From: Eric Botcazou <ebotcazou@adacore.com>
To: Oleg Endo <oleg.endo@t-online.de>
Cc: gcc@gcc.gnu.org
Subject: Re: Combine Pass Behavior
Date: Wed, 12 Oct 2011 09:44:00 -0000 [thread overview]
Message-ID: <201110121008.09411.ebotcazou@adacore.com> (raw)
In-Reply-To: <1318301863.19997.311.camel@yam-132-YW-E178-FTW>
> The actual problem (or maybe my misunderstanding) is that it
> combines the two original insns, then does some substitutions and tries
> to match the combined and transformed insn against those defined in the
> machine description. If it can't find anything there it reverts
> everything and proceeds with the next insn pair. It never tries out the
> straight forward option in the first place (which is not to transform
> the combination).
The combiner pass does some form of canonicalization to better combine, see
expand_compound_operation and make_compound_operation.
> Is the scenario above intended behavior of the combine pass or an
> accident? Or maybe even something else wrong in the machine description
> that makes it behave like that?
See how the i386 back-end copes with this for its "test" instruction.
--
Eric Botcazou
prev parent reply other threads:[~2011-10-12 8:12 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-11 12:21 Oleg Endo
2011-10-12 9:44 ` Eric Botcazou [this message]
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