64bit: weak symbols

[Corinna Vinschen]

On Mar 30 22:32, Yaakov (Cygwin/X) wrote:
> The following code links behaves on i686 and x86_64:
> 
> /* from gcc config/weakref.m4, used in libitm */
> extern void fNotToBeFound(void) __attribute__((weak));
> int main ()
> {
>   if (fNotToBeFound)
>     return 1;
>   else
>     return 0;
> }
> 
> On i686 with gcc-4.5.3, this links and returns 0.  On x86_64 with
> gcc-4.8.0, this produces an error:
> 
> /tmp/ccPWiz9s.o:test.c:(.rdata$.refptr.fNotToBeFound[.refptr.fNotToBeFound]+0x0):
> undefined reference to `fNotToBeFound'
> collect2: error: ld returned 1 exit status

There's a recent discussion aboout this very problem on the gcc mailing
list.  Kai claims that weak symbols cannot work this way on PE/COFF
targets, and the fact that they worked on i686 is kind of a coincidence,
Dave Korn thinks everything's ok as is.  IIUC, the symbol is an absolute
reference to the 0 address on i686, but a pc-relative reference with
undefined displacement on x86_64.  Personally I think the x86_64 gcc
should create absolute references for weak symbols as well, but I'm not
fluent enough in the bowels of gcc to have a say in the matter.

Kai?  Dave?


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
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