[PATCH] D23214: [MDA] Treat invariant.start as non-dependence

[Daniel Berlin via llvm-commits]

>
> > That said, in theory:
> >
> > %a = add 5, 3
> > bar = invariant.start(foo, 5)
> > load foo
> > invariant.end(bar)
> >
> > to
> > bar = invariant.start(foo, 5)
> > %a = add 5, 3
> > load foo
> > invariant.end(bar)
> >
> > is probably also illegal, and has the same issues :)
>
> I don't understand why that ^ is a problem.
>
>
You have extended the scope :)

Even if the extension is not meaningful :)


> >     With the memdep changes we will allow reordering loads across the
> >     invarinat_start call:
> >
> >        int val = *ptr;
> >        invariant_start(ptr)
> >
> >     <==>
> >
> >        int val = *ptr;
> >        invariant_start(ptr)
> >
> >
> > (you meant for the first one to have the load after invariant start, i
> > presume)
>
> I meant given what we're teaching MDA, the two are equivalent (hence
> the "bidirectional arrow" :)).


I meant that the examples are identical
 :)


>
>
> > I'm willing to give it a shot if you want, i'm just in the "we'll
> > probably break something unexpected" camp :)
>
> I now think it is better to add one more special case to AliasAnalysis
> around invariant_start s, like we do for assumes.  It's a hack, but at
> least a localized hack.


I already added the same hack to memoryssa, so i can't complain :)


>
>
> -- Sanjoy
>
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