[Bug target/108338] New: use mtvsrws for lowpart DI->SF conversion on P9

[guojiufu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108338

            Bug ID: 108338
           Summary: use mtvsrws for lowpart DI->SF conversion on P9
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: guojiufu at gcc dot gnu.org
  Target Milestone: ---

In a mail-list discussion,
https://gcc.gnu.org/pipermail/gcc-patches/2022-December/609054.html, as Segher
points out, we could use 'mtvsrws' for the conversion from lowpart DI to SF on
P9;  and use 'mtvsrd' for the conversion from highpart of DI to SF.

float sf_from_di_off0 (long l)
{
  char buff[16];
  *(long*)buff = l;
  float f = *(float*)(buff);
  return f;
}

float sf_from_di_off4 (long l)
{
  char buff[16];
  *(long*)buff = l;
  float f = *(float*)(buff + 4);
  return f;
}

With trunk, -O2 -mcpu=power9,  the asm code could be optimized.

        sldi 9,3,32
        mtvsrd 1,9
        xscvspdpn 1,1
        blr
==> mtvsrws;xscvspdpn  (p9 LE) lowpart


        srdi 3,3,32
        sldi 9,3,32
        mtvsrd 1,9
        xscvspdpn 1,1
        blr
==> (+ clean lowpart to 0) mtvsrd;xscvspdpn   highpart