[PATCH] D75285: Mark restrict pointer or reference to const as invariant

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D75285#1903284 <https://reviews.llvm.org/D75285#1903284>, @yaxunl wrote:

> In D75285#1902835 <https://reviews.llvm.org/D75285#1902835>, @jeroen.dobbelaere wrote:
>
> > In D75285#1902788 <https://reviews.llvm.org/D75285#1902788>, @Anastasia wrote:
> >
> > > In D75285#1896610 <https://reviews.llvm.org/D75285#1896610>, @rjmccall wrote:
> > >
> > > > Are you sure `restrict` alone isn't good enough?  It doesn't directly tell you that the memory is invariant, but it's usually simple to prove that the memory isn't modified within the `restrict` scope, which might be sufficient.
> > >
> > >
> > > Do you mean to prove in analysis passes? Should we emit some sort of hints from the frontend to indicate what to look for?
> >
> >
> > Not sure what you mean with 'hints from the frontend', but D68484 <https://reviews.llvm.org/D68484> (and later) contain a significant improvement to clang's handling of restrict. That could make the restrict path feasible (if that would support the actual use case).
>
>
> I think there are cases that noalias is not sufficient to prove invariance. For example, a global variable, even if we mark it as restrict and we do not modify it in a function, the compiler is still not sure it is invariant in that function, since it may be modified by another thread. In this case, if a user knows that it is invariant in that function, he would just want to mark it as `__invariant__` or `__immutable__`.


That is not true for two reasons: first, `restrict` guarantees that the variable is not accessed through any non-derived l-value within its scope, and that would certainly include from other threads; and second, it is undefined behavior for two threads to access the same object without synchronizing anyway (unless they're both just reading from it).


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