[PATCH v2 3/9] Introduce can_vector_compare_p function

[Ilya Leoshkevich]

> Am 23.08.2019 um 13:24 schrieb Richard Biener <richard.guenther@gmail.com>:
> 
> On Fri, Aug 23, 2019 at 12:43 PM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>> 
>> Ilya Leoshkevich <iii@linux.ibm.com> writes:
>>> @@ -3819,6 +3820,82 @@ can_compare_p (enum rtx_code code, machine_mode mode,
>>>   return 0;
>>> }
>>> 
>>> +/* can_vector_compare_p presents fake rtx binary operations to the the back-end
>>> +   in order to determine its capabilities.  In order to avoid creating fake
>>> +   operations on each call, values from previous calls are cached in a global
>>> +   cached_binops hash_table.  It contains rtxes, which can be looked up using
>>> +   binop_keys.  */
>>> +
>>> +struct binop_key {
>>> +  enum rtx_code code;        /* Operation code.  */
>>> +  machine_mode value_mode;   /* Result mode.     */
>>> +  machine_mode cmp_op_mode;  /* Operand mode.    */
>>> +};
>>> +
>>> +struct binop_hasher : pointer_hash_mark<rtx>, ggc_cache_remove<rtx> {
>>> +  typedef rtx value_type;
>>> +  typedef binop_key compare_type;
>>> +
>>> +  static hashval_t
>>> +  hash (enum rtx_code code, machine_mode value_mode, machine_mode cmp_op_mode)
>>> +  {
>>> +    inchash::hash hstate (0);
>>> +    hstate.add_int (code);
>>> +    hstate.add_int (value_mode);
>>> +    hstate.add_int (cmp_op_mode);
>>> +    return hstate.end ();
>>> +  }
>>> +
>>> +  static hashval_t
>>> +  hash (const rtx &ref)
>>> +  {
>>> +    return hash (GET_CODE (ref), GET_MODE (ref), GET_MODE (XEXP (ref, 0)));
>>> +  }
>>> +
>>> +  static bool
>>> +  equal (const rtx &ref1, const binop_key &ref2)
>>> +  {
>>> +    return (GET_CODE (ref1) == ref2.code)
>>> +        && (GET_MODE (ref1) == ref2.value_mode)
>>> +        && (GET_MODE (XEXP (ref1, 0)) == ref2.cmp_op_mode);
>>> +  }
>>> +};
>>> +
>>> +static GTY ((cache)) hash_table<binop_hasher> *cached_binops;
>>> +
>>> +static rtx
>>> +get_cached_binop (enum rtx_code code, machine_mode value_mode,
>>> +               machine_mode cmp_op_mode)
>>> +{
>>> +  if (!cached_binops)
>>> +    cached_binops = hash_table<binop_hasher>::create_ggc (1024);
>>> +  binop_key key = { code, value_mode, cmp_op_mode };
>>> +  hashval_t hash = binop_hasher::hash (code, value_mode, cmp_op_mode);
>>> +  rtx *slot = cached_binops->find_slot_with_hash (key, hash, INSERT);
>>> +  if (!*slot)
>>> +    *slot = gen_rtx_fmt_ee (code, value_mode, gen_reg_rtx (cmp_op_mode),
>>> +                         gen_reg_rtx (cmp_op_mode));
>>> +  return *slot;
>>> +}
>> 
>> Sorry, I didn't mean anything this complicated.  I just meant that
>> we should have a single cached rtx that we can change via PUT_CODE and
>> PUT_MODE_RAW for each new query, rather than allocating a new rtx each
>> time.
>> 
>> Something like:
>> 
>> static GTY ((cache)) rtx cached_binop;
>> 
>> rtx
>> get_cached_binop (machine_mode mode, rtx_code code, machine_mode op_mode)
>> {
>>  if (cached_binop)
>>    {
>>      PUT_CODE (cached_binop, code);
>>      PUT_MODE_RAW (cached_binop, mode);
>>      PUT_MODE_RAW (XEXP (cached_binop, 0), op_mode);
>>      PUT_MODE_RAW (XEXP (cached_binop, 1), op_mode);
>>    }
>>  else
>>    {
>>      rtx reg1 = gen_raw_REG (op_mode, LAST_VIRTUAL_REGISTER + 1);
>>      rtx reg2 = gen_raw_REG (op_mode, LAST_VIRTUAL_REGISTER + 2);
>>      cached_binop = gen_rtx_fmt_ee (code, mode, reg1, reg2);
>>    }
>>  return cached_binop;
>> }
> 
> Hmm, maybe we need  auto_rtx (code) that constructs such
> RTX on the stack instead of wasting a GC root (and causing
> issues for future threading of GCC ;)).

Do you mean something like this?

union {
  char raw[rtx_code_size[code]];
  rtx rtx;
} binop;

Does this exist already (git grep auto.*rtx / rtx.*auto doesn't show
anything useful), or should I implement this?