选项 -split-ldp-stp

说明

识别某些性能表现差的 ldp/stp，将其拆分成2个 ldr 和 str。

使用方法

使用-fsplit-ldp-stp选项使能优化。

• 使用参数--param=param-ldp-dependency-search-range=[1,32]控制搜索范围，默认16。

注：依赖-O1及以上优化等级。

结果

测试用例如下：

int __RTL (startwith ("split_complex_instructions"))
simple_ldp_after_store ()
{
(function "simple_ldp_after_store"
  (insn-chain
    (block 2
      (edge-from entry (flags "FALLTHRU"))
      (cnote 3 [bb 2] NOTE_INSN_BASIC_BLOCK)
      (cinsn 228 (set (reg/i:DI sp)
                   (reg/i:DI x0)))
      (cinsn 238 (set (reg/i:DI x1)
                   (reg/i:DI x0)))
      (cinsn 101 (set (mem/c:DI
                        (plus:DI (reg/f:DI sp)
                          (const_int 8))[1 S4 A32])(reg:DI x0)))
      (cinsn 10 (parallel [
        (set (reg:DI x29)
          (mem:DI (plus:DI (reg/f:DI sp) (const_int 8)) [1 S4 A32]))
        (set (reg:DI x30)
          (mem:DI (plus:DI (reg/f:DI sp)
            (const_int 16)) [1 S4 A32]))]))
      (cinsn 102 (set (mem/c:DI (plus:DI (reg/f:DI x1)
                                          (const_int -16)) [1 S4 A32])
                      (reg:DI x0)))
      (cinsn 11 (parallel [
        (set (reg:DI x3)
          (mem:DI (plus:DI (reg/f:DI x1) (const_int -16)) [1 S4 A32]))
        (set (reg:DI x4)
          (mem:DI (plus:DI (reg/f:DI x1) (const_int -8)) [1 S4 A32]))
      ]))
      (cinsn 103 (set (mem/c:DI (reg/f:DI x1) [1 S4 A32])
                      (reg:DI x0)))
      (cinsn 12 (parallel [
        (set (reg:DI x5) (mem:DI (reg/f:DI x1) [1 S4 A32]))
        (set (reg:DI x6) (mem:DI (plus:DI (reg/f:DI x1)
                                          (const_int 8)) [1 S4 A32]))
      ]))
      (cinsn 13 (use (reg/i:DI sp)))
      (cinsn 14 (use (reg/i:DI cc)))
      (cinsn 15 (use (reg/i:DI x29)))
      (cinsn 16 (use (reg/i:DI x30)))
      (cinsn 17 (use (reg/i:DI x0)))
      (cinsn 18 (use (reg/i:DI x3)))
      (cinsn 19 (use (reg/i:DI x4)))
      (cinsn 20 (use (reg/i:DI x5)))
      (cinsn 21 (use (reg/i:DI x6)))
      (edge-to exit (flags "FALLTHRU"))
    ) ;; block 2
  ) ;; insn-chain
) ;; function "simple_ldp_after_store"
}

测试命令：

gcc -O1 -fsplit-ldp-stp -S test.c -o test.s

相比选项未打开时，选项打开后，生成的汇编代码指令不存在ldp指令，而是拆分为两个ldr指令。