diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -372,6 +372,15 @@
                                       QualType ResultTy,
                                       ArrayRef<QualType> Args);
 
+  /// The maximum alignment, same as in llvm::Value. We duplicate them here
+  /// because that allows us not to duplicate the constants in clang code,
+  /// which we must to since we can't directly use the llvm constants.
+  ///
+  /// This is the greatest alignment value supported by load, store, and alloca
+  /// instructions, and global values.
+  static const unsigned MaxAlignmentExponent = 29;
+  static const unsigned MaximumAlignment = 1u << MaxAlignmentExponent;
+
 public:
   typedef OpaquePtr<DeclGroupRef> DeclGroupPtrTy;
   typedef OpaquePtr<TemplateName> TemplateTy;
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3665,11 +3665,9 @@
           return;
         }
 
-        // Alignment calculations can wrap around if it's greater than 2**29.
-        unsigned MaximumAlignment = 536870912;
-        if (I > MaximumAlignment)
+        if (I > Sema::MaximumAlignment)
           Diag(Arg->getExprLoc(), diag::warn_assume_aligned_too_great)
-              << Arg->getSourceRange() << MaximumAlignment;
+              << Arg->getSourceRange() << Sema::MaximumAlignment;
       }
     }
   }
@@ -5394,11 +5392,9 @@
       return Diag(TheCall->getBeginLoc(), diag::err_alignment_not_power_of_two)
              << Arg->getSourceRange();
 
-    // Alignment calculations can wrap around if it's greater than 2**29.
-    unsigned MaximumAlignment = 536870912;
-    if (Result > MaximumAlignment)
+    if (Result > Sema::MaximumAlignment)
       Diag(TheCall->getBeginLoc(), diag::warn_assume_aligned_too_great)
-          << Arg->getSourceRange() << MaximumAlignment;
+          << Arg->getSourceRange() << Sema::MaximumAlignment;
   }
 
   if (NumArgs > 2) {
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1626,11 +1626,9 @@
       return;
     }
 
-    // Alignment calculations can wrap around if it's greater than 2**29.
-    unsigned MaximumAlignment = 536870912;
-    if (I > MaximumAlignment)
+    if (I > Sema::MaximumAlignment)
       Diag(CI.getLoc(), diag::warn_assume_aligned_too_great)
-          << CI.getRange() << MaximumAlignment;
+          << CI.getRange() << Sema::MaximumAlignment;
   }
 
   if (OE) {
@@ -3815,13 +3813,9 @@
     }
   }
 
-  // Alignment calculations can wrap around if it's greater than 2**28.
-  unsigned MaxValidAlignment =
-      Context.getTargetInfo().getTriple().isOSBinFormatCOFF() ? 8192
-                                                              : 268435456;
-  if (AlignVal > MaxValidAlignment) {
-    Diag(AttrLoc, diag::err_attribute_aligned_too_great) << MaxValidAlignment
-                                                         << E->getSourceRange();
+  if (AlignVal > Sema::MaximumAlignment) {
+    Diag(AttrLoc, diag::err_attribute_aligned_too_great)
+        << Sema::MaximumAlignment << E->getSourceRange();
     return;
   }
 
diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
--- a/llvm/lib/IR/Attributes.cpp
+++ b/llvm/lib/IR/Attributes.cpp
@@ -143,7 +143,7 @@
 }
 
 Attribute Attribute::getWithAlignment(LLVMContext &Context, Align A) {
-  assert(A <= 0x40000000 && "Alignment too large.");
+  assert(A <= llvm::Value::MaximumAlignment && "Alignment too large.");
   return get(Context, Alignment, A.value());
 }
 
@@ -1525,7 +1525,7 @@
   if (!Align)
     return *this;
 
-  assert(*Align <= 0x40000000 && "Alignment too large.");
+  assert(*Align <= llvm::Value::MaximumAlignment && "Alignment too large.");
 
   Attrs[Attribute::Alignment] = true;
   Alignment = Align;