gdb: add HtabPrinter to gdb-gdb.py.in

When debugging GDB, I find it a bit tedious to inspect htab_t objects.
It is possible to find the entries by poking at the fields, but it's
annoying to do each time.  I think a pretty printer would help.  Add a
basic one to gdb-gdb.py.

The pretty printer advertises itself as "array-like", and the result
looks like:

    (top-gdb) p bfcache
    $3 = htab_t with 3 elements = {0x6210003252a0, 0x62100032caa0, 0x62100033baa0}

The htab_t itself doesn't know about the type of pointed objects.  But
it's easy enough to cast the addresses to the right type to use them:

    (top-gdb) print *((btrace_frame_cache *) 0x6210003252a0)
    $6 = {tp = 0x61700002ed80, frame = 0x6210003251e0, bfun = 0x62000000b390}

Change-Id: Ia692e3555fe7a117b7ec087840246b1260a704c6
Reviewed-By: Tom Tromey <tom@tromey.com>

1 files changed, 31 insertions, 0 deletions

diff --git a/gdb/gdb-gdb.py.in b/gdb/gdb-gdb.py.in
index 95b7d84..56d063c 100644
--- a/gdb/gdb-gdb.py.in
+++ b/gdb/gdb-gdb.py.in
@@ -369,6 +369,35 @@ class IntrusiveListPrinter:
         return self._children_generator()
 
 
+class HtabPrinter:
+    """Pretty-printer for htab_t hash tables."""
+
+    def __init__(self, val):
+        self._val = val
+
+    def display_hint(self):
+        return "array"
+
+    def to_string(self):
+        n = int(self._val["n_elements"]) - int(self._val["n_deleted"])
+        return "htab_t with {} elements".format(n)
+
+    def children(self):
+        size = int(self._val["size"])
+        entries = self._val["entries"]
+
+        child_i = 0
+        for entries_i in range(size):
+            entry = entries[entries_i]
+            # 0 (NULL pointer) means there's nothing, 1 (HTAB_DELETED_ENTRY)
+            # means there was something, but is now deleted.
+            if int(entry) in (0, 1):
+                continue
+
+            yield (str(child_i), entry)
+            child_i += 1
+
+
 def type_lookup_function(val):
     """A routine that returns the correct pretty printer for VAL
     if appropriate.  Returns None otherwise.
@@ -383,6 +412,8 @@ def type_lookup_function(val):
         return CoreAddrPrettyPrinter(val)
     elif tag is not None and tag.startswith("intrusive_list<"):
         return IntrusiveListPrinter(val)
+    elif name == "htab_t":
+        return HtabPrinter(val)
     return None