Re: generated decoder code question

["Chuan-Hua Chang" <chuanhua.chang@gmail.com>]

>
> Please point out any cases where the test is not needed and I will see
> if I can eliminate it in those cases.
>

Using sid/component/cgen-cpu/mt/mt-decode.cxx as an example:

---------------------------------------------------------------------------------------------
void
mt_scache::decode (mt_cpu* current_cpu, PCADDR pc, mt_insn_word
base_insn, mt_insn_word entire_insn)
{
  /* Result of decoder.  */
  MT_INSN_TYPE itype;

  {
    mt_insn_word insn = base_insn;

    {
      unsigned int val = (((insn >> 24) & (255 << 0)));
      switch (val)
      {
      case 0 :
        if ((entire_insn & 0xff000fff) == 0x0)
          { itype = MT_INSN_ADD; mt_extract_sfmt_add (this,
current_cpu, pc, base_insn, entire_insn); goto done; }
        itype = MT_INSN_X_INVALID; mt_extract_sfmt_empty (this,
current_cpu, pc, base_insn, entire_insn); goto done;
      case 1 :
        if ((entire_insn & 0xff000000) == 0x1000000)
          { itype = MT_INSN_ADDI; mt_extract_sfmt_addi (this,
current_cpu, pc, base_insn, entire_insn); goto done; }
        itype = MT_INSN_X_INVALID; mt_extract_sfmt_empty (this,
current_cpu, pc, base_insn, entire_insn); goto done;
      case 2 :
        if ((entire_insn & 0xff000fff) == 0x2000000)
          { itype = MT_INSN_ADDU; mt_extract_sfmt_addu (this,
current_cpu, pc, base_insn, entire_insn); goto done; }
        itype = MT_INSN_X_INVALID; mt_extract_sfmt_empty (this,
current_cpu, pc, base_insn, entire_insn); goto done;
      case 3 :
        if ((entire_insn & 0xff000000) == 0x3000000)
          { itype = MT_INSN_ADDUI; mt_extract_sfmt_addui (this,
current_cpu, pc, base_insn, entire_insn); goto done; }
        itype = MT_INSN_X_INVALID; mt_extract_sfmt_empty (this,
current_cpu, pc, base_insn, entire_insn); goto done;
      case 4 :
        if ((entire_insn & 0xff000fff) == 0x4000000)
          { itype = MT_INSN_SUB; mt_extract_sfmt_add (this,
current_cpu, pc, base_insn, entire_insn); goto done; }
        itype = MT_INSN_X_INVALID; mt_extract_sfmt_empty (this,
current_cpu, pc, base_insn, entire_insn); goto done;
      case 5 :
        if ((entire_insn & 0xff000000) == 0x5000000)
          { itype = MT_INSN_SUBI; mt_extract_sfmt_addi (this,
current_cpu, pc, base_insn, entire_insn); goto done; }
        itype = MT_INSN_X_INVALID; mt_extract_sfmt_empty (this,
current_cpu, pc, base_insn, entire_insn); goto done;
---------------------------------------------------------------------------------------------------

The top level switch/case statement tests the upper 8 bits of the
instruction ((>> 24) & 255). For case 0, it is true that additional
lower 12 bits are being tested (& 0xff000fff). However, for case 1, no
additional bits are being tested (& 0xff000000), this is where the
redundancy comes in. I would expect CGEN to detect this case and not
generate unnecessary IF checking here. (e.g similar to M32R code in
the CGEN release.)


>>
>> When looking at the M32R decode function, the IF-statement is absent
>> from the decoder code.  This lets me wonder that if there is a way to
>> remove this redundant IF-statement check in the CGEN flow.
>>
>This is likely a case where all of the decodable bits have already been
>tested in reaching that particular case.
>

May I ask that if the released M32R code is generated by the current
CGEN utils-sim.scm file or generated by another version or hand-edited
to remove the IF checking statement mentioned in my question? I am
asking this since from the the "-gen-decode-insn-entry" function I do
not see any conditional construct to control the generation of the IF
checking code. It seems that the IF checking code is always generated.
I have also re-generated the M32R simulator to confirm my guess, and
the newly-generated M32R decode function DOES have the IF checking
statement now. Am I missing something?

>
>Do you have a sense of how frequently such a test is completely
>redundant, and how much additional time this test takes?
>

For high locality code, the decode function may not be invoked that
frequently, but for low locality code, the decode function will be
invoked a lot more often. it seems that this kind of redundancy should
be made as few as possible.

Thanks for your help.