From: Segher Boessenkool <segher@kernel.crashing.org>
To: Jeff Law <law@redhat.com>
Cc: Wilco Dijkstra <wdijkstr@arm.com>,
"'GCC Patches'" <gcc-patches@gcc.gnu.org>
Subject: Re: RFC: Combine of compare & and oddity
Date: Thu, 03 Sep 2015 19:06:00 -0000 [thread overview]
Message-ID: <20150903190523.GD13559@gate.crashing.org> (raw)
In-Reply-To: <55E89496.4080202@redhat.com>
On Thu, Sep 03, 2015 at 12:42:30PM -0600, Jeff Law wrote:
> Note huge parts of combine are structured around the needs of processors
> from the late 80s and early 90s. The canonical forms selected for those
> processors may not be optimal for today's processors.
Or, more precisely, for the backends for those processors. Some of the
canonicalisation rules are very inconvenient for some backends.
> The problem (of course) is changing the canonical forms can be a ton of
> work in both the backends as well as combine itself to ensure quality of
> code doesn't regress.
Yes exactly. Even more so than with other combine changes, before we
do such changes we need to evaluate 1) what this changes, on what targets;
and 2) how big the impact of that is.
Without a proposed patch, all I can say is "most targets will need changes".
> >>But the change from AND to zero_extract is already changing semantics...
> >
> >Oh? It is not supposed to!
> Combine should never change semantics. It can change form and may
> change what happens to "don't care" bits. But it should never change
> visible semantics.
And in the reverse transform (in change_zero_ext), it is hard to tell
what those "don't care" bits are (so there are no such bits).
Segher
next prev parent reply other threads:[~2015-09-03 19:05 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-02 17:09 Wilco Dijkstra
2015-09-02 18:49 ` Segher Boessenkool
2015-09-02 19:56 ` Segher Boessenkool
2015-09-03 11:51 ` Wilco Dijkstra
2015-09-03 13:57 ` Segher Boessenkool
2015-09-03 15:00 ` Wilco Dijkstra
2015-09-03 15:21 ` Andrew Pinski
2015-09-03 16:15 ` Jeff Law
2015-09-03 16:40 ` Segher Boessenkool
2015-09-03 18:56 ` Wilco Dijkstra
2015-09-03 19:05 ` Jeff Law
2015-09-03 19:20 ` Richard Sandiford
2015-09-03 21:20 ` Jeff Law
2015-09-03 19:22 ` Segher Boessenkool
2015-09-03 16:24 ` Segher Boessenkool
2015-09-03 16:36 ` Kyrill Tkachov
2015-09-03 16:44 ` Wilco Dijkstra
2015-09-03 17:27 ` Segher Boessenkool
2015-09-03 17:30 ` Oleg Endo
2015-09-03 18:53 ` Wilco Dijkstra
2015-09-03 18:42 ` Jeff Law
2015-09-03 19:06 ` Segher Boessenkool [this message]
2015-09-03 15:52 ` Jeff Law
2015-09-02 20:45 ` Jeff Law
2015-09-02 21:05 ` Segher Boessenkool
2015-09-03 15:26 ` Jeff Law
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