[LAA] LLE 2/6: Fix a NoDep case that should be a Forward dependence

Summary:
When the dependence distance in zero then we have a loop-independent
dependence from the earlier to the later access.

No current client of LAA uses forward dependences so other than
potentially hitting the MaxDependences threshold earlier, this change
shouldn't affect anything right now.

This and the previous patch were tested together for compile-time
regression.  None found in LNT/SPEC.

Reviewers: hfinkel

Subscribers: rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D13255

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251973 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 0 deletions

diff --git a/include/llvm/Analysis/LoopAccessAnalysis.h b/include/llvm/Analysis/LoopAccessAnalysis.h
index 97a72b3d1cd..ea080bc7f87 100644
--- a/include/llvm/Analysis/LoopAccessAnalysis.h
+++ b/include/llvm/Analysis/LoopAccessAnalysis.h
@@ -136,6 +136,14 @@ public:
       // We couldn't determine the direction or the distance.
       Unknown,
       // Lexically forward.
+      //
+      // FIXME: If we only have loop-independent forward dependences (e.g. a
+      // read and write of A[i]), LAA will locally deem the dependence "safe"
+      // without querying the MemoryDepChecker.  Therefore we can miss
+      // enumerating loop-independent forward dependences in
+      // getInterestingDependences.  Note that as soon as there are different
+      // indices used to access the same array, the MemoryDepChecker *is*
+      // queried and the dependence list is complete.
       Forward,
       // Forward, but if vectorized, is likely to prevent store-to-load
       // forwarding.