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Re: PATCH: Remove SLOW_BYTE_ACCESS from get_best_mode
- From: Paul Brook <paul at codesourcery dot com>
- To: gcc-patches at gcc dot gnu dot org
- Cc: "H. J. Lu" <hjl at lucon dot org>, Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>, Eric Christopher <echristo at apple dot com>
- Date: Thu, 14 Sep 2006 00:20:31 +0100
- Subject: Re: PATCH: Remove SLOW_BYTE_ACCESS from get_best_mode
- References: <4796D556-1CD8-40E6-9A96-1D506F4D9799@apple.com> <4507ABF2.3030006@lu.unisi.ch> <20060913230858.GA25261@lucon.org>
> Here is the updated patch. I ran a micro benchmark with code difference
> similar to twolf:
>
> - movl (%rdi), %eax
> - xorb %ah, %ah
> - subl $1, %eax
> + movq (%rdi), %rax
> + andl $4294902015, %eax
> + subq $1, %rax
> jne .L8
>
> There is no speed difference on Nocona. I checked my SPEC results.
> twolf isn't very stable on my Nocona.
The former is significantly smaller though (7 bytes vs. 12 bytes).
Paul