RFC: New mechanism for hard reg operands to inline asm

[Richard Sandiford]

Paul Koning via Gcc <gcc@gcc.gnu.org> writes:
>> 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.

+1 FWIW.  I think this would also avoid the common confusion around
the semantics of register asms.

We would need to check whether { is used as a constraint string by
any target (hepefully not).

Richard