Re: [PATCH] __cacheline_aligned always in own section

[Matt Mackall <mpm@selenic.com>]

On Thu, Feb 26, 2004 at 03:21:39PM -0800, Andrew Morton wrote:
> Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua> 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