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From: Paul Koning <paulkoning@comcast.net>
To: coypu@sdf.org
Cc: Jeff Law <law@redhat.com>, gcc@gcc.gnu.org, port-vax@netbsd.org
Subject: Re: syncing the GCC vax port, atomic issue
Date: Sat, 21 Sep 2019 18:27:00 -0000	[thread overview]
Message-ID: <CCB28096-DF84-4D2F-AB59-6541D1870144@comcast.net> (raw)
In-Reply-To: <20190921011835.GA5931@SDF.ORG>



> On Sep 20, 2019, at 9:18 PM, coypu@sdf.org wrote:
> 
> On Fri, Sep 20, 2019 at 10:07:59PM +0000, coypu@sdf.org wrote:
>> Introducing the reversed jbb* patterns doesn't seem to help with the
>> original issue. It crashes building libatomic.
> 
> My loose understanding of what is going on:
> - GCC emits this atomic in expand.
> - When cleaning up, it looks for optimizations.
> - It decides this is a branch to another branch situation, so maybe
>  can be improved.
> - This fails to output an instruction for unrelated reasons.
> - Hit an assert.
> 
> I don't think that we should be trying to combine regular branch +
> atomic branch in very generic code.
> My guess is that, if it didn't crash now, it might emit a different kind
> of branch which loses the atomic qualities, and result in wrong code.

Or it might leave the atomic branch, in a place where it isn't really wanted.

I wonder if this could be avoided by representing the atomic branch by an UNSPEC rather than by a branch, since it isn't a "normal branch" that GCC knows about.

	paul

  reply	other threads:[~2019-09-21 18:27 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-30  9:03 syncing the GCC vax port coypu
2019-03-31 17:26 ` Paul Koning
2019-03-31 17:41   ` coypu
2019-04-01 15:33   ` Jeff Law
2019-04-29 20:08 ` Jeff Law
2019-09-20 11:15   ` coypu
2019-09-20 21:04     ` syncing the GCC vax port, atomic issue coypu
2019-09-20 21:45       ` Jeff Law
2019-09-20 22:08         ` coypu
2019-09-21  1:18           ` coypu
2019-09-21 18:27             ` Paul Koning [this message]
2019-10-01 19:44               ` Jeff Law
2019-10-01 19:43             ` Jeff Law
2019-10-02  9:39               ` Richard Earnshaw (lists)
2019-10-02 17:17                 ` Segher Boessenkool

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