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Re: builtin assembler constraints
On Sat, Feb 9, 2013 at 7:37 AM, Georg-Johann Lay <gjl@gcc.gnu.org> wrote:
> Ian Lance Taylor schrieb:
>
>> As far as I know there is no constraint that describes only the even
>> registers of the general register set. It would not be very difficult
>> to add one to the PPC backend. I can't think of any other way to
>> approach this.
>
>
> AFAIK that does not work because the respective register class would have
> holes. You cannot have a register class that contains only even registers
> and omits the odd ones. Or can IRA / reload handle this now?
To the best of my knowledge it always could. Register classes
routinely have holes. I'm not sure what sort of problem you are
thinking of.
> One way would be to use HARD_REGNO_MODE_OK and restrict 64-bit modes to even
> registers, but that would be an ABI change...
Yeah, don't do that.
> From the C level a local register variable could be use, thus it is possible
> to describe but the results will be suboptimal because the respective
> operand is not available for register allocation.
Yes.
Ian