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

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

Hi!

Richard Biener <rguenther@suse.de> writes:

> On Tue, 8 Mar 2022, Jiufu Guo wrote:
>
>> Jiufu Guo <guojiufu@linux.ibm.com> writes:
>> 
>> Hi!
>> 
>> > 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.
>> 
>> I drafted a new patch that replace rtx_cost with insn_cost for cse.cc.
>> Different from the previous partial patch, this patch replaces all usage
>> of rtx_cost. It may be better/aggressive than previous one.
>
> I think there's no advantage for using insn_cost over rtx_cost for
> the simple SET case.

Thanks for your comments and raise this concern.

For those targets which do not implement insn_cost, insn_cost calls
rtx_cost through pattern_cost, then insn_cost is equal to rtx_cost.

While, for those targets which have insn_cost, it seems insn_cost would
be better(or say more accurate/consistent?) than rtx_cost. Since:
- insn_cost recog the insn first, and compute cost through something
(like length/cost attributes from .md file) for the 'machine insn'.
- rtx_cost estimates the cost through analyzing the 'rtx content'.
The accurate estimation relates to the context.

For a special example: "%r100 = C", as a previous patch, by tunning
target's rtx_cost hook, cost could be computed according to the value
of C. insn_cost may just model the cost in the define of the machine
instruction.

These reasons are my initial thoughts.  Segher may have better
explain. :-) 

To replace rtx_cost with insn_cost, this patch build a SET instruction:
"%r = rtx_expr", then using "%r = rtx_expr" from insn_cost to simulate
the cost of "rtx_expr" from rtx_cost.


BR,
Jiufu

>
> Richard.
>
>> With this patch, bootstrap pass.
>> From regtest, only output of fusion-p10-ldcmpi.c is changed, and the
>> change seems as expected.
>> 
>> 
>> BR,
>> Jiufu
>> 
>> diff --git a/gcc/cse.cc b/gcc/cse.cc
>> index a18b599d324..e623ad298db 100644
>> --- a/gcc/cse.cc
>> +++ b/gcc/cse.cc
>> @@ -262,6 +262,9 @@ static struct qty_table_elem *qty_table;
>>  static rtx_insn *this_insn;
>>  static bool optimize_this_for_speed_p;
>>  
>> +/* Used for insn_cost. */
>> +static rtx_insn *estimate_insn;
>> +
>>  /* Index by register number, gives the number of the next (or
>>     previous) register in the chain of registers sharing the same
>>     value.
>> @@ -445,7 +448,7 @@ struct table_elt
>>  /* Compute cost of X, as stored in the `cost' field of a table_elt.  Fixed
>>     hard registers and pointers into the frame are the cheapest with a cost
>>     of 0.  Next come pseudos with a cost of one and other hard registers with
>> -   a cost of 2.  Aside from these special cases, call `rtx_cost'.  */
>> +   a cost of 2.  Aside from these special cases, call `insn_cost'.  */
>>  
>>  #define CHEAP_REGNO(N)							\
>>    (REGNO_PTR_FRAME_P (N)						\
>> @@ -698,18 +701,33 @@ preferable (int cost_a, int regcost_a, int cost_b, int regcost_b)
>>     from COST macro to keep it simple.  */
>>  
>>  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*/)
>>  {
>>    scalar_int_mode int_mode, inner_mode;
>> -  return ((GET_CODE (x) == SUBREG
>> -	   && REG_P (SUBREG_REG (x))
>> -	   && is_int_mode (mode, &int_mode)
>> -	   && is_int_mode (GET_MODE (SUBREG_REG (x)), &inner_mode)
>> -	   && GET_MODE_SIZE (int_mode) < GET_MODE_SIZE (inner_mode)
>> -	   && subreg_lowpart_p (x)
>> -	   && TRULY_NOOP_TRUNCATION_MODES_P (int_mode, inner_mode))
>> -	  ? 0
>> -	  : rtx_cost (x, mode, outer, opno, optimize_this_for_speed_p) * 2);
>> +  if (GET_CODE (x) == SUBREG && REG_P (SUBREG_REG (x))
>> +      && is_int_mode (mode, &int_mode)
>> +      && is_int_mode (GET_MODE (SUBREG_REG (x)), &inner_mode)
>> +      && GET_MODE_SIZE (int_mode) < GET_MODE_SIZE (inner_mode)
>> +      && subreg_lowpart_p (x)
>> +      && TRULY_NOOP_TRUNCATION_MODES_P (int_mode, inner_mode))
>> +    return 0;
>> +
>> +  if (estimate_insn == NULL)
>> +    {
>> +      estimate_insn = make_insn_raw (
>> +	gen_rtx_SET (gen_rtx_REG (mode, LAST_VIRTUAL_REGISTER + 1), x));
>> +      SET_PREV_INSN (estimate_insn) = NULL_RTX;
>> +      SET_NEXT_INSN (estimate_insn) = NULL_RTX;
>> +      INSN_LOCATION (estimate_insn) = 0;
>> +    }
>> +  else
>> +    {
>> +      /* Update for new context.  */
>> +      INSN_CODE (estimate_insn) = -1;
>> +      PUT_MODE (SET_DEST (PATTERN (estimate_insn)), mode);
>> +      SET_SRC (PATTERN (estimate_insn)) = x;
>> +    }
>> +  return insn_cost (estimate_insn, optimize_this_for_speed_p);
>>  }
>>  
>>  \f
>> @@ -6667,6 +6685,7 @@ cse_main (rtx_insn *f ATTRIBUTE_UNUSED, int nregs)
>>  
>>    init_recog ();
>>    init_alias_analysis ();
>> +  estimate_insn = NULL;
>>  
>>    reg_eqv_table = XNEWVEC (struct reg_eqv_elem, nregs);
>>  
>>