[gcc(refs/vendors/riscv/heads/gcc-14-with-riscv-opts)] [PATCH] RISC-V: Optimize the cost of the DFmode register move for RV32.

[Jeff Law]

https://gcc.gnu.org/g:82d58b78291d4108435da4411e8ebfa840ee4dc9

commit 82d58b78291d4108435da4411e8ebfa840ee4dc9
Author: Xianmiao Qu <cooper.qu@linux.alibaba.com>
Date:   Sun Sep 1 22:28:13 2024 -0600

    [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.
    
    (cherry picked from commit eca320bfe340be2222ec9267bdb6021c7b387111)

Diff:
---
 gcc/config/riscv/riscv.cc                        |  5 +++++
 gcc/testsuite/gcc.target/riscv/rv32-movdf-cost.c | 13 +++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 5e9870a574c..e634173bd53 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -3600,6 +3600,11 @@ riscv_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno ATTRIBUTE_UN
       if (outer_code == INSN
 	  && register_operand (SET_DEST (x), GET_MODE (SET_DEST (x))))
 	{
+	  if (REG_P (SET_SRC (x)) && TARGET_DOUBLE_FLOAT && mode == DFmode)
+	    {
+	      *total = COSTS_N_INSNS (1);
+	      return true;
+	    }
 	  riscv_rtx_costs (SET_SRC (x), mode, outer_code, opno, total, speed);
 	  return true;
 	}
diff --git a/gcc/testsuite/gcc.target/riscv/rv32-movdf-cost.c b/gcc/testsuite/gcc.target/riscv/rv32-movdf-cost.c
new file mode 100644
index 00000000000..cb679e7b95f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/rv32-movdf-cost.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv32imafdc -mabi=ilp32d" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" } } */
+
+double foo (int t, double a, double b)
+{
+  if (t > 0)
+    return a;
+  else
+    return b;
+}
+
+/* { dg-final { scan-assembler-not "fsd\t" } } */