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

[reames]

Summary

(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.

[reames]

post holiday ping!

[• dberlin]

Either this is correct, or we have bugs elsewhere, so LGTM.

lib/Analysis/MemoryDependenceAnalysis.cpp

#endif
     // need to continue scanning until the malloc call.
     if (isa<AllocaInst>(Inst) || isNoAliasFn(Inst, TLI)) {
       const Value *AccessPtr = GetUnderlyingObject(MemLoc.Ptr, DL);
 
       if (AccessPtr == Inst || AA->isMustAlias(Inst, AccessPtr))
         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);
+      // 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.
-      if (isa<AllocaInst>(Inst) ||
-          isMallocLikeFn(Inst, TLI) || isCallocLikeFn(Inst, TLI))
+          (isa<AllocaInst>(Inst) || isMallocLikeFn(Inst, TLI) ||
+           isCallocLikeFn(Inst, TLI)))
         continue;
     }
 
     if (isInvariantLoad)
        continue;
 
     // See if this instruction (e.g. a call or vaarg) mod/ref's the pointer.
     ModRefInfo MR = AA->getModRefInfo(Inst, MemLoc);