From: Richard Biener <rguenther@suse.de>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Jason Merrill <jason@redhat.com>,
gcc-patches@gcc.gnu.org,
"Joseph S. Myers" <joseph@codesourcery.com>
Subject: Re: [PATCH][RFC] c/94392 - only enable -ffinite-loops for C++
Date: Thu, 2 Apr 2020 11:41:22 +0200 (CEST) [thread overview]
Message-ID: <nycvar.YFH.7.76.2004021133250.830@zhemvz.fhfr.qr> (raw)
In-Reply-To: <20200402091746.GP2212@tucnak>
On Thu, 2 Apr 2020, Jakub Jelinek wrote:
> On Thu, Apr 02, 2020 at 11:12:48AM +0200, Richard Biener wrote:
> > > "The implementation may assume that any thread will eventually do one of the
> > > following:
> > > — terminate,
> > > — make a call to a library I/O function,
> > > — perform an access through a volatile glvalue, or
> > > — perform a synchronization operation or an atomic operation.
> > > [Note: This is intended to allow compiler transformations such as removal of
> > > empty loops, even when termination cannot be proven. — end note]"
>
> With -ffinite-loops, do we actually not optimize if the loop has volatile accesses
> or atomics or library I/O calls?
We don't remove the loop then, yes. All of those are considered
side-effects by GCC and thus they are considered needed and keep
the loop live. From a technical point finite_loop_p will still
return true for those which is not correct but harmless at the
moment (I hope). I read the above as
volatile int i;
int __attribute__((const,noinline)) baz(int i) { return i; }
int foo(int a)
{
do
{
i;
if (a != baz(a)) return 1;
}
while (1);
}
being a valid endles loop which we indeed preserve. But we notice
the opportunity to unswitch on the condition and elide _that_ loop:
<bb 2> [local count: 118111600]:
_1 = baz (a_4(D));
if (_1 != a_4(D))
goto <bb 4>; [11.00%]
else
goto <bb 3>; [89.00%]
<bb 3> [local count: 955630224]:
vol.0_5 ={v} i;
goto <bb 3>; [100.00%]
<bb 4> [local count: 118111600]:
vol.0_3 ={v} i;
return 1;
thus turn it into
if (a != baz(a)) { i; return 1; }
do { i; } while (1);
Richard.
next prev parent reply other threads:[~2020-04-02 9:41 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-17 4:17 [PATCH] Remove empty loop with assumed finiteness (PR tree-optimization/89713) 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
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 [this message]
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
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