[gdb] Add pretty printer for Expected

[dblaikie]

Thanks for adding to/improving the GDB pretty printer support! Just a few questions/thoughts to try to understand how this works similarly/differently from other related printers!

utils/gdb-scripts/prettyprinters.py

134

dereference() is done here, but not in the Optional pretty printer below - any idea if there's a reason for that inconsistency? (maybe they're implemented differently in a way that I'm missing/not understanding right now)

137

llvm::Optional pretty printer doesn't include the template argument (& neither does the built-in std::vector printer, I think?). Probably, for consistency & to avoid being overly verbose, do similarly here & skip the template argument?

[MaskRay]

Make OptionalPrinter inherit from Iterator to remove boilerplate

[MaskRay]

.

[dblaikie]

Still not quite sure why the change in implementation of Optional - what the difference is between how it was done & how it's done now/with this patch, etc. But I'm not too fussed - if it works for you :)

[MaskRay]

less verbose

[MaskRay]

In D48512#1142960, @dblaikie wrote:

Still not quite sure why the change in implementation of Optional - what the difference is between how it was done & how it's done now/with this patch, etc. But I'm not too fussed - if it works for you :)

I find the same trick is used in /usr/share/gcc-8/python/libstdcxx/v6/printers.py. The base class defines some boilerplate (__iter__, Python 2 compatible def next) so that the nested _iterator class can be flattened into the outer class.

utils/gdb-scripts/prettyprinters.py

 import gdb.printing
+
+class Iterator:
+  def __iter__(self):
+    return self
+
+  # Python 2 compatibility
+  def next(self):
+    return self.__next__()
+
+  def children(self):
+    return self
+
+
 class SmallStringPrinter:
   """Print an llvm::SmallString object."""
 
   def __init__(self, val):
     self.val = val
 
   def to_string(self):
     begin = self.val['BeginX']
def children(self):
     return self._iterator(data, data + self.val['Length'])
 
   def to_string(self):
     return 'llvm::ArrayRef of length %d' % (self.val['Length'])
 
   def display_hint (self):
     return 'array'
 
-class OptionalPrinter:
-  """Print an llvm::Optional object."""
+class ExpectedPrinter(Iterator):
+  """Print an llvm::Expected object."""
 
-  def __init__(self, value):
-    self.value = value
+  def __init__(self, val):
+    self.val = val
 
-  class _iterator:
-    def __init__(self, member, empty):
-      self.member = member
-      self.done = empty
+  def __next__(self):
+    val = self.val
+    if val is None:
+      raise StopIteration
+    self.val = None
+    if val['HasError']:
+      return ('error', val['ErrorStorage'].address.cast(
+          gdb.lookup_type('llvm::ErrorInfoBase').pointer()).dereference())
+    return ('value', val['TStorage'].address.cast(
+        val.type.template_argument(0).pointer()).dereference())
 
-    def __iter__(self):
-      return self
+  def to_string(self):
+    return 'llvm::Expected is {}'.format('error' if self.val['HasError'] else 'expected')
 
-    def next(self):
-      if self.done:
-        raise StopIteration
-      self.done = True
-      return ('value', self.member.dereference())
+class OptionalPrinter(Iterator):
+  """Print an llvm::Optional object."""
 
-  def children(self):
-    if not self.value['Storage']['hasVal']:
-      return self._iterator('', True)
-    return self._iterator(self.value['Storage']['storage']['buffer'].address.cast(
-        self.value.type.template_argument(0).pointer()), False)
+  def __init__(self, val):
+    self.val = val
+
+  def __next__(self):
+    val = self.val
+    if val is None:
+      raise StopIteration
+    self.val = None
+    if not val['Storage']['hasVal']:
+      raise StopIteration
+    return ('value', val['Storage']['storage']['buffer'].address.cast(
+        val.type.template_argument(0).pointer()).dereference())
 
   def to_string(self):
-    return 'llvm::Optional is %sinitialized' % ('' if self.value['Storage']['hasVal'] else 'not ')
+    return 'llvm::Optional is %sinitialized' % ('' if self.val['Storage']['hasVal'] else 'not ')
 
 class DenseMapPrinter:
   "Print a DenseMap"
 
   class _iterator:
     def __init__(self, key_info_t, begin, end):
       self.key_info_t = key_info_t
       self.cur = begin
class TwinePrinter:
   def to_string(self):
     return self.string_from_twine_object(self._val)
 
 pp = gdb.printing.RegexpCollectionPrettyPrinter("LLVMSupport")
 pp.add_printer('llvm::SmallString', '^llvm::SmallString<.*>$', SmallStringPrinter)
 pp.add_printer('llvm::StringRef', '^llvm::StringRef$', StringRefPrinter)
 pp.add_printer('llvm::SmallVectorImpl', '^llvm::SmallVector(Impl)?<.*>$', SmallVectorPrinter)
 pp.add_printer('llvm::ArrayRef', '^llvm::(Const)?ArrayRef<.*>$', ArrayRefPrinter)
+pp.add_printer('llvm::Expected', '^llvm::Expected<.*>$', ExpectedPrinter)
 pp.add_printer('llvm::Optional', '^llvm::Optional<.*>$', OptionalPrinter)
 pp.add_printer('llvm::DenseMap', '^llvm::DenseMap<.*>$', DenseMapPrinter)
 pp.add_printer('llvm::Twine', '^llvm::Twine$', TwinePrinter)
 gdb.printing.register_pretty_printer(gdb.current_objfile(), pp)