Re: [PATCH] __cacheline_aligned always in own section

From: Matt Mackall
Date: Thu Feb 26 2004 - 20:10:12 EST


On Thu, Feb 26, 2004 at 03:21:39PM -0800, Andrew Morton wrote:
> Denis Vlasenko <vda@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > I compile my kernels for 486 but buils system aligns
> > functions and labels to 16 bytes, with results like this:
> >
> > 00000730 <islpci_eth_tx_timeout>:
> > 730: 55 push %ebp
> > 731: 89 e5 mov %esp,%ebp
> > 733: 8b 45 08 mov 0x8(%ebp),%eax
> > 736: 8b 40 64 mov 0x64(%eax),%eax
> > 739: 05 14 03 00 00 add $0x314,%eax
> > 73e: ff 40 14 incl 0x14(%eax)
> > 741: 5d pop %ebp
> > 742: c3 ret
> > 743: 90 nop
> > 744: 90 nop
> > 745: 90 nop
> > 746: 90 nop
> > 747: 90 nop
> > 748: 90 nop
> > 749: 90 nop
> > 74a: 90 nop
> > 74b: 90 nop
> > 74c: 90 nop
> > 74d: 90 nop
> > 74e: 90 nop
> > 74f: 90 nop
> >
> > Losing on average 15/2 bytes to alignment, my kernel lose
> > # echo $((`cat System.map | grep '0 ' | wc -l`*15/2))
> > 149632
> > bytes only due to function alignment, not counting jump target
> > alighment.
> >
> > Is there any way to prevent this?
>
> Yes, there are ways of turning off a lot of this alignment padding and it
> makes a significant different in code size. You need to dig around in the
> gcc documentation for the several -*align* options.
>
> IIRC, not all of the padding could be turned off (the gcc options were not
> complete) but it's a couple of years since I investigated this.

Denis, there's a patch in -tiny to let you experiment with overriding
CFLAGS. See http://selenic.com/tiny-about/

You probably want something like:

-falign-functions=1 -falign-jumps=1 -falign-labels=1 -falign-loops=1

--
Matt Mackall : http://www.selenic.com : Linux development and consulting
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