Index: include/llvm/Analysis/AliasSetTracker.h
===================================================================
--- include/llvm/Analysis/AliasSetTracker.h
+++ include/llvm/Analysis/AliasSetTracker.h
@@ -468,6 +468,7 @@
   /// pointer.
   AliasSet &mergeAllAliasSets();
 
+public: // HACK: fixme
   AliasSet *findAliasSetForUnknownInst(Instruction *Inst);
 };
 
Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -658,6 +658,7 @@
     if (CI->mayThrow())
       return false;
 
+#if 0
     if (Function *F = CI->getCalledFunction())
         switch (F->getIntrinsicID()) {
         default: break;
@@ -666,7 +667,7 @@
           // Assumes don't actually alias anything or throw
           return true;
         };
-    
+
     // Handle simple cases by querying alias analysis.
     FunctionModRefBehavior Behavior = AA->getModRefBehavior(CI);
     if (Behavior == FMRB_DoesNotAccessMemory)
@@ -683,17 +684,38 @@
             return false;
         return true;
       }
+      
 
       // If this call only reads from memory and there are no writes to memory
       // in the loop, we can hoist or sink the call as appropriate.
       if (isReadOnly(CurAST))
         return true;
     }
+#endif
 
-    // FIXME: This should use mod/ref information to see if we can hoist or
-    // sink the call.
+    AliasSet *AS = CurAST->findAliasSetForUnknownInst(CI);
+    if (!AS)
+      // Was considered by AST not to alias any possible memory location
+      return true;
+    if (!AS->isMod())
+      // The call (which must be readonly per AST) does read memory clobbered
+      // in the loop.
+      return true;
 
+#if 1
+    // May contain a clobber, give up!
     return false;
+#else
+    // Now check for a uniquely writing instruction within the loop.  In this
+    // case, the value written can't be overwritten or read within the loop.
+    // NOTE: Drop in first patch.
+    auto *UniqueI = AS->getUniqueInstruction();
+    if (!UniqueI)
+      // other memory op, give up
+      return false;
+    assert(UniqueI == CI && "AS must contain FI");
+    return true;
+#endif
   } else if (auto *FI = dyn_cast<FenceInst>(&I)) {
     // Fences alias (most) everything to provide ordering.  For the moment,
     // just give up if there are any other memory operations in the loop.