Index: lib/IR/Value.cpp
===================================================================
--- lib/IR/Value.cpp
+++ lib/IR/Value.cpp
@@ -548,9 +548,14 @@
       if (EltTy->isSized())
         Align = DL.getABITypeAlignment(EltTy);
     }
-  } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(this))
+  } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(this)) {
     Align = AI->getAlignment();
-  else if (auto CS = ImmutableCallSite(this))
+    if (Align == 0) {
+      Type *AllocatedType = AI->getAllocatedType();
+      if (AllocatedType->isSized())
+        Align = DL.getPrefTypeAlignment(AllocatedType);
+    }
+  } else if (auto CS = ImmutableCallSite(this))
     Align = CS.getAttributes().getParamAlignment(AttributeSet::ReturnIndex);
   else if (const LoadInst *LI = dyn_cast<LoadInst>(this))
     if (MDNode *MD = LI->getMetadata(LLVMContext::MD_align)) {
Index: test/Analysis/ValueTracking/memory-dereferenceable.ll
===================================================================
--- test/Analysis/ValueTracking/memory-dereferenceable.ll
+++ test/Analysis/ValueTracking/memory-dereferenceable.ll
@@ -3,7 +3,7 @@
 ; Uses the print-deref (+ analyze to print) pass to run
 ; isDereferenceablePointer() on many load instruction operands
 
-target datalayout = "e"
+target datalayout = "e-i32:32:64"
 
 %TypeOpaque = type opaque
 
@@ -133,6 +133,12 @@
     %load26 = load i32, i32* %d4_unaligned_load, align 16
     %load27 = load i32, i32* %d4_aligned_load, align 16
 
+   ; Alloca with no explicit alignment is aligned to preferred alignment of
+   ; the type (specified by datalayout string).
+; CHECK: %alloca.noalign{{.*}}(aligned)
+    %alloca.noalign = alloca i32
+    %load28 = load i32, i32* %alloca.noalign, align 8
+
     ret void
 }