mackall <email@example.com>|
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
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