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

[H. Peter Anvin]

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.

	-hpa
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