RFC: New mechanism for hard reg operands to inline asm

Andreas Krebbel krebbel@linux.ibm.com
Fri Jun 4 18:02:27 GMT 2021


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


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