Fix memdep's code for reasoning about dependences between two calls. A Ref

response from getModRefInfo is not useful here. Instead, check for identical
calls only in the NoModRef case.

Reapply r110270, and strengthen it to compensate for the memdep changes.
When both calls are readonly, there is no dependence between them.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110382 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 6bb84d4e45..11cba991a9 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -425,8 +425,13 @@ BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
   ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
   if (CS2B == DoesNotAccessMemory) return NoModRef;
   
-  // If they both only read from memory, just return ref.
+  // If they both only read from memory, there is no dependence.
   if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
+    return NoModRef;
+
+  // If CS1 only reads memory, the only dependence on CS2 can be
+  // from CS1 reading memory written by CS2.
+  if (CS1B == OnlyReadsMemory)
     return Ref;
   
   // Otherwise, fall back to NoAA (mod+ref).