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.
This patch (finally!) makes it so that trying to use XNEW with a type
that requires "new" will cause a compilation error. The criterion I
initially used to allow a type to use XNEW (which calls malloc in the
end) was std::is_trivially_constructible, but then realized that gcc 4.8
did not have it. Instead, I went with:
using IsMallocatable = std::is_pod<T>;
which is just a bit more strict, which doesn't hurt. A similar thing is
done for macros that free instead of allocated, the criterion is:
using IsFreeable = gdb::Or<std::is_trivially_destructible<T>, std::is_void<T>>;
Trying to use XNEW on a type that requires new will result in an error
like this:
In file included from /home/simark/src/binutils-gdb/gdb/common/common-utils.h:26:0,
from /home/simark/src/binutils-gdb/gdb/common/common-defs.h:78,
from /home/simark/src/binutils-gdb/gdb/defs.h:28,
from /home/simark/src/binutils-gdb/gdb/lala.c:1:
/home/simark/src/binutils-gdb/gdb/common/poison.h: In instantiation of ‘T* xnew() [with T = bar]’:
/home/simark/src/binutils-gdb/gdb/lala.c:13:3: required from here
/home/simark/src/binutils-gdb/gdb/common/poison.h:103:3: error: static assertion failed: Trying to use XNEW with a non-POD data type. Use operator new instead.
static_assert (IsMallocatable<T>::value, "Trying to use XNEW with a non-POD\
^~~~~~~~~~~~~
Generated-code-wise, it adds one more function call (xnew<T>) when using
XNEW and building with -O0, but it all goes away with optimizations
enabled.
gdb/ChangeLog:
* common/common-utils.h: Include poison.h.
(xfree): Remove declaration, add definition with static_assert.
* common/common-utils.c (xfree): Remove.
* common/poison.h (IsMallocatable): Define.
(IsFreeable): Define.
(free): Delete for non-freeable types.
(xnew): New.
(XNEW): Undef and redefine.
(xcnew): New.
(XCNEW): Undef and redefine.
(xdelete): New.
(XDELETE): Undef and redefine.
(xnewvec): New.
(XNEWVEC): Undef and redefine.
(xcnewvec): New.
(XCNEWVEC): Undef and redefine.
(xresizevec): New.
(XRESIZEVEC): Undef and redefine.
(xdeletevec): New.
(XDELETEVEC): Undef and redefine.
(xnewvar): New.
(XNEWVAR): Undef and redefine.
(xcnewvar): New.
(XCNEWVAR): Undef and redefine.
(xresizevar): New.
(XRESIZEVAR): Undef and redefine.
This patch catches invalid initialization of non-POD types with
memset, at compile time.
This is what I used to catch the problems fixed by the previous
patches in the series:
$ make -k 2>&1 | grep "deleted function"
src/gdb/breakpoint.c:951:53: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = bp_location; <template-parameter-1-2> = void; size_t = long unsigned int]’
src/gdb/breakpoint.c:7325:32: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = bp_location; <template-parameter-1-2> = void; size_t = long unsigned int]’
src/gdb/btrace.c:1153:42: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = btrace_insn; <template-parameter-1-2> = void; size_t = long unsigned int]’
...
gdb/ChangeLog:
2017-04-25 Pedro Alves <palves@redhat.com>
* common/common-defs.h: Include "common/poison.h".
* common/function-view.h: (Not, Or, Requires): Move to traits.h
and adjust.
* common/poison.h: New file.
* common/traits.h: Include <type_traits>.
(Not, Or, Requires): New, moved from common/function-view.h.
