[PATCH] D15146: [InstCombine] Look through PHIs, GEPs, IntToPtrs and PtrToInts to expose more constants when comparing GEPs

[Hal Finkel via llvm-commits]

hfinkel added inline comments.

================
Comment at: lib/Transforms/InstCombine/InstCombineCompares.cpp:905
@@ +904,3 @@
+  //   OFFSET1 cmp OFFSET2
+  Value *NewRHS = RewriteGEPAsOffset(RHS, PtrBase, DL, Nodes);
+  return new ICmpInst(ICmpInst::getSignedPredicate(Cond), Index, NewRHS);
----------------
sbaranga wrote:
> hfinkel wrote:
> > Still don't find this clear. It seems to me that:
> > 
> >   std::tie(PtrBase, Index) = GetAsConstantIndexedAddress(GEPLHS, DL);
> > 
> > returns PtrBase and Index such that GEP(PtrBase,  Index) == GEPLHS. And then:
> > 
> >   RewriteGEPAsOffset(RHS, PtrBase)
> > 
> > returns GEP(PtrBase, Something) == RHS. And then we compare:
> > 
> >   Index icmp GEP(PtrBase, Something)
> > 
> > which must be wrong. What am I missing?
> > 
> RewriteGEPAsOffset returns the offset, not the GEP.
Ah, I understand now. The GEP it builds is not returned, but is for use by other users of the original GEP being replaced.


http://reviews.llvm.org/D15146