[PATCH] D20665: Claim NoAlias if two GEPs index different fields of the same struct

[Taewook Oh via llvm-commits]

twoh added inline comments.

================
Comment at: llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp:842
@@ +841,3 @@
+  // of the same struct, they do not alias.
+  if (GEP1->isInBounds() && GEP2->isInBounds()) {
+    auto Opi1 = GEP1->op_begin() + 1;
----------------
eli.friedman wrote:
> You aren't allowed to make aliasing assumptions based on the type of a GEP; it's just pointer math.  "inbounds" just means that the result has to be in bounds relative to the allocation as a whole.  See http://llvm.org/docs/LangRef.html#getelementptr-instruction .
> 
> Even if a frontend generates "nice" code, LLVM itself performs transformations which will break any assumptions based on the type of a GEP (for example, LLVM will transform a bitcast into a GEP).
@eli.friedman Thanks for the comment. I'm afraid I didn't understand the point of your comments. This patch is for the case where two GEPs have the same pointer operand and access through the same indices until a certain point. For such case, not only types, but also indexed values should be identical at that point. If so, when the next index values are different, it means that two GEPs will point different fields of the same struct. 

It would be great if you could suggest an example that this patch generates wrong result. 


Repository:
  rL LLVM

http://reviews.llvm.org/D20665