[Bug target/104976] New: [avx512fp16] lowpart_subreg return NULL_RTX cause ICE

[crazylht at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104976

            Bug ID: 104976
           Summary: [avx512fp16] lowpart_subreg return NULL_RTX cause ICE
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: crazylht at gmail dot com
  Target Milestone: ---

void
foo (int a, float b, double* c)
{
  union {
    _Float16 f[2];
    int i;} x;
  x.i = a;
  c[0] = x.f[0] * 0.2;
  c[1] = x.f[1] * 0.2;
}

 7165(define_expand "extendv2hfv2df2"
 7166  [(set (match_operand:V2DF 0 "register_operand")
 7167        (float_extend:V2DF
 7168          (match_operand:V2HF 1 "nonimmediate_operand")))]
 7169  "TARGET_AVX512FP16 && TARGET_AVX512VL"
 7170{
 7171  if (!MEM_P (operands[1]))
 7172    {
 7173      operands[1] = lowpart_subreg (V8HFmode, operands[1], V2HFmode);
 7174      emit_insn (gen_avx512fp16_float_extend_phv2df2
 7175                 (operands[0], operands[1]));
 7176      DONE;
 7177    }
 7178})

here operands[1] is (subreg:V2HF (reg:SI) 0) which is valid, but not for
(subreg:v8hf (reg:SI 0) and return NULL cause invalid rtl.

101(insn 8 7 9 (set (reg:V2DF 88 [ vect__1.3 ])
102        (float_extend:V2DF (vec_select:V2HF (nil)
103                (parallel [
104                        (const_int 0 [0])
105                        (const_int 1 [0x1])
106                    ])))) "test3.c":8:13 -1
107     (nil))