From: Marc Glisse <marc.glisse@inria.fr>
To: Michael Matz <matz@suse.de>
Cc: Richard Biener <richard.guenther@gmail.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Jeff Law <law@redhat.com>,
Feng Xue OS <fxue@os.amperecomputing.com>
Subject: Re: [PATCH] Remove empty loop with assumed finiteness (PR tree-optimization/89713)
Date: Tue, 21 May 2019 08:06:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.21.1905210947090.13137@stedding.saclay.inria.fr> (raw)
In-Reply-To: <alpine.LSU.2.21.1905201442460.8064@wotan.suse.de>
On Mon, 20 May 2019, Michael Matz wrote:
> On Mon, 20 May 2019, Richard Biener wrote:
>
>>> The C++ standard says that do{}while(1) is __builtin_unreachable(), we
>>> don't have to preserve it. There is no mention of anything like a
>>> "nontrivial exit condition". Other languages may have a different
>>> opinion though, so it would probably need a flag indeed... But I am
>>> curious what the point of such a loop is.
>>
>> busy wait until wakeup by signal or interrupt.
>
> I'd actually turn it around from what C++ says. If the user wrote, as
> is, "do{}while(1);" or "while(1);" or "for(;;);" then we can assume
> something funky going on and not remove the loop. For any other loop we
> assume that they are finite. I.e. we mark loops as to-be-preserved (which
> we set on a few known patterns), and just remove all other loops when they
> contain no observable side effects after optimization.
Seems sensible, although marking the trivial infinite loops in gimple
seems simpler than doing it in the front-ends, and a good enough
approximation unless we are willing to replace some other infinite loops
with unreachable (or trap).
> And of course we'd still have to determine what acceptable side effects
> are. E.g. in a pointer chasing loop containing no body, is the
> segfault when the pointer chain is not in fact circular, a side effect we
> should retain, or should we be allowed to remove the loop? I'd say we
> should remove the loop, of course.
That may depend on flags like -fnon-call-exceptions (maybe not the right
one) I guess, although I would also want the removal to happen in as many
cases as possible. We do usually remove memory reads if the value read is
unused.
> (And yes, I've always found our obsession with preserving infinite loops,
> outside of the above "obvious" cases, overly anal as well)
--
Marc Glisse
next prev parent reply other threads:[~2019-05-21 8:06 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-17 4:17 Feng Xue OS
2019-05-17 16:47 ` Jeff Law
2019-05-17 18:50 ` Richard Biener
2019-05-18 14:00 ` Marc Glisse
2019-05-20 7:50 ` Richard Biener
2019-05-20 8:27 ` Feng Xue OS
2019-05-20 9:19 ` Richard Biener
2019-05-20 9:48 ` Feng Xue OS
2019-05-20 11:54 ` Richard Biener
2019-05-20 14:00 ` Feng Xue OS
2019-05-20 14:04 ` Richard Biener
2019-05-20 14:51 ` Feng Xue OS
2019-05-21 10:12 ` Richard Biener
2019-05-21 14:24 ` Richard Biener
2019-05-22 13:44 ` Michael Matz
2019-05-24 16:02 ` [PATCH V3] " Feng Xue OS
2019-05-24 9:15 ` [PATCH V2] " Feng Xue OS
2019-05-29 11:16 ` Richard Biener
2019-06-04 6:49 ` [PATCH V4] " Feng Xue OS
2019-06-04 8:24 ` Marc Glisse
2019-06-04 15:16 ` [PATCH V5] " Feng Xue OS
2019-06-04 15:24 ` [PATCH V6] " Feng Xue OS
2019-06-05 11:05 ` Richard Biener
2019-06-06 10:00 ` [PATCH V7] " Feng Xue OS
2019-06-11 2:40 ` [PATCH V8] " Feng Xue OS
2019-06-12 9:43 ` Richard Biener
2019-06-15 12:05 ` [committed][nvptx, libgomp] Update pr85381-{2,4}.c test-cases Tom de Vries
2019-05-20 13:04 ` [PATCH] Remove empty loop with assumed finiteness (PR tree-optimization/89713) Marc Glisse
2019-05-20 13:26 ` Richard Biener
2019-05-20 14:49 ` Michael Matz
2019-05-21 8:06 ` Marc Glisse [this message]
2020-04-01 13:36 ` [PATCH][RFC] c/94392 - only enable -ffinite-loops for C++ Richard Biener
2020-04-01 13:47 ` Jakub Jelinek
2020-04-01 13:52 ` Richard Biener
2020-04-01 15:56 ` Jan Hubicka
2020-04-01 16:59 ` Richard Biener
2020-04-01 19:15 ` Jason Merrill
2020-04-02 9:12 ` Richard Biener
2020-04-02 9:17 ` Jakub Jelinek
2020-04-02 9:41 ` Richard Biener
2020-04-03 8:29 ` Revert "[nvptx, libgomp] Update pr85381-{2, 4}.c test-cases" [PR89713, PR94392] (was: [PATCH][RFC] c/94392 - only enable -ffinite-loops for C++) Thomas Schwinge
2020-04-03 9:36 ` Revert "[nvptx, libgomp] Update pr85381-{2,4}.c " Richard Biener
2020-04-03 10:34 ` Jakub Jelinek
2020-10-30 14:09 ` Revert "[nvptx, libgomp] Update pr85381-{2, 4}.c " Thomas Schwinge
2020-10-30 14:16 ` Revert "[nvptx, libgomp] Update pr85381-{2,4}.c " Jakub Jelinek
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.DEB.2.21.1905210947090.13137@stedding.saclay.inria.fr \
--to=marc.glisse@inria.fr \
--cc=fxue@os.amperecomputing.com \
--cc=gcc-patches@gcc.gnu.org \
--cc=law@redhat.com \
--cc=matz@suse.de \
--cc=richard.guenther@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).