Re: [PATCH v2] x86: page: get_order() optimization

From: Ingo Molnar
Date: Tue Mar 29 2011 - 03:27:17 EST



* H. Peter Anvin <hpa@xxxxxxxxx> wrote:

> On 03/27/2011 01:45 AM, Maksym Planeta wrote:
> > For x86 architecture get_order function can be optimized due to
> > assembler instruction bsr.
> >
> > This is second version of patch where for constants gcc precompute the
> > result.
> >
> > Signed-off-by: Maksym Planeta <mcsim.planeta@xxxxxxxxx>
>
> gcc 4.x has an intrinsic, __builtin_clz(), which does the opposite of
> the bsr instruction; specifically:
>
> __builtin_clz(x) ^ 31
>
> ... generates a bsrl instruction if x is variable. This tends to
> generate much better code than any assembly hacks.

Indeed, that should work better and should be tried - and it can probably
propagate the flags result sensibly (which GCC's asm() cannot, unfortunately).

Thanks,

Ingo
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