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Re: [PATCH 2/8] Introduce vcond_supported_p hook


Ilya Leoshkevich <iii@linux.ibm.com> writes:
>> Am 20.08.2019 um 12:13 schrieb Richard Sandiford <richard.sandiford@arm.com>:
>> 
>> Ilya Leoshkevich <iii@linux.ibm.com> writes:
>>> z13 supports only non-signaling vector comparisons.  This means we
>>> cannot vectorize LT, LE, GT, GE and LTGT when compiling for z13.
>>> However, we cannot express this restriction today: the code only checks
>>> whether vcond$a$b optab, which does not contain information about the
>>> operation.
>>> 
>>> Introduce a hook that tells whether target supports certain vector
>>> comparison operations with certain modes.
>>> 
>>> gcc/ChangeLog:
>>> 
>>> 2019-08-09  Ilya Leoshkevich  <iii@linux.ibm.com>
>>> 
>>> 	* doc/tm.texi (TARGET_VCOND_SUPPORTED_P): Document.
>>> 	* doc/tm.texi.in (TARGET_VCOND_SUPPORTED_P): Document.
>>> 	* optabs-tree.c (expand_vec_cond_expr_p): Use vcond_supported_p
>>> 	in addition to get_vcond_icode.
>>> 	* target.def (vcond_supported_p): New hook.
>>> 	* targhooks.c (default_vcond_supported_p): Likewise.
>>> 	* targhooks.h (default_vcond_supported_p): Likewise.
>> 
>> IMO it'd be cleaner to have a new optabs-query.[hc] helper that uses
>> the predicate for operand 3 to test whether a particular comparison
>> is supported.  I guess this would require a cached rtx to avoid
>> generating too much garbage rtl though (via GTY((cache))).
>
> How can I implement such a predicate?  Would calling maybe_gen_insn with
> a fake rtx be reasonable?  In this case, what would be the best way to
> generate fake input operands?  The existing code that calls
> maybe_gen_insn gets the corresponding rtxes from upper layers.

I was thinking of something like optabs.c:can_compare_p, but with
some caching to reduce the overhead, and comparing registers rather
than constants.  E.g.:

static rtx cached_binop GTY ((cached));

rtx
get_cached_binop (machine_mode mode, rtx_code code, machine_mode op_mode)
{
  ...create or modify cached_binop, with register operands...;
  return cached_binop;
}

/* ...maybe a different name if the overload seems like a bad idea... */
bool
vcond_supported_p (machine_mode value_mode, rtx_code cmp,
		   machine_mode cmp_op_mode)
{
  bool unsigned = (cmp == LTU || cmp == LEU || cmp == GTU || cmp == GEU);
  insn_code icode = get_vcond_icode (value_mode, cmp_op_mode, unsigned);
  if (icode == CODE_FOR_nothing)
    return false;

  if (insn_operand_predicate_fn pred = insn_data[icode].operand[3].predicate)
    {
      machine_mode cmp_mode = insn_data[icode].operand[3].mode;
      rtx cmp = get_cached_binop (cmp_mode, cmp, cmp_op_mode);
      if (!pred (cmp, cmp_mode))
	return false;
    }

  return true;
}

(not attached to the names).

Then the target pattern would use a normal match_operator for operand 3,
with the predicate on the match_operand only allowing through the
comparisons it actually supports.

The get_cached_binop stuff makes it a bit cumbersome, but that part
would be reusable by other queries in future (and by can_compare_p
probably).

Thanks,
Richard


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