From: Jakub Jelinek <jakub@redhat.com>
To: Sudi Das <Sudi.Das@arm.com>
Cc: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
Marcus Shawcroft <Marcus.Shawcroft@arm.com>,
Richard Earnshaw <Richard.Earnshaw@arm.com>,
James Greenhalgh <James.Greenhalgh@arm.com>,
nd <nd@arm.com>
Subject: Re: [PATCH][GCC] Simplification of 1U << (31 - x)
Date: Wed, 12 Apr 2017 17:06:00 -0000 [thread overview]
Message-ID: <20170412170638.GA1809@tucnak> (raw)
In-Reply-To: <VI1PR08MB26887B0900BF829A25462F0698000@VI1PR08MB2688.eurprd08.prod.outlook.com>
On Wed, Apr 12, 2017 at 09:29:55AM +0000, Sudi Das wrote:
> Hi all
>
> This is a fix for PR 80131
> Currently the code A << (B - C) is not simplified.
> However at least a more specific case of 1U << (C -x) where C = precision(type) - 1 can be simplified to (1 << C) >> x.
Is that always a win though?
Some constants have higher costs than others on various targets, some
significantly higher. This change might be beneficial only
if if C is as expensive as 1, then you get rid of a one (typically cheap)
operation.
Which makes me wonder whether this should be done at GIMPLE time and not
at RTL time (expansion or combine etc.) when one can compare the RTX costs.
Or do this at match.pd as canonicalization and then have RTL transformation
to rewrite such (1U << C) >> X as 1U << (C - X) if the latter is faster (or
shorter).
Jakub
next prev parent reply other threads:[~2017-04-12 17:06 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-12 9:30 Sudi Das
2017-04-12 17:06 ` Jakub Jelinek [this message]
2017-04-12 18:16 ` Segher Boessenkool
2017-04-12 18:59 ` Jakub Jelinek
2017-04-12 19:34 ` Segher Boessenkool
2017-04-13 11:16 Wilco Dijkstra
2017-04-13 11:21 ` Jakub Jelinek
2017-04-13 11:33 ` Wilco Dijkstra
2017-04-13 11:41 ` Jakub Jelinek
2017-04-13 11:49 ` Richard Biener
2017-04-13 11:55 ` Jakub Jelinek
2017-04-13 12:01 ` Wilco Dijkstra
2017-08-01 9:15 ` Sudi Das
2017-08-04 10:16 ` Richard Biener
2017-09-26 12:44 ` Sudi Das
2017-09-26 13:06 ` Jakub Jelinek
2017-09-26 13:20 ` Wilco Dijkstra
2017-10-06 17:00 ` Sudi Das
2017-10-09 12:05 ` Richard Biener
2017-10-09 13:04 ` Wilco Dijkstra
2017-10-10 11:12 ` Sudi Das
2017-11-07 12:37 ` Wilco Dijkstra
2017-11-07 14:48 ` Christophe Lyon
2017-11-07 14:49 ` Wilco Dijkstra
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