Re: RFA: Add an FPIC set of multilibs to the MN10300 port

[Jeff Law <law at redhat dot com>]

Nick Clifton wrote:
> Hi Jeff, Hi Alex,
>
>   I believe that there is a need for a new set of multilibs for the
>   MN10300 port.  The -fpic and -fPIC command line options change the
>   ABI by making register r6 fixed instead of call_saved.  This
>   resulted in the problem with __main() in crt1.c that I reported
>   earlier and I now have a test case which demonstrates the problem in
>   normal C code:
>   
>     #include <stdlib.h>
>     #include <stdio.h>
>
>     #define array_size 128
>
>     int array[array_size];
>     int ascend = 1;
>
>     static int 
>     comp (const void * _a, const void * _b)
>     {
>       int * a = (int *) _a;
>       int * b = (int *) _b;
>
>       return ascend ? *a - *b : *b - *a;
>     }
>
>     int 
>     main (void)
>     {
>       int i;
>
>       for (i = array_size; i--;)
>         array[i] = i;
>
>       qsort (array, array_size, sizeof * array, comp);
>
>       for (i = array_size; i--;)
>         if (array[i] != (ascend ? i : (array_size - i) - 1))
>           abort ();
>
>       exit (0);
>     }
>
>   Compile this program without -fPIC and run it and it will exit
>   successfully, compile it with -fPIC and it will seg fault because
>   the comp() function will corrupt the value held in register a2 which
>   the qsort() library function expects will be preserved.
>
>   I have attached a patch to add a set of pic multilibs.  Please may I
>   apply it ?
>
> Cheers
>   Nick
>
> gcc/ChangeLog
> 2008-10-14  Nick Clifton  <nickc@redhat.com>
>
> 	* config/mn10300/t-mn10300 (MULTILIB_OPTIONS): Add fPIC.
> 	(MULTILIB_DIRNAMES): Add pic.
> 	(MULTILIB_MATCHES): New.  Treat -fpic as -fPIC.
>   
Something doesn't make sense here.   a2 is call-saved and thus a 
function can reasonably expect its value to be preserved across calls -- 
which ought to apply to PIC code as well.

So ISTM that if comp is modifying a2 in a way which is visible to qsort, 
then the problem would be that a2 isn't being saved/restored within comp.

At least that's the way it seems to me and how things work on other 
ports.  Alex -- is there something special in the mn103 ABI for PIC code 
that I'm not aware of?

Jeff