[PATCH] RISC-V: Optimize the cost of the DFmode register move for RV32.

Currently, in RV32, even with the D extension enabled, the cost of DFmode
register moves is still set to 'COSTS_N_INSNS (2)'. This results in the
'lower-subreg' pass splitting DFmode register moves into two SImode SUBREG
register moves, leading to the generation of many redundant instructions.

As an example, consider the following test case:
  double foo (int t, double a, double b)
  {
    if (t > 0)
      return a;
    else
      return b;
  }

When compiling with -march=rv32imafdc -mabi=ilp32d, the following code is generated:
          .cfi_startproc
          addi    sp,sp,-32
          .cfi_def_cfa_offset 32
          fsd     fa0,8(sp)
          fsd     fa1,16(sp)
          lw      a4,8(sp)
          lw      a5,12(sp)
          lw      a2,16(sp)
          lw      a3,20(sp)
          bgt     a0,zero,.L1
          mv      a4,a2
          mv      a5,a3
  .L1:
          sw      a4,24(sp)
          sw      a5,28(sp)
          fld     fa0,24(sp)
          addi    sp,sp,32
          .cfi_def_cfa_offset 0
          jr      ra
          .cfi_endproc

After adjust the DFmode register move's cost to 'COSTS_N_INSNS (1)', the
generated code is as follows, with a significant reduction in the number
of instructions.
          .cfi_startproc
          ble     a0,zero,.L5
          ret
  .L5:
          fmv.d   fa0,fa1
          ret
          .cfi_endproc

gcc/
	* config/riscv/riscv.cc (riscv_rtx_costs): Optimize the cost of the
	DFmode register move for RV32.

gcc/testsuite/
	* gcc.target/riscv/rv32-movdf-cost.c: New test.

0 files changed, 0 insertions, 0 deletions