register variable is clobbered by passing parameter to a function

[Dave Korn]

> -----Original Message-----
> From: gcc-owner On Behalf Of Ian Lance Taylor

> "Dave Korn" <dk@artimi.com> writes:
> 
> >   Wolfgang, your best bet is probably to just add 
> -fixed-r16 to your 
> > compile-time flags.  And don't call any library functions that were 
> > compiled without it.
> 
> This is AVR, and he is trying to use r16.  According to
> function_arg_regno_p() in avr.c, that register can be used to 
> pass parameters to functions.  gcc can't handle declaring a 
> register used for parameter passing as fixed.

  Ah.  Then my advice should have been "Change r16 to some safer register."
Hopefully that should then suffice to make his global-register-var work
correctly and there isn't actually a bug in gcc.

> Trying to do that is an implicit ABI change anyhow.  Maybe 
> gcc should warn about this, but it doesn't.
> 
> Ian

  Fair enough.  It would be simple enough for the code that implements
-fixed-rXX to clear the corresponding entries from the CALL_USED_REGISTERS
and REG_ALLOC_ORDER arrays at the same time as it sets the entry in
FIXED_REGISTERS; but I guess there's no general way to allow for what
backends might do with registers in prolog/epilog or varargs codegeneration.

  As to the implicit ABI change, well, we're a long way into the realms of
"don't do this unless you know exactly what you're doing" anyway.  Issuing a
warning seems tricky, without having some knowledge of what goes on inside
functions in the backend.  The best bet might be to add a bit more
documentation to the register-vars pages, indicating that registers chosen
should not be call-used or fixed-for-other-purposes-by-the-abi.


    cheers, 
      DaveK
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