diff --git a/llvm/include/llvm/Analysis/MemoryBuiltins.h b/llvm/include/llvm/Analysis/MemoryBuiltins.h
--- a/llvm/include/llvm/Analysis/MemoryBuiltins.h
+++ b/llvm/include/llvm/Analysis/MemoryBuiltins.h
@@ -100,7 +100,10 @@
 /// insertion or speculative execution of allocation routines.
 bool isAllocRemovable(const CallBase *V, const TargetLibraryInfo *TLI);
 
-/// Gets the alignment argument for an aligned_alloc-like function
+/// Gets the alignment argument for an aligned_alloc-like function, using either
+/// built-in knowledge based on fuction names/signatures or allocalign
+/// attributes. Note: the Value returned may not indicate a valid alignment, per
+/// the definition of the allocalign attribute.
 Value *getAllocAlignment(const CallBase *V, const TargetLibraryInfo *TLI);
 
 /// Return the size of the requested allocation.  With a trivial mapper, this is
diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -334,10 +334,15 @@
   assert(isAllocationFn(V, TLI));
 
   const Optional<AllocFnsTy> FnData = getAllocationData(V, AnyAlloc, TLI);
-  if (!FnData.hasValue() || FnData->AlignParam < 0) {
-    return nullptr;
+  if (FnData.hasValue() && FnData->AlignParam >= 0) {
+    return V->getOperand(FnData->AlignParam);
+  }
+  unsigned AllocAlignParam;
+  if (V->getAttributes().hasAttrSomewhere(Attribute::AllocAlign,
+                                          &AllocAlignParam)) {
+    return V->getOperand(AllocAlignParam-1);
   }
-  return V->getOperand(FnData->AlignParam);
+  return nullptr;
 }
 
 /// When we're compiling N-bit code, and the user uses parameters that are