[PATCH] D22793: IR: Introduce inbounds attribute on getelementptr indices.

[Eli Friedman via llvm-commits]

eli.friedman added inline comments.

================
Comment at: docs/LangRef.rst:7547
@@ +7546,3 @@
+element selected by the index marked as ``inrange``. The result of a pointer
+comparison is undefined if either operand was derived from an ``inrange``
+GEP, with the exception of comparisons where both operands were derived from
----------------
I think this is supposed to be "if both operands were derived from an inrange GEP"?

================
Comment at: docs/LangRef.rst:7548
@@ +7547,3 @@
+comparison is undefined if either operand was derived from an ``inrange``
+GEP, with the exception of comparisons where both operands were derived from
+a GEP which was evaluated with identical operands up to and including the
----------------
hfinkel wrote:
> eli.friedman wrote:
> > Hmm... this sort of restricts the possible uses of the marking.  For example, given:
> > 
> >     struct X {
> >         int x, y[2];
> >     };
> >     int f(struct X *x, int i) { return x->y[i]; }
> >     int *f2(struct X *x) { return x->y; }
> > 
> > We can mark the GEP in `x->y[i]` inrange, but we can't mark the GEP in `x->y` inrange because it could be used in a pointer comparison.  I guess that's okay (the stated form is convenient for performing SROA).  That said, inevitably someone is going to push for a version which doesn't make comparisons undefined so it can be used more aggressively in C code.
> > 
> > You should probably state explicitly that the result of converting an inrange pointer to an integer is undef; you can't actually do anything useful with the result anyway.
> What's the motivation for the current language? I don't think we can have anything that strict.
The motivation here is that we want to be able to say "if every use of a given global is an inrange GEP, SROA is legal (i.e. we can split the global)".  That requires some sort of restriction on comparisons because we could change the result of a comparison in an unpredictable way.


https://reviews.llvm.org/D22793