[gcc/devel/sphinx] RISC-V: Add RTX costs for `if_then_else' expressions

[Martin Liska <marxin@gcc.gnu.org>]

https://gcc.gnu.org/g:449563eb5eedcabefa34441936c2f6bac710d68f

commit 449563eb5eedcabefa34441936c2f6bac710d68f
Author: Maciej W. Rozycki <macro@embecosm.com>
Date:   Wed Jul 27 11:09:42 2022 +0100

    RISC-V: Add RTX costs for `if_then_else' expressions
    
    Fix a performance regression from commit 391500af1932 ("Do not ignore
    costs of jump insns in combine."), a part of the m68k series for MODE_CC
    conversion (<https://gcc.gnu.org/ml/gcc-patches/2019-11/msg01028.html>),
    observed in soft-fp code in libgcc used by some of the embench-iot
    benchmarks.
    
    The immediate origin of the regression is the middle end, which in the
    absence of cost information from the backend estimates the cost of an
    RTL expression by assuming a single machine instruction for each of the
    expression's subexpression.
    
    So for `if_then_else', which takes 3 operands, the estimated cost is 3
    instructions (i.e. 12 units) even though a branch instruction evaluates
    it in a single machine cycle (ignoring the cost of actually taking the
    branch of course, which is handled elsewhere).  Consequently an insn
    sequence like:
    
    (insn 595 594 596 43 (set (reg:DI 305)
            (lshiftrt:DI (reg/v:DI 160 [ R_f ])
                (const_int 55 [0x37]))) ".../libgcc/soft-fp/adddf3.c":46:3 216 {lshrdi3}
         (nil))
    (insn 596 595 597 43 (set (reg:DI 304)
            (and:DI (reg:DI 305)
                (const_int 1 [0x1]))) ".../libgcc/soft-fp/adddf3.c":46:3 109 {anddi3}
         (expr_list:REG_DEAD (reg:DI 305)
            (nil)))
    (jump_insn 597 596 598 43 (set (pc)
            (if_then_else (eq (reg:DI 304)
                    (const_int 0 [0]))
                (label_ref:DI 1644)
                (pc))) ".../libgcc/soft-fp/adddf3.c":46:3 237 {*branchdi}
         (expr_list:REG_DEAD (reg:DI 304)
            (int_list:REG_BR_PROB 536870916 (nil)))
     -> 1644)
    
    does not (anymore, as from the commit referred) get combined into:
    
    (note 595 594 596 43 NOTE_INSN_DELETED)
    (note 596 595 597 43 NOTE_INSN_DELETED)
    (jump_insn 597 596 598 43 (parallel [
                (set (pc)
                    (if_then_else (eq (zero_extract:DI (reg/v:DI 160 [ R_f ])
                                (const_int 1 [0x1])
                                (const_int 55 [0x37]))
                            (const_int 0 [0]))
                        (label_ref:DI 1644)
                        (pc)))
                (clobber (scratch:DI))
            ]) ".../libgcc/soft-fp/adddf3.c":46:3 243 {*branch_on_bitdi}
         (int_list:REG_BR_PROB 536870916 (nil))
     -> 1644)
    
    This is because the new cost is incorrectly calculated as 28 units while
    the cost of the original 3 instructions was 24:
    
    rejecting combination of insns 595, 596 and 597
    original costs 4 + 4 + 16 = 24
    replacement cost 28
    
    Before the commit referred the cost of jump instruction was ignored and
    considered 0 (i.e. unknown) and a sequence of instructions of a known
    cost used to win:
    
    allowing combination of insns 595, 596 and 597
    original costs 4 + 4 + 0 = 0
    replacement cost 28
    
    Add the missing costs for the 3 variants of `if_then_else' expressions
    we currently define in the backend.
    
    With the fix in place the cost of this particular `if_then_else' pattern
    is 2 instructions or 8 units (because of the shift operation) and
    therefore the ultimate cost of the original 3 RTL insns will work out at
    16 units (4 + 4 + 8), however the replacement single RTL insn will cost
    8 units only.
    
            gcc/
            * config/riscv/riscv.cc (riscv_rtx_costs) <IF_THEN_ELSE>: New
            case.

Diff:
---
 gcc/config/riscv/riscv.cc | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 2e83ca07394..5a0adffb5ce 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -1853,6 +1853,33 @@ riscv_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno ATTRIBUTE_UN
       /* Otherwise use the default handling.  */
       return false;
 
+    case IF_THEN_ELSE:
+      if (TARGET_SFB_ALU
+	  && register_operand (XEXP (x, 1), mode)
+	  && sfb_alu_operand (XEXP (x, 2), mode)
+	  && comparison_operator (XEXP (x, 0), VOIDmode))
+	{
+	  /* For predicated conditional-move operations we assume the cost
+	     of a single instruction even though there are actually two.  */
+	  *total = COSTS_N_INSNS (1);
+	  return true;
+	}
+      else if (LABEL_REF_P (XEXP (x, 1)) && XEXP (x, 2) == pc_rtx)
+	{
+	  if (equality_operator (XEXP (x, 0), mode)
+	      && GET_CODE (XEXP (XEXP (x, 0), 0)) == ZERO_EXTRACT)
+	    {
+	      *total = COSTS_N_INSNS (SINGLE_SHIFT_COST + 1);
+	      return true;
+	    }
+	  if (order_operator (XEXP (x, 0), mode))
+	    {
+	      *total = COSTS_N_INSNS (1);
+	      return true;
+	    }
+	}
+      return false;
+
     case NOT:
       *total = COSTS_N_INSNS (GET_MODE_SIZE (mode) > UNITS_PER_WORD ? 2 : 1);
       return false;