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Re: [PATCH] teach emit_store_flag to use clz/ctz

From: Richard Henderson <rth at redhat dot com>
To: Paolo Bonzini <bonzini at gnu dot org>
Cc: Richard Guenther <richard dot guenther at gmail dot com>, Richard Guenther <rguenther at suse dot de>, gcc-patches at gcc dot gnu dot org, amodra at gmail dot com
Date: Thu, 03 May 2012 08:10:19 -0700
Subject: Re: [PATCH] teach emit_store_flag to use clz/ctz
References: <1335520129-28934-1-git-send-email-bonzini@gnu.org> <Pine.LNX.4.64.1204271308530.23071@jbgna.fhfr.qr> <4F9A948A.1040903@gnu.org> <CAFiYyc20Cqah92yxsk8CNNpE=w4yWB6bC_b1E=y9ryf-_33TGA@mail.gmail.com>

On Fri, Apr 27, 2012 at 2:43 PM, Paolo Bonzini<bonzini@gnu.org> wrote:
Interesting, first time I hear about this...
... except that no target sets the macros to 2, and all of them could
(as far as I could see).  Looks like the code trumps the documentation;
how does this look?


"No target sets to 2"?  You mean like mips?  You forgot to look at the
corresponding CLZ_DEFINED_VALUE_AT_ZERO.

I'll admit I hadn't been paying attention when sandra added the patch
in question though...

        * optabs.c (expand_ffs): Check CTZ_DEFINED_VALUE_AT_ZERO
        against 1.
        * doc/tm.texi (Misc): Invert meaning of 1 and 2 for
        CLZ_DEFINED_VALUE_AT_ZERO and CTZ_DEFINED_VALUE_AT_ZERO.

So... no, I don't think this is a good idea.

r~

Follow-Ups:
- Re: [PATCH] teach emit_store_flag to use clz/ctz
  - From: Paolo Bonzini

References:
- Re: [PATCH] teach emit_store_flag to use clz/ctz
  - From: Richard Guenther

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