Introduce obstack_new, poison other "typed" obstack functions

Since we use obstacks with objects that are not default constructible,
we sometimes need to manually call the constructor by hand using
placement new:

  foo *f = obstack_alloc (obstack, sizeof (foo));
  f = new (f) foo;

It's possible to use allocate_on_obstack instead, but there are types
that we sometimes want to allocate on an obstack, and sometimes on the
regular heap.  This patch introduces a utility to make this pattern
simpler if allocate_on_obstack is not an option:

  foo *f = obstack_new<foo> (obstack);

Right now there's only one usage (in tdesc_data_init).

To help catch places where we would forget to call new when allocating
such an object on an obstack, this patch also poisons some other methods
of allocating an instance of a type on an obstack:

  - OBSTACK_ZALLOC/OBSTACK_CALLOC
  - XOBNEW/XOBNEW
  - GDBARCH_OBSTACK_ZALLOC/GDBARCH_OBSTACK_CALLOC

Unfortunately, there's no way to catch wrong usages of obstack_alloc.

By pulling on that string though, it tripped on allocating struct
template_symbol using OBSTACK_ZALLOC.  The criterion currently used to
know whether it's safe to "malloc" an instance of a struct is whether it
is a POD.  Because it inherits from struct symbol, template_symbol is
not a POD.  This criterion is a bit too strict however, it should still
safe to allocate memory for a template_symbol and memset it to 0.  We
didn't use is_trivially_constructible as the criterion in the first
place only because it is not available in gcc < 5.  So here I considered
two alternatives:

1. Relax that criterion to use std::is_trivially_constructible and add a
   bit more glue code to make it work with gcc < 5
2. Continue pulling on the string and change how the symbol structures
   are allocated and initialized

I managed to do both, but I decided to go with #1 to keep this patch
simpler and more focused.  When building with a compiler that does not
have is_trivially_constructible, the check will just not be enforced.

gdb/ChangeLog:

	* common/traits.h (HAVE_IS_TRIVIALLY_COPYABLE): Define if
	compiler supports std::is_trivially_constructible.
	* common/poison.h: Include obstack.h.
	(IsMallocable): Define to is_trivially_constructible if the
	compiler supports it, define to true_type otherwise.
	(xobnew): New.
	(XOBNEW): Redefine.
	(xobnewvec): New.
	(XOBNEWVEC): Redefine.
	* gdb_obstack.h (obstack_zalloc): New.
	(OBSTACK_ZALLOC): Redefine.
	(obstack_calloc): New.
	(OBSTACK_CALLOC): Redefine.
	(obstack_new): New.
	* gdbarch.sh: Include gdb_obstack in gdbarch.h.
	(gdbarch_obstack): New declaration in gdbarch.h, definition in
	gdbarch.c.
	(GDBARCH_OBSTACK_CALLOC, GDBARCH_OBSTACK_ZALLOC): Use
	obstack_calloc/obstack_zalloc.
	(gdbarch_obstack_zalloc): Remove.
	* target-descriptions.c (tdesc_data_init): Use obstack_new.

1 files changed, 32 insertions, 4 deletions

diff --git a/gdb/gdb_obstack.h b/gdb/gdb_obstack.h
index 1011008..29cad93 100644
--- a/gdb/gdb_obstack.h
+++ b/gdb/gdb_obstack.h
@@ -24,12 +24,40 @@
 
 /* Utility macros - wrap obstack alloc into something more robust.  */
 
-#define OBSTACK_ZALLOC(OBSTACK,TYPE) \
-  ((TYPE *) memset (obstack_alloc ((OBSTACK), sizeof (TYPE)), 0, sizeof (TYPE)))
+template <typename T>
+static inline T*
+obstack_zalloc (struct obstack *ob)
+{
+  static_assert (IsMallocable<T>::value, "Trying to use OBSTACK_ZALLOC with a \
+non-POD data type.  Use obstack_new instead.");
+  return ((T *) memset (obstack_alloc (ob, sizeof (T)), 0, sizeof (T)));
+}
+
+#define OBSTACK_ZALLOC(OBSTACK,TYPE) obstack_zalloc<TYPE> ((OBSTACK))
+
+template <typename T>
+static inline T *
+obstack_calloc (struct obstack *ob, size_t number)
+{
+  static_assert (IsMallocable<T>::value, "Trying to use OBSTACK_CALLOC with a \
+non-POD data type.  Use obstack_new instead.");
+  return ((T *) memset (obstack_alloc (ob, number * sizeof (T)), 0,
+			number * sizeof (T)));
+}
 
 #define OBSTACK_CALLOC(OBSTACK,NUMBER,TYPE) \
-  ((TYPE *) memset (obstack_alloc ((OBSTACK), (NUMBER) * sizeof (TYPE)), \
-		    0, (NUMBER) * sizeof (TYPE)))
+  obstack_calloc<TYPE> ((OBSTACK), (NUMBER))
+
+/* Allocate an object on OB and call its constructor.  */
+
+template <typename T, typename... Args>
+static inline T*
+obstack_new (struct obstack *ob, Args&&... args)
+{
+  T* object = (T *) obstack_alloc (ob, sizeof (T));
+  object = new (object) T (std::forward<Args> (args)...);
+  return object;
+}
 
 /* Unless explicitly specified, GDB obstacks always use xmalloc() and
    xfree().  */