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

[tmsriram at google dot com]

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

--- Comment #38 from Sriraman Tallam <tmsriram at google dot com> 2013-01-18 19:53:16 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?  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:

Without bringing in your patch, I removed this line with patch:


--- gcc/c-family/c-common.c    (revision 195302)
+++ gcc/c-family/c-common.c    (working copy)
@@ -8763,8 +8763,7 @@
      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 ())
+                              flags))
     *no_add_attrs = true;

   return NULL_TREE;

and then tried the new compiler on the following example:

int foo ();
int foo () __attribute__ ((target("mmx")));

int main ()
{
  return foo ();
}

int
foo ()
{
  return 0;
}

int __attribute__ ((target("mmx")))
foo ()
{
  return 0;
}

and with -mno-mmx added to the compile options, everything is fine. However,
with -mmmx in the compile options, I get:

fe_example.cc: In function ‘int foo()’:
fe_example.cc:16:1: error: redefinition of ‘int foo()’
 foo ()
 ^
fe_example.cc:10:1: error: ‘int foo()’ previously defined here
 foo ()

Reason is the stripping of target attributes that do not make sense. But, for
MV that creates duplicate functions. 

I can change this to only keep the attribute tagged it is recognized by the
target. That way I will strip out erroneous values for target attribute.



> 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.