ORFPX32/ORFPX64 Comparison Corrections (P19) 2025-03-19 - Stafford Horne

When Jed Liu was working on the mor1kx FPU verirfication PR#164 he noticed some shortcomings in the ORFPX32/ORFPX64 specification regarding unordered comparisons these are:

The text (mostly from Jed) being:

• For = and ≠ (lf.sfeq.*, lf.sfne.*; pp.151–2, 161–2), the spec says to raise an exception when a signalling NaN is given, but doesn’t explicitly specify the behaviour on quiet NaNs. We should specify how for quiet NaN’s that they are never equal to anything, any comparison comparing NaNs should clear the SR[F] flag.
• For unordered or = (lf.sfueq.*; pp.163–4), the spec says to set the compare flag on any NaN, but the implementation yields an exception on signalling NaNs. As per IEEE 754 we should specify this in the specification.

• For unordered or ≠ (lf.sfune.d and lf.sfune.s) are missing from the spec, although the machine code reference in §18 lists both instructions. These should be added to the spec similar to lf.sfne.*. Should these be removed, are they useful?

  • These are missing in QEMU also.
  • These are missing in GCC
  • These defined in gas binutils-gdb/cpu/or1korfpx.cpu
• For the unordered comparisons (lf.sfult.*, lf.sfule.*, lf.sfugt.*, lf.sfuge.*; pp.165–72), the spec says to set the compare flag on any NaN, but IEEE 754 says to signal an exception for signalling NaNs. This should be specified in the spec and implementations should be updated to do so. QEMU already does this.