Avoid fragile type lookups in GDB pretty printer

[MoritzS]

Summary

Instead of using the type llvm::StringMapEntry<{stringified_value_type}>
use only the base class llvm::StringMapEntryBase and calculate the
offsets of the member variables manually. The approach with stringifying
the name of the value type is pretty fragile as it can easily break with
local and dependent types.

[csigg]

Yes, that does look safer.

[MoritzS]

Thanks for the review! Can you commit this for me, please?

llvm/utils/gdb-scripts/prettyprinters.py

class StringMapPrinter:
 
   def __init__(self, val):
     self.val = val
 
   def children(self):
     it = self.val['TheTable']
     end = (it + self.val['NumBuckets'])
     value_ty = self.val.type.template_argument(0)
-    entry_ty = gdb.lookup_type('llvm::StringMapEntry<{}>'.format(value_ty.name))
+    entry_base_ty = gdb.lookup_type('llvm::StringMapEntryBase')
     tombstone = gdb.parse_and_eval('llvm::StringMapImpl::TombstoneIntVal');
 
     while it != end:
       it_deref = it.dereference()
       if it_deref == 0 or it_deref == tombstone:
         it = it + 1
         continue
 
-      entry_ptr = it_deref.cast(entry_ty.pointer())
+      entry_ptr = it_deref.cast(entry_base_ty.pointer())
       entry = entry_ptr.dereference()
 
       str_len = entry['keyLength']
-      str_data = (entry_ptr + 1).cast(gdb.lookup_type('char').const().pointer())
+      value_ptr = (entry_ptr + 1).cast(value_ty.pointer())
+      str_data = (entry_ptr + 1).cast(gdb.lookup_type('uintptr_t')) + max(value_ty.sizeof, entry_base_ty.alignof)
+      str_data = str_data.cast(gdb.lookup_type('char').const().pointer())
       string_ref = gdb.Value(struct.pack('PN', int(str_data), int(str_len)), gdb.lookup_type('llvm::StringRef'))
       yield 'key', string_ref
 
-      value = entry['second']
+      value = value_ptr.dereference()
       yield 'value', value
 
       it = it + 1
 
   def to_string(self):
     return 'llvm::StringMap with %d elements' % (self.val['NumItems'])
 
   def display_hint(self):