[Bug c++/55742] [4.8 regression] __attribute__ in class function declaration cause "prototype does not match" errors.

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55742

--- Comment #32 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-01-18 14:18:58 UTC ---
Created attachment 29207
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29207
gcc48-pr55742.patch

This bug is open for way too long given its severity, so let's start talking
over patches.

This patch attempts to implement what I understand from Jason's comments, just
with "default" instead of "any", because it is indeed the default target
attribute (whatever you specify on the command line).

Say on:
void foo ();
void foo () __attribute__((target ("avx")));
void foo () __attribute__((target ("default")));
__attribute__((target ("default"))) void foo ()
{
}
__attribute__((target ("avx"))) void foo ()
{
}
void (*fn) () = foo;

first we merge the first two decls, because only if target attribute is present
on both, we consider it for multi-versioning, for compatibility with 4.7 and
older.  On e.g.
void foo ();
void foo () __attribute__((target ("sse4")));
void foo () __attribute__((target ("default")));
void foo ()
{
}
we reject the last fn definition, because at that point foo is already known to
be multi-versioned, thus it is required that target attribute is specified for
foo (either "default", or some other).  Unfortunately, for this case the error
is reported twice for some reason.

The #c0 testcase now compiles.

Now, the issues I discovered with multiversioning, still unfixed by the patch:
1) the mv*.C testcases should be moved, probably to g++.dg/ext/mv*.C
2) can you please explain the mess in handle_target_attribute?
  /* Do not strip invalid target attributes for targets which support function
     multiversioning as the target string is used to determine versioned
     functions.  */
  else if (! targetm.target_option.valid_attribute_p (*node, name, args,
                                                      flags)
           && ! targetm.target_option.supports_function_versions ())
    *no_add_attrs = true;
Why do you need that?  Consider complete garbage in target attribute arguments,
which is errored about, but the above for i386/x86_64 keeps the target
attribute around anyway, leading to lots of ICEs everywhere:
Consider e.g.:
__attribute__((target ("default"))) void foo (void)
{
}
__attribute__((target (128))) void foo (void)
{
}
3) the multiversioning code assumes that target has a single argument, but it
can have more than one.  Say for:
__attribute__((target ("avx,popcnt"))) void foo (void)
{
}
__attribute__((target ("popcnt","avx"))) void bar (void)
{
}
the compiler handles those two as equivalent, but with -Dbar=foo
multi-versioning only considers the first string out of that.