[MDA] Don't be quite as conservative for noalias functions

If we encounter a noalias call that alias analysis can't analyse, we can fall down into the generic call handling rather than giving up entirely. I noticed this while reading through the code for another purpose.

I can't seem to write a test case which changes; that sorta makes sense given any test case would have to be an inconsistency in AA. Suggestions welcome.

Differential Revision: http://reviews.llvm.org/D15825

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256802 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index 06dcf94676c34b04afb58416454fb2a86e07d9d1..6918360536a300b58ccdb8dca5e5dc6676092348 100644 (file)
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -685,13 +685,13 @@ MemDepResult MemoryDependenceAnalysis::getSimplePointerDependencyFrom(
         return MemDepResult::getDef(Inst);
       if (isInvariantLoad)
         continue;
-      // Be conservative if the accessed pointer may alias the allocation.
-      if (AA->alias(Inst, AccessPtr) != NoAlias)
-        return MemDepResult::getClobber(Inst);
-      // If the allocation is not aliased and does not read memory (like
-      // strdup), it is safe to ignore.
-      if (isa<AllocaInst>(Inst) ||
-          isMallocLikeFn(Inst, TLI) || isCallocLikeFn(Inst, TLI))
+      // Be conservative if the accessed pointer may alias the allocation -
+      // fallback to the generic handling below.
+      if ((AA->alias(Inst, AccessPtr) == NoAlias) &&
+          // If the allocation is not aliased and does not read memory (like
+          // strdup), it is safe to ignore.
+          (isa<AllocaInst>(Inst) || isMallocLikeFn(Inst, TLI) ||
+           isCallocLikeFn(Inst, TLI)))
         continue;
     }