gcc optimises out test of value in register-only loop

[Andrew Haley]

On 10/15/2011 04:19 AM, Ian Lance Taylor wrote:
> Andrew Haley <aph@redhat.com> writes:
> 
>> On 10/14/2011 05:41 AM, Ian Lance Taylor wrote:
>>> MikeW <mw_phil@yahoo.co.uk> writes:
>>>
>>>>   volatile register int stop_loop __asm("r5")__;
>>>
>>> gcc does not support volatile register variables.  I think it could be
>>> meaningful, but it has never worked.
>>
>> It would be really nasty to implement: we'd have to check for volatility
>> on every access to a hard register.  The benefits don't seem worth it,
>> IMO.
> 
> No, it's not that bad.  At the tree level, we would only have to check
> on every reference to a DECL for which DECL_REGISTER is true.  When
> expanding to RTL, we would replace each reference with a special
> UNSPEC_VOLATILE which we handle specially when generating asm code.  Or
> perhaps simply expand each reference to a specially handled asm
> statement.

Ah yes, that would indeed work.

Still doesn't seem worth it!  :-)

Andrew.