[PATCH] D15825: [MDA] Don't be quite as conservative for noalias functions

[Philip Reames via llvm-commits]

reames created this revision.
reames added reviewers: dberlin, hfinkel, sanjoy.
reames added a subscriber: llvm-commits.

(I just want a second look at this patch for sanity sake.)

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.

http://reviews.llvm.org/D15825

Files:
  lib/Analysis/MemoryDependenceAnalysis.cpp

Index: lib/Analysis/MemoryDependenceAnalysis.cpp
===================================================================
--- lib/Analysis/MemoryDependenceAnalysis.cpp
+++ lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -685,13 +685,13 @@
         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;
     }
 


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