gcc - 在avr-gcc中寻找_delay_ms调用的__builtin_avr_delay_cycles的源代码

Question

我正在调查 delay_ms avr-gcc 的功能.在 delay.h我找到了它的定义:

void _delay_ms(double __ms)
{
    double __tmp ;
#if __HAS_DELAY_CYCLES && defined(__OPTIMIZE__) && \
    !defined(__DELAY_BACKWARD_COMPATIBLE__) &&       \
    __STDC_HOSTED__

    uint32_t __ticks_dc;
    extern void __builtin_avr_delay_cycles(unsigned long);
    __tmp = ((F_CPU) / 1e3) * __ms;

    #if defined(__DELAY_ROUND_DOWN__)
        __ticks_dc = (uint32_t)fabs(__tmp);

    #elif defined(__DELAY_ROUND_CLOSEST__)
        __ticks_dc = (uint32_t)(fabs(__tmp)+0.5);

    #else
          //round up by default
          __ticks_dc = (uint32_t)(ceil(fabs(__tmp)));
    #endif

    __builtin_avr_delay_cycles(__ticks_dc);

#else
    ...
}

我感兴趣的是 __builtin_avr_delay_cycles函数在内部看起来像它在哪里定义？我在哪里可以找到来源？

Answer 1

正如我在对关于electronics.SE 的这个问题的评论中所说:

Compiler builtins are kinda funky to find, always, because they are not just C functions, but things that get inserted while parsing/compiling the code (at various levels of abstraction from the textual representation of the code itself. compiler theory stuff). What you're looking for is the function avr_expand_builtin in the GCC source tree. There's a case AVR_BUILTIN_DELAY_CYCLES in there. Look for what happens there.

这是:

/* Implement `TARGET_EXPAND_BUILTIN'.  */
/* Expand an expression EXP that calls a built-in function,
   with result going to TARGET if that's convenient
   (and in mode MODE if that's convenient).
   SUBTARGET may be used as the target for computing one of EXP's operands.
   IGNORE is nonzero if the value is to be ignored.  */

static rtx
avr_expand_builtin (tree exp, rtx target,
                    rtx subtarget ATTRIBUTE_UNUSED,
                    machine_mode mode ATTRIBUTE_UNUSED,
                    int ignore)
{
  tree fndecl = TREE_OPERAND (CALL_EXPR_FN (exp), 0);
  const char *bname = IDENTIFIER_POINTER (DECL_NAME (fndecl));
  unsigned int id = DECL_FUNCTION_CODE (fndecl);
  const struct avr_builtin_description *d = &avr_bdesc[id];
  tree arg0;
  rtx op0;

  gcc_assert (id < AVR_BUILTIN_COUNT);

  switch (id)
    {
    case AVR_BUILTIN_NOP:
      emit_insn (gen_nopv (GEN_INT (1)));
      return 0;

    case AVR_BUILTIN_DELAY_CYCLES:
      {
        arg0 = CALL_EXPR_ARG (exp, 0);
        op0 = expand_expr (arg0, NULL_RTX, VOIDmode, EXPAND_NORMAL);

        if (!CONST_INT_P (op0))
          error ("%s expects a compile time integer constant", bname);
        else
          avr_expand_delay_cycles (op0);

        return NULL_RTX;
      }
…

因此，您正在寻找的功能是 avr_expand_delay_cycles在同一个文件中:

static void
avr_expand_delay_cycles (rtx operands0)
{
  unsigned HOST_WIDE_INT cycles = UINTVAL (operands0) & GET_MODE_MASK (SImode);
  unsigned HOST_WIDE_INT cycles_used;
  unsigned HOST_WIDE_INT loop_count;

  if (IN_RANGE (cycles, 83886082, 0xFFFFFFFF))
    {
      loop_count = ((cycles - 9) / 6) + 1;
      cycles_used = ((loop_count - 1) * 6) + 9;
      emit_insn (gen_delay_cycles_4 (gen_int_mode (loop_count, SImode),
                                     avr_mem_clobber()));
      cycles -= cycles_used;
    }

  if (IN_RANGE (cycles, 262145, 83886081))
    {
      loop_count = ((cycles - 7) / 5) + 1;
      if (loop_count > 0xFFFFFF)
        loop_count = 0xFFFFFF;
      cycles_used = ((loop_count - 1) * 5) + 7;
      emit_insn (gen_delay_cycles_3 (gen_int_mode (loop_count, SImode),
                                     avr_mem_clobber()));
      cycles -= cycles_used;
    }

  if (IN_RANGE (cycles, 768, 262144))
    {
      loop_count = ((cycles - 5) / 4) + 1;
      if (loop_count > 0xFFFF)
        loop_count = 0xFFFF;
      cycles_used = ((loop_count - 1) * 4) + 5;
      emit_insn (gen_delay_cycles_2 (gen_int_mode (loop_count, HImode),
                                     avr_mem_clobber()));
      cycles -= cycles_used;
    }

  if (IN_RANGE (cycles, 6, 767))
    {
      loop_count = cycles / 3;
      if (loop_count > 255)
        loop_count = 255;
      cycles_used = loop_count * 3;
      emit_insn (gen_delay_cycles_1 (gen_int_mode (loop_count, QImode),
                                     avr_mem_clobber()));
      cycles -= cycles_used;
    }

  while (cycles >= 2)
    {
      emit_insn (gen_nopv (GEN_INT (2)));
      cycles -= 2;
    }

  if (cycles == 1)
    {
      emit_insn (gen_nopv (GEN_INT (1)));
      cycles--;
    }
}

这里最有趣的是，这会修改抽象语法树中的一个节点，并在那里发出指令。