http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55742
--- Comment #36 from Sriraman Tallam <tmsriram at google dot com> 2013-01-18
18:03:21 UTC ---
(In reply to comment #32)
> Created attachment 29207 [details]
> 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?
This was added because previously if I had two declarations of foo like this:
void foo ();
void foo __target__(("sse4.2")));
int main ()
{
foo ();
}
void foo ()
{
}
__target__(("sse4.2")));
void foo ()
{
}
The call to foo in main will be treated like 2 different versions of foo exist.
However with -msse4.2 on the command-line, the target attribute will be
stripped off the second declaration which makes foo no longer multi-versioned
when the call to foo is processed. The call to foo without -msse4.2 is
multi-versioned and with -msse4.2 is not. I wanted to avoid this behaviour.
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.
