Here’s Mike’s version of the function:

qword SimdFloatToHalf( vf32 a_, vf32 b_ )
{
  const qword shuf_hi16  = (qword)(vu16){ 0x0001, 0x0405, 0x0809, 0x0c0d, 0x1011, 0x1415, 0x1819, 0x1c1d };

  const qword s_mask     = (qword)(vu16){ 0x8000, 0x8000, 0x8000, 0x8000, 0x8000, 0x8000, 0x8000, 0x8000 };
  const qword blah       = (qword)(vu16){ 0x4000, 0x4000, 0x4000, 0x4000, 0x4000, 0x4000, 0x4000, 0x4000 };
  const qword exp_off    = (qword)(vu16){ 0x3800, 0x3800, 0x3800, 0x3800, 0x3800, 0x3800, 0x3800, 0x3800 };

  qword a_rot5 = si_shlqbii((qword)a_, 3);
  qword b_rot5 = si_shlqbii((qword)b_, 3);

  qword mid = si_shufb(a_rot5, b_rot5, shuf_hi16);        // mid = | e5  e4  e3  e2  e1  e0  m23 m22 m21 m20 m19 m18 m17 m16 m15 m14 | 
  qword hi  = si_shufb((qword)a_, (qword)b_, shuf_hi16);  // hi  = |  s  e7  e6  e5  e4  e3  e2  e1  e0  m23 m22 m21 m20 m19 m18 m17 | 

  qword h = si_xor(mid, blah);                            // h   = | e5  e4' e3  e2  e1  e0  m23 m22 m21 m20 m19 m18 m17 m16 m15 m14 | 
  qword e = si_andc(hi, s_mask);                          // e   = |  0  e7  e6  e5  e4  e3  e2  e1  e0  m23 m22 m21 m20 m19 m18 m17 | 
  h = si_selb(h, hi, s_mask);                             // h   = |  s  e4' e3  e2  e1  e0  m23 m22 m21 m20 m19 m18 m17 m16 m15 m14 | 
  qword d_mask = si_cgth(exp_off, e);                                                                                                  
  h = si_andc(h, d_mask);                                 // set to 0 if underflow                                                     

  return h;
}


And here’s what it compiles into:
00000000 <_ZN3FXV15SimdFloatToHalfEU8__vectorfS0_>:
   0:      04 00 01 86       ori         $6,$3,0
   4:      35 80 00 0f        hbr        40 <_ZN3FXV15SimdFloatToHalfEU8__vectorfS0_+0x40>,$0
   8:      41 c0 00 08       ilh         $8,32768           # 8000
   c:      30 80 00 07       lqa        $7,0
  10:      3f 60 c2 02        shlqbii   $2,$4,3
  14:      3f 60 c1 83        shlqbii   $3,$3,3
  18:      40 20 00 7f        nop       $127
  1c:      b0 c1 03 07       shufb    $6,$6,$4,$7
  20:      41 9c 00 04       ilh         $4,14336           # 3800
  24:      b0 60 81 87       shufb    $3,$3,$2,$7
  28:      41 a0 00 02       ilh         $2,16384           # 4000
  2c:      58 22 03 05       andc     $5,$6,$8
  30:      48 20 81 83       xor        $3,$3,$2
  34:      49 01 42 04       cgth      $4,$4,$5
  38:      80 61 81 88       selb      $3,$3,$6,$8
  3c:      58 21 01 83       andc     $3,$3,$4
  40:      35 00 00 00       bi          $0
  44:      00 20 00 00       lnop