Re: [PATCH] Check if loading const from mem is faster

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi Sehger,

Segher Boessenkool <segher@kernel.crashing.org> writes:

> On Tue, Mar 01, 2022 at 10:28:57PM +0800, Jiufu Guo wrote:
>> Segher Boessenkool <segher@kernel.crashing.org> writes:
>> > No.  insn_cost is only for correct, existing instructions, not for
>> > made-up nonsense.  I created insn_cost precisely to get away from that
>> > aspect of rtx_cost (and some other issues, like, it is incredibly hard
>> > and cumbersome to write a correct rtx_cost).
>> 
>> Thanks! The implementations of hook insn_cost are align with this
>> design, they are  checking insn's attributes and COSTS_N_INSNS.
>> 
>> One question on the speciall case: 
>> For instruction: "r119:DI=0x100803004101001"
>> Would we treat it as valid instruction?
>
> Currently we do, alternative 6 in *movdi_internal64: we allow any r<-n.
> This is costed as 5 insns (cost=20).
>
> It generally is better to split things into patterns close to the
> eventual machine isntructions as early as possible: all the more generic
> optimisations can take advantage of that then.
Get it!
>
>> A patch, which is attached the end of this mail, accepts
>> "r119:DI=0x100803004101001" as input of insn_cost.
>> In this patch, 
>> - A tmp instruction is generated via make_insn_raw.
>> - A few calls to rtx_cost (in cse_insn) is replaced by insn_cost.
>> - In hook of insn_cost, checking the special 'constant' instruction.
>> Are these make sense?
>
> I'll review that patch inline.
>
>> > That is one reason why it is better to generate (close to) machine
>> > insns as early as possible: it makes it much easier to estimate
>> > realistic costs.  (Another important reason is it allows other
>> > optimisations, without us having to do any work for it!)
>> Get it!  In the middle of an optimization pass, 'interim'
>> instruction maybe acceptable.  While it would better to outputs
>> only contains 'valid machine insn' from any RTL passes.
>
> Acceptable only if there is a very good reason for it, really :-(
>
>> --- a/gcc/config/rs6000/rs6000.cc
>> +++ b/gcc/config/rs6000/rs6000.cc
>> @@ -22131,6 +22131,16 @@ rs6000_debug_rtx_costs (rtx x, machine_mode mode, int outer_code,
>>  static int
>>  rs6000_insn_cost (rtx_insn *insn, bool speed)
>>  {
>> +  /* Handle special 'constant int' insn. */
>> +  rtx set = PATTERN (insn);
>> +  if (GET_CODE (set) == SET && CONSTANT_P (SET_SRC (set)))
>> +    {
>> +      rtx src = SET_SRC (set);
>> +      machine_mode mode = GET_MODE (SET_DEST (set));
>> +      if (CONST_INT_P (src) || CONST_WIDE_INT_P (src))
>> +	return COSTS_N_INSNS (num_insns_constant (src, mode));
>> +    }
>> +  
>>    if (recog_memoized (insn) < 0)
>>      return 0;
>
> Why would such a set not recog()?
Thanks.  This code is not need at the top of function insn_cost.
recog_memoized could check insn_code on 'insn'.
>
> Needs a comment in any case, to say what this is a workaround for.
>
>> +static int insn_cost_x (rtx_insn *, rtx);
>
> Don't declare functions, just put their definitions before their first
> use.  (And use a better name please :-) )
Get it. :-)
>
>>  static int
>> -notreg_cost (rtx x, machine_mode mode, enum rtx_code outer, int opno)
>> +notreg_cost (rtx x, machine_mode mode, enum rtx_code outer, int opno,
>> +	     rtx_insn *insn = NULL)
>
> Don't use default arguments like this, it is an abomination.
Thanks.
>
>> @@ -709,9 +713,21 @@ notreg_cost (rtx x, machine_mode mode, enum rtx_code outer, int opno)
>>  	   && subreg_lowpart_p (x)
>>  	   && TRULY_NOOP_TRUNCATION_MODES_P (int_mode, inner_mode))
>>  	  ? 0
>> +	  : insn != NULL ? insn_cost_x (insn, x)
>>  	  : rtx_cost (x, mode, outer, opno, optimize_this_for_speed_p) * 2);
>>  }
>
> You can just always use insn_cost?  insn_cost -> pattern_cost ->
> set_src_cost -> rtx_cost.  That works for COST at least, not sure about
> COST_IN, maybe that needs a little more care (cse.c works with invalid
> insns all over the place :-( )
>
This experiement patch just replace part of rtx_cost with insn_cost.
In case, COST is called outside cse_insn, 'insn' may not be set, and
then 'insn_cost' may not work.   This would need to be enhanced.
>>  
>> +/* Internal function, to get cost when use X to replace source of insn
>> +   which is a SET.  */
>> +
>> +static int
>> +insn_cost_x (rtx_insn *insn, rtx x)
>> +{
>> +  INSN_CODE (insn) = -1;
>> +  SET_SRC (PATTERN (insn)) = x;
>> +  return insn_cost (insn, optimize_this_for_speed_p);
>> +}
>
> You need to restore stuff as well?
In this patch, this function is called on a tmp_insn, so I did not
restore it.  If using the original 'insn' of cse_insn to invoked
'insn_cost_x', fields of 'insn' should be restored.
>
>> @@ -4603,6 +4619,7 @@ cse_insn (rtx_insn *insn)
>>  
>>       Nothing in this loop changes the hash table or the register chains.  */
>>  
>> +  rtx_insn *tmp_insn = NULL;
>>    for (i = 0; i < n_sets; i++)
>>      {
>>        bool repeat = false;
>> @@ -4638,6 +4655,10 @@ cse_insn (rtx_insn *insn)
>>        mode = GET_MODE (src) == VOIDmode ? GET_MODE (dest) : GET_MODE (src);
>>        sets[i].mode = mode;
>>  
>> +      if (tmp_insn == NULL_RTX && src && dest && dest != pc_rtx
>> +	  && src != pc_rtx)
>> +	tmp_insn = make_insn_raw (gen_rtx_SET (copy_rtx (dest), copy_rtx(src)));
>
> src and dest are always non-nil here.  I'll have to read the code better
> to know about the (pc) stuff.
>
>> @@ -5103,7 +5124,7 @@ cse_insn (rtx_insn *insn)
>>  	    src_cost = src_regcost = -1;
>>  	  else
>>  	    {
>> -	      src_cost = COST (src, mode);
>> +	      src_cost = COST_SRC (tmp_insn, src, mode);
>
> I think you can just leave this as COST?
>
Yes, it would be better to just use COST, and update COST macro
to use insn_cost.

Thanks a lot for your greate help!

BR,
Jiufu

>
> Segher