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, 7 insertions, 3 deletions

diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index b3a15c9..09edcd5 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -38,6 +38,7 @@
 #include <vector>
 #include "frame.h"
 #include "dis-asm.h"
+#include "gdb_obstack.h"
 
 struct floatformat;
 struct ui_file;
@@ -1705,14 +1706,17 @@ extern struct gdbarch *gdbarch_alloc (const struct gdbarch_info *info, struct gd
 
 extern void gdbarch_free (struct gdbarch *);
 
+/* Get the obstack owned by ARCH.  */
+
+extern obstack *gdbarch_obstack (gdbarch *arch);
 
 /* Helper function.  Allocate memory from the ``struct gdbarch''
    obstack.  The memory is freed when the corresponding architecture
    is also freed.  */
 
-extern void *gdbarch_obstack_zalloc (struct gdbarch *gdbarch, long size);
-#define GDBARCH_OBSTACK_CALLOC(GDBARCH, NR, TYPE) ((TYPE *) gdbarch_obstack_zalloc ((GDBARCH), (NR) * sizeof (TYPE)))
-#define GDBARCH_OBSTACK_ZALLOC(GDBARCH, TYPE) ((TYPE *) gdbarch_obstack_zalloc ((GDBARCH), sizeof (TYPE)))
+#define GDBARCH_OBSTACK_CALLOC(GDBARCH, NR, TYPE)   obstack_calloc<TYPE> (gdbarch_obstack ((GDBARCH)), (NR))
+
+#define GDBARCH_OBSTACK_ZALLOC(GDBARCH, TYPE)   obstack_zalloc<TYPE> (gdbarch_obstack ((GDBARCH)))
 
 /* Duplicate STRING, returning an equivalent string that's allocated on the
    obstack associated with GDBARCH.  The string is freed when the corresponding