> On Jun 4, 2021, at 2:02 PM, Andreas Krebbel via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi,
>
> I wonder if we could replace the register asm construct for
> inline assemblies with something a bit nicer and more obvious.
> E.g. turning this (real world example from IBM Z kernel code):
>
> int diag8_response(int cmdlen, char *response, int *rlen)
> {
> register unsigned long reg2 asm ("2") = (addr_t) cpcmd_buf;
> register unsigned long reg3 asm ("3") = (addr_t) response;
> register unsigned long reg4 asm ("4") = cmdlen | 0x40000000L;
> register unsigned long reg5 asm ("5") = *rlen; /* <-- */
> asm volatile(
> " diag %2,%0,0x8\n"
> " brc 8,1f\n"
> " agr %1,%4\n"
> "1:\n"
> : "+d" (reg4), "+d" (reg5)
> : "d" (reg2), "d" (reg3), "d" (*rlen): "cc");
> *rlen = reg5;
> return reg4;
> }
>
> into this:
>
> int diag8_response(int cmdlen, char *response, int *rlen)
> {
> unsigned long len = cmdlen | 0x40000000L;
>
> asm volatile(
> " diag %2,%0,0x8\n"
> " brc 8,1f\n"
> " agr %1,%4\n"
> "1:\n"
> : "+{r4}" (len), "+{r5}" (*rlen)
> : "{r2}" ((addr_t)cpcmd_buf), "{r3}" ((addr_t)response), "d"
> (*rlen): "cc");
> return len;
> }
>
> Apart from being much easier to read because the hard regs become part
> of the inline assembly it solves also a couple of other issues:
>
> - function calls might clobber register asm variables see BZ100908
> - the constraints for the register asm operands are superfluous
> - one register asm variable cannot be used for 2 different inline
> assemblies if the value is expected in different hard regs
>
> I've started with a hackish implementation for IBM Z using the
> TARGET_MD_ASM_ADJUST hook and let all the places parsing constraints
> skip over the {} parts. But perhaps it would be useful to make this a
> generic mechanism for all targets?!
>
> Andreas
Yes, I would think this should be made a general mechanism that any target
could use.
I wonder if instead of creating a new mechanism you could do this simply by
creating new constraint names, where each name matches exactly one hard
register. That's roughly what this amounts to, isn't it?
paul
