[Bug target/92488] New: GCC generates to calls to __dpd_trunctdsd2 with -mhard-dfp

[murphyp at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92488

            Bug ID: 92488
           Summary: GCC generates to calls to __dpd_trunctdsd2 with
                    -mhard-dfp
           Product: gcc
           Version: 9.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: murphyp at linux dot vnet.ibm.com
  Target Milestone: ---

While cleaning up libdfp, I noticed GCC still leans on libgcc to perform
_Decimal128 to _Decimal32 truncation.

This can be inlined with 4-5 instructions depending on ISA using the "round to
prepare for shorter precision" rounding mode supported on all PPC DFP targets:

    mffs    fp4     /* Save current rounding mode.  */
    mtfsfi  7, 7, 1 /* Set round to prepare for shorter.  */
    drdpq   fp0,fp2 /* Initial round to _Decimal64.  */
    mtfsf   0xff,fp4,1,0 /* Restore previous rounding mode.  */
    drsp    fp1,fp0 /* Round result to _Decimal32.  */

mffs/mtsfi could be substituted for "mffscdrni fp4, 7" on ISA 3.0 and above.

e.g the following trivial example:

_Decimal32
truncd128(_Decimal128 d)
{
  return d;
}