From: Jeff Law <law@redhat.com>
To: Richard Guenther <richard.guenther@gmail.com>
Cc: Andrew MacLeod <amacleod@redhat.com>,
"Joseph S. Myers" <joseph@codesourcery.com>,
Aldy Hernandez <aldyh@redhat.com>,
Richard Henderson <rth@redhat.com>,
gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [cxx-mem-model] disallow load data races (1 of some)
Date: Thu, 31 Mar 2011 13:32:00 -0000 [thread overview]
Message-ID: <4D948158.1010601@redhat.com> (raw)
In-Reply-To: <AANLkTinOBUuGnCuOH8-ghwog4jSeBhqv8vxKceoeFWa1@mail.gmail.com>
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On 03/31/11 07:26, Richard Guenther wrote:
> On Thu, Mar 31, 2011 at 3:08 PM, Jeff Law <law@redhat.com> wrote:
> On 03/24/11 17:57, Andrew MacLeod wrote:
>>>> My fault for not being specific about it... I tend to just use data race
>>>> as a catch all for all these types of things when talking about them
>>>> with Aldy.
>>>>
>>>> the testcase should have for (x=0; x< limit; x++) sum[x] = global;
>>>> rather than x<5, so that its not a known loop count.
>>>>
>>>> The hoisted load is then not allowed as it become a speculative load of
>>>> 'global' on the main path which would not otherwise have been there.
>>>> When the value of limit < 0, this can cause a load race detection on
>>>> systems with either a software or hardware data race detector.
> We generally don't allow this kind of code motion anyway -- though it's
> more because we haven't built any kind of analysis to determine that
> this kind of speculative load is often profitable and can't result in a
> fault.
>
> However, having a switch we can throw to avoid this behaviour (assuming
> we add this kind of speculative code motion one day) is, IMHO, a good thing.
>
>> If we know the access to global cannot trap I see no reason to disallow
>> speculative reading of it.
Without trip count estimates the speculative read can introduce a
performance regression.
jeff
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next prev parent reply other threads:[~2011-03-31 13:28 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-24 17:33 Aldy Hernandez
2011-03-24 19:58 ` Richard Henderson
2011-03-24 21:31 ` Aldy Hernandez
2011-03-24 21:51 ` Joseph S. Myers
2011-03-24 23:57 ` Andrew MacLeod
2011-03-25 10:03 ` Richard Guenther
2011-03-25 12:58 ` Aldy Hernandez
2011-03-25 15:27 ` Michael Matz
2011-03-25 15:33 ` Jeff Law
2011-03-25 15:35 ` Jakub Jelinek
2011-03-25 15:46 ` Richard Guenther
2011-03-29 16:52 ` Aldy Hernandez
2011-03-31 13:12 ` Jeff Law
2011-03-31 13:32 ` Richard Guenther
2011-03-31 13:32 ` Jeff Law [this message]
2011-03-31 21:07 ` Michael Matz
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