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Re: Towards GNU11


On 10/07/14 15:07, Marek Polacek wrote:
Hi!

I'd like to kick off a discussion about moving the default standard
for C from gnu89 to gnu11.

This really shouldn't be much of a surprise: the docs mention that
gnu11 is intended future default for a year now.  I would presume now
is a good time to make this move: together with the new naming scheme
this should make GCC more modern (C89 really is as old as the hills).
And we're still in stage1.

Prerequisites should be largely complete at this point:
- we have -Wc90-c99-compat option that warns about features not present
   in ISO C90, but present in ISO C99,
- we have -Wc99-c11-compat option that warns about features not present
   in ISO C99, but present in ISO C11,
- the testsuite has been adjusted so all the test that pass with gnu89
   default should pass with gnu11 default as well (see my recent batch
   of cleanup patches).  This unfortunately isn't correct for all archs,
   I just don't have enough resources to test everything.  But generally
   the fallout from moving to gnu11 is easy to fix: just add proper decls
   and return types (to fix defaulting to int), or for inline stuff use
   -fgnu89-inline/gnu_inline attribute.  I'd appreciate testing on other
   architectures than x86_64/ppc64.

The things I had to fix in the testsuite nicely reflect what we can expect
in the real life: mostly bunch of new warnings about missing declarations
and defaulting to int (this is probably going to be a pain with -Werror,
but I feel that people really should write proper declarations), different
inline semantics (in C99 semantics, the TU has to have the body of the inline
function etc.), new "return with no value, in function returning non-void"
warnings.  Different rules for constant expressions, the fact that in C90
non-lvalue arrays do not decay to pointers, slightly different rules for
compatible types (?) might come in game as well.

In turn, you can use all C99 and C11 features even with -pedantic.

Comments?

Regtested/bootstrapped on powerpc64-linux and x86_64-linux.

2014-10-07  Marek Polacek  <polacek@redhat.com>

	* doc/invoke.texi: Update to reflect that GNU11 is the default
	mode for C.
	* c-common.h (c_language_kind): Update comment.
c-family/
	* c-opts.c (c_common_init_options): Make -std=gnu11 the default for C.
I like it. And one could reasonably argue that now is the time to change since that maximizes the time for folks to find broken code.

I'd go so far as to conditionally approve -- if other maintainers don't shout out in the next week or so against, then I feel this should go forward.

I know it's really early, but a "porting to ..." document ought to be started and have something in it about these changes. Both how to fix the broken code or how to go back to c89.

Jeff


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