On Thu, 19 Jan 2012, BELBACHIR Selim wrote: > In fact my final purpose is to replace $INP by a register bank > in order to be able to read several inputs using pipelined > instructions (and instruction scheduler). The fixed reg solution > will prevent me from doing this. Is there another way to prevent > the use of some registers during the reload pass without turning > them into fixed register ?
I can't think of another way than to use different (narrower) register constraints for the destination operand than the source operands for the instructions which can take $INP as a source operand, with the source operand register class to include the register (sub)class for $INP, but this latter register class must not be included in GENERAL_REGS nor of course the destination register class. (Though, the source register class can be a superclass that includes GENERAL_REGS and the register class for $INP, whatever applies.) This wouldn't "prevent the use of some registers during the reload pass" but that wasn't the original goal nor a reasonably doable one AFAIK. With reservations for possibly not understanding your problem. > -----Message d'origine----- > De : Ian Lance Taylor [mailto:i...@google.com] > Envoyé : jeudi 19 janvier 2012 00:17 > À : BELBACHIR Selim > Cc : gcc@gcc.gnu.org > Objet : Re: readonly register > > BELBACHIR Selim <selim.belbac...@fr.thalesgroup.com> writes: > > > I'm trying to support an 'in' instruction which reads a value on a > > peripheral and writes it into a $INP register. > > The $INP register can be used in almost every insn as input operand (add, > > sub, mul ...). > > I defined a builting to access the 'in' instruction. > > > > How should I express to gcc that the $INP register can only be 'read' and > > must never be written? > > > > For the moment, I encounter a problem during IRA pass where some of my > > 'classic' registers are reloaded into $INP (apparently because I have no > > register left). > > If you have a builtin to access the value, then you should be able to > make it a fixed register. The register allocator will never try to > allocate a fixed register. brgds, H-P
