[Builtin] Add _isValidAddress pointer-value check.

And adopt it for StringGuts, as it is more robust on 32-bit systems.

Author: milseman

stdlib/public/core/Builtin.swift

@@ -363,6 +363,14 @@ internal func _class_getInstancePositiveExtentSize(_ theClass: AnyClass) -> Int
 #endif
 }
 
+@_inlineable
+@_versioned
+internal
+func _isValidAddress(_ address: UInt) -> Bool {
+  // TODO: define (and use) ABI max valid pointer value
+  return address >= _swift_abi_LeastValidPointerValue
+}
+
 //===--- Builtin.BridgeObject ---------------------------------------------===//
 
 // TODO(<rdar://problem/34837023>): Get rid of superfluous UInt constructor

stdlib/public/core/StringGuts.swift

@@ -29,8 +29,6 @@ import SwiftShims
 // FIXME: what about ppc64 and s390x?
 //
 
-
-
 @_fixed_layout
 public // FIXME
 struct _StringGuts {
@@ -78,7 +76,7 @@ extension _StringGuts {
     } else if _object.isUnmanaged {
     } else if _object.isCocoa {
       if _object.isContiguous {
-        _sanityCheck(_otherBits != 0) // TODO: in ABI's address space
+        _sanityCheck(_isValidAddress(_otherBits))
       } else {
         _sanityCheck(_otherBits == 0)
       }
@@ -238,7 +236,7 @@ extension _StringGuts {
     @inline(__always)
     get {
       _sanityCheck(_object.isContiguousCocoa)
-      _sanityCheck(Int(bitPattern: _otherBits) >= 1) // TODO: ABI's min address
+      _sanityCheck(_isValidAddress(_otherBits))
       return UnsafeRawPointer(
         bitPattern: _otherBits
       )._unsafelyUnwrappedUnchecked