Re: builtin assembler constraints

[Georg-Johann Lay <gjl at gcc dot gnu dot org>]

Ian Lance Taylor schrieb:
> On Fri, Feb 8, 2013 at 10:15 AM, Grobman Alexander-R53196
> <R53196@freescale.com> wrote:
> > I'm trying to incorporate new PPC instructions into a C code. These instructions use limited set of GPRs as operands i.e. only even registers can be used as a source,
> >
> > Is there way to specify that in the asm() statement?
>
> 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?

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...

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.

Johann