[cxx-mem-model] disallow load data races (1 of some)

[Jeff Law]

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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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