[Bug target/111139] New: RISC-V: improve scalar constants cost model

[Bug target/111139] New: RISC-V: improve scalar constants cost model

[vineetg at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111139

            Bug ID: 111139
           Summary: RISC-V: improve scalar constants cost model
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vineetg at gcc dot gnu.org
                CC: jeffreyalaw at gmail dot com, kito.cheng at gmail dot com,
                    palmer at dabbelt dot com
  Target Milestone: ---

The current const cost determination in riscv_rtx_costs () and its children
such as riscv_const_insns () needs improvements.

1. There's some likely inaccuracies with various heuristics.
2. Those heuristics are distributed in a bunch of places and better be
consolidated.
3. We need to make const cost cpu/uarch tunable as hardware widgets like macro
fusions could ammortize multi-insn const costs.


Some of the heuristics to cleanup/revisit:

1a. riscv_rtx_cost() returns 1 insn even if riscv_const_insns () returns > 1.

    case CONST:
      if ((cost = riscv_const_insns (x)) > 0)
        {
          if (cost == 1 && outer_code == SET)
            *total = COSTS_N_INSNS (1);
          else if (outer_code == SET || GET_MODE (x) == VOIDmode)
            *total = COSTS_N_INSNS (1);
        }

1b. riscv_const_insns () caps the const cost to 4 even if it higher with intent
to force const pool. RV backend in general no longer favors const pools for
large constants since 2e886eef7f2b5a ("RISC-V: Produce better code with complex
constants [PR95632] [PR106602]"). This heuristic needs to be revisited.

1c. riscv_split_integer_cost () adds 2 to initial cost computed.

[Bug target/111139] RISC-V: improve scalar constants cost model

[palmer at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111139

palmer at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-08-24
     Ever confirmed|0                           |1
                 CC|                            |palmer at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from palmer at gcc dot gnu.org ---
I think maybe Jeff said he was going to look at this?  IMO even just a
refactoring here would be great, like Vineet is pointing out it's become a bit
of a mess.

Also: Edwin is refactoring the rest of the types.

[Bug target/111139] RISC-V: improve scalar constants cost model

[vineetg at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111139

--- Comment #2 from Vineet Gupta <vineetg at gcc dot gnu.org> ---
Test case to help drive some of this:

unsigned long long f5(unsigned long long i)
{
  return i * 0x0202020202020202ULL;
}