Re: [PATCH] MIPS: Use ARCH_USE_BUILTIN_BSWAP.

[Ralf Baechle <ralf@linux-mips.org>]

On Tue, Sep 29, 2015 at 02:34:12PM +0100, Måns Rullgård wrote:

> >> >>    text    data     bss     dec     hex filename
> >> >> 3996071  155804   88992 4240867  40b5e3 vmlinux         ip22 baseline
> >> >> 3985687  159900   88992 4234579  409d53 vmlinux         ip22 + bswap patch
> >> >> 6913157  378552  251024 7542733  7317cd vmlinux         ip27 baseline
> >> >> 6878581  378552  251024 7508157  7290bd vmlinux         ip27 + bswap patch
> >> >> 5773777  268752  187424 6229953  5f0fc1 vmlinux         malta baseline
> >> >> 5773401  268752  187424 6229577  5f0e49 vmlinux         malta + bswap patch
> >> >
> >> > A still unexplained effect of this patch and the reason why I have not
> >> > committed this patch is the increase of the data size for the ip22
> >> > configuration by 4096 bytes.  There is no change in data size expected.
> >> > Also this affects only the test with ip22_defconfig not any of the others
> >> > I've tried.
> >> 
> >> Have you checked which object file(s) the increase comes from?
> >
> > The data size difference is created in the final link stage.  data for
> > all inividual .o files even vmlinux.o; only vmlinux differs.
> 
> Could it be an alignment thing.  Check if the alignment of some data
> section has changed from something smaller to 4096.

Almost.  Alignment has changed but 8k alignment of init_thread_union
forced the linker to waste an extra 4k.

I always knew this could happen but never actually observed this.  Hard to
avoid but it's yet another reason to minimize stack size which on MIPS
may be up to 4 pages for 64 bit kernels with 4k pages.

  Ralf