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Re: moving constants out of loops
- To: kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)
- Subject: Re: moving constants out of loops
- From: Jeffrey A Law <law at cygnus dot com>
- Date: Tue, 30 Jun 1998 17:39:14 -0600
- cc: Haj dot Ten dot Brugge at net dot hcc dot nl, egcs-patches at cygnus dot com, gcc-patches at gnu dot org
- Reply-To: law at cygnus dot com
In message <9806302327.AA14100@vlsi1.ultra.nyu.edu>you write:
> Furthermore, we found that exposing more givs and invariants wasn't
> always a win. In fact, it was a huge loss on the PA for a while (see
> alias discussion below).
>
> My feeling is a bit different here. There is currently code that attempts
> to decide which givs are worth doing something about. I feel that the
> more choice it has to pick from, the better job it wil do. But that's
> not relevant in this case, since what's being found aren't "givs",
> but constants.
Now that we agree that we're really enabling the loop optimizer to
find more givs hopefully my comment makes more sense. ie, the
existing alias code in gcc2 isn't up to the task of dealing with
the more complex givs (in particular those used for memory addresses).
The net result is severe impact on the scheduler's ability to reorder
memory references within loops.
jeff