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SubjectRe: [PATCH] __cacheline_aligned always in own section
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.

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