Messages in this thread | | | Date | Wed, 5 Jul 2006 14:58:26 -0700 | From | "Randy.Dunlap" <> | Subject | Re: [patch] uninline init_waitqueue_*() functions |
| |
On Wed, 5 Jul 2006 23:45:02 +0200 Ingo Molnar wrote:
> > * Linus Torvalds <torvalds@osdl.org> wrote: > > > On Wed, 5 Jul 2006, Ingo Molnar wrote: > > > > > > i had CONFIG_DEBUG_INFO (and UNWIND_INFO) disabled in all these build > > > tests. > > > > Good, because I just verified: those two together will on their own > > increase "text size" by about 17% for me. > > > > I still think Andrew is right: I don't see how an initializer that > > should basically be three instructions can possibly be 35 bytes larger > > than a function call that should be a minimum of two instructions > > (argument setup in %eax and the actual call - and that's totally > > ignoring the deleterious effects of a function call on register > > liveness). > > > > The fact that with allnoconfig the kernel is _smaller_ (but, quite > > franlkly, within the noise) with the inlined version would seem to > > back up Andrews position that it really shouldn't matter. > > well, the allnoconfig thing is artificial (and the uninteresting) for a > number of reasons:
hm, I'd have to say that allyesconfig is also artificial and the savings numbers are somewhat uninteresting in that case too.
> - it has REGPARM disabled which penalizes function calls > > - it's UP and hence the inlining cost of init_wait_queue_head() is > significantly smaller. > > - allnoconfig has smaller average function size - increasing the cost of > uninlining > > > So I'm left wondering why it matters for you, and what triggers it. > > Maybe there is some secondary issue that could show us an even more > > interesting optimization (or some compiler behaviour that we should > > try to encourage). > > yeah, i'd not want to skip over some interesting and still unexplained > effect either, but 35 bytes isnt all that outlandish and from everything > i've seen it's a real win. Here is an actual example: > > c0fb6137: c7 44 24 08 00 00 00 movl $0x0,0x8(%esp) > c0fb613e: 00 > c0fb613f: c7 44 24 08 01 00 00 movl $0x1,0x8(%esp) > c0fb6146: 00 > c0fb6147: c7 43 60 00 00 00 00 movl $0x0,0x60(%ebx) > c0fb614e: 8b 44 24 08 mov 0x8(%esp),%eax > c0fb6152: 89 43 5c mov %eax,0x5c(%ebx) > c0fb6155: 8d 43 64 lea 0x64(%ebx),%eax > c0fb6158: 89 40 04 mov %eax,0x4(%eax) > c0fb615b: 89 43 64 mov %eax,0x64(%ebx) > > versus: > > c0fb070e: 8d 43 5c lea 0x5c(%ebx),%eax > c0fb0711: e8 94 98 18 ff call c0139faa <init_waitqueue_head> > > so 39 bytes versus 8 bytes - 31 bytes saved. It's a similar win in other > cases i checked too. (the only exception is for smaller functions that i > mentioned before: where the parameters are not pre-calculated yet so > there's no good integration for the function call. In that case it's > break even, or in some cases a 3-4 bytes loss.)
--- ~Randy - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |