-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 03/31/11 07:26, Richard Guenther wrote: > On Thu, Mar 31, 2011 at 3:08 PM, Jeff Law <l...@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.
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