Add a feature for Builtin.assumeAlignment

Fixes rdar://86785846

Author: DougGregor

include/swift/Basic/Features.def

@@ -58,5 +58,6 @@ LANGUAGE_FEATURE(BuiltinMove, 0, "Builtin.move()", true)
 LANGUAGE_FEATURE(BuiltinCopy, 0, "Builtin.copy()", true)
 LANGUAGE_FEATURE(BuiltinStackAlloc, 0, "Builtin.stackAlloc", true)
 LANGUAGE_FEATURE(SpecializeAttributeWithAvailability, 0, "@_specialize attribute with availability", true)
+LANGUAGE_FEATURE(BuiltinAssumeAlignment, 0, "Builtin.assumeAlignment", true)
 
 #undef LANGUAGE_FEATURE

lib/AST/ASTPrinter.cpp

@@ -2891,6 +2891,10 @@ static bool usesFeatureBuiltinStackAlloc(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureBuiltinAssumeAlignment(Decl *decl) {
+  return false;
+}
+
 /// Determine the set of "new" features used on a given declaration.
 ///
 /// Note: right now, all features we check for are "new". At some point, we'll

stdlib/public/core/UnsafeRawPointer.swift

@@ -357,11 +357,15 @@ public struct UnsafeRawPointer: _Pointer {
 
     let rawPointer = (self + offset)._rawValue
 
+#if compiler(>=5.5) && $BuiltinAssumeAlignment
     // TODO: to support misaligned raw loads, simply remove this assumption.
     let alignedPointer =
       Builtin.assumeAlignment(rawPointer,
                               MemoryLayout<T>.alignment._builtinWordValue)
     return Builtin.loadRaw(alignedPointer)
+#else
+  return Builtin.loadRaw(rawPointer)
+#endif
   }
 
 }
@@ -909,11 +913,15 @@ public struct UnsafeMutableRawPointer: _Pointer {
 
     let rawPointer = (self + offset)._rawValue
 
+#if compiler(>=5.5) && $BuiltinAssumeAlignment
     // TODO: to support misaligned raw loads, simply remove this assumption.
     let alignedPointer =
       Builtin.assumeAlignment(rawPointer,
                               MemoryLayout<T>.alignment._builtinWordValue)
     return Builtin.loadRaw(alignedPointer)
+#else
+    return Builtin.loadRaw(rawPointer)
+#endif
   }
 
   /// Stores the given value's bytes into raw memory at the specified offset.