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. --- gcc/config/riscv/riscv.cc | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) gcc-riscv-rtx-costs-if-then-else.diff Index: gcc/gcc/config/riscv/riscv.cc =================================================================== --- gcc.orig/gcc/config/riscv/riscv.cc +++ gcc/gcc/config/riscv/riscv.cc @@ -1853,6 +1853,33 @@ riscv_rtx_costs (rtx x, machine_mode mod /* 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;