llvm.noalias - isSafeToLoadUnconditionally can look through them

[hfinkel]

Summary

This is part of the series started by D9375, and teaches isSafeToLoadUnconditionally to look through them.

I don't have a test case for this, but as a general utility, it makes sense.

[reames]

Resigning as a reviewer to get a very stale review off my list of blocking tasks in phabricator. Please readd when desired.

[hfinkel]

This change should not be needed if the intrinsics are stripped by default.

lib/Analysis/Loads.cpp

bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
   // want to check to see if the pointer is already being loaded or stored
   // from/to.  If so, the previous load or store would have already trapped,
   // so there is no harm doing an extra load (also, CSE will later eliminate
   // the load entirely).
   BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
 
   // We can at least always strip pointer casts even though we can't use the
   // base here.
-  V = V->stripPointerCasts();
+  V = V->stripPointerCasts(/*LookThroughNoAlias*/true);
 
   while (BBI != E) {
     --BBI;
 
     // If we see a free or a call which may write to memory (i.e. which might do
     // a free) the pointer could be marked invalid.
     if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
         !isa<DbgInfoIntrinsic>(BBI))
       return false;
 
     Value *AccessedPtr;
     if (LoadInst *LI = dyn_cast<LoadInst>(BBI))
       AccessedPtr = LI->getPointerOperand();
     else if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
       AccessedPtr = SI->getPointerOperand();
     else
       continue;
 
     // Handle trivial cases.
     if (AccessedPtr == V)
       return true;
 
     auto *AccessedTy = cast<PointerType>(AccessedPtr->getType());
-    if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
+    if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(true), V) &&
         LoadSize <= DL.getTypeStoreSize(AccessedTy->getElementType()))
       return true;
   }
   return false;
 }
 
 /// \brief Scan the ScanBB block backwards to see if we have the value at the
 /// memory address *Ptr locally available within a small number of instructions.