当我尝试编译这段代码时:

#include <iostream>
#include <limits.h>

// End recursive template-expansion of function select below.
template <typename Type>
static inline constexpr Type select(unsigned index)
{ return Type(); }

// Select one of the items passed to it.
// e.g. select(0, a, b, c) = a; select(1, a, b, c) = b; etc.
template <typename Type, typename... Params>
[[gnu::always_inline]]
static inline constexpr Type select(unsigned index, Type value, Params... values)
{ return index == 0 ? value : select<Type>(index - 1, values...); }

template <typename Type>
[[gnu::always_inline]]
static inline constexpr Type reflect_mask_helper_1(Type mask, Type shift, Type value)
{ return ((value & mask) >> shift) | ((value << shift) & mask); }

template <typename Type>
[[gnu::always_inline]]
static inline constexpr Type reflect_mask_helper_0(unsigned i, Type value)
{
  return i == 0
    ? value
    : reflect_mask_helper_0(
        i - 1,
        reflect_mask_helper_1<Type>(
          select(i - 1, 0xaaaaaaaaaaaaaaaa, 0xcccccccccccccccc, 0xf0f0f0f0f0f0f0f0,
                        0xff00ff00ff00ff00, 0xffff0000ffff0000, 0xffffffff00000000),
          1 << (i - 1),
          value));
}

template <typename Type>
[[gnu::flatten]]
static inline constexpr Type reflect_mask(Type value)
{ return reflect_mask_helper_0(__builtin_ctz(sizeof(Type) * CHAR_BIT), value); }

int main(void) {
  for (int i = 0; i < 65536; i++) {
    std::cout << reflect_mask<uint16_t>(i) << std::endl;
  }
}

gcc 给我一个错误，说函数 reflect_mask_helper_0 不能被内联，因为它是递归的。然而，函数 select 也是递归的，但 gcc 没有提示地将它内联。我在这里缺少什么？

(我需要它是递归的，因为 constexpr 函数在 C++11 下不能包含循环。)

错误信息:

% g++ test.cpp -O3 -march=native -c
test.cpp: In function ‘constexpr Type reflect_mask_helper_0(unsigned int, Type) [with Type = short unsigned int]’:
test.cpp:23:30: error: inlining failed in call to always_inline ‘constexpr Type reflect_mask_helper_0(unsigned int, Type) [with Type = short unsigned int]’: recursive inlining
   23 | static inline constexpr Type reflect_mask_helper_0(unsigned i, Type value)
      |                              ^~~~~~~~~~~~~~~~~~~~~
test.cpp:27:28: note: called from here
   27 |     : reflect_mask_helper_0(
      |       ~~~~~~~~~~~~~~~~~~~~~^
   28 |         i - 1,
      |         ~~~~~~              
   29 |         reflect_mask_helper_1<Type>(
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   30 |           select(i - 1, 0xaaaaaaaaaaaaaaaa, 0xcccccccccccccccc, 0xf0f0f0f0f0f0f0f0,
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   31 |                         0xff00ff00ff00ff00, 0xffff0000ffff0000, 0xffffffff00000000),
      |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   32 |           1 << (i - 1),
      |           ~~~~~~~~~~~~~     
   33 |           value));
      |           ~~~~~~~           
test.cpp: In function ‘int main()’:
test.cpp:23:30: error: inlining failed in call to always_inline ‘constexpr Type reflect_mask_helper_0(unsigned int, Type) [with Type = short unsigned int]’: recursive inlining
   23 | static inline constexpr Type reflect_mask_helper_0(unsigned i, Type value)
      |                              ^~~~~~~~~~~~~~~~~~~~~
test.cpp:27:28: note: called from here
   27 |     : reflect_mask_helper_0(
      |       ~~~~~~~~~~~~~~~~~~~~~^
   28 |         i - 1,
      |         ~~~~~~              
   29 |         reflect_mask_helper_1<Type>(
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   30 |           select(i - 1, 0xaaaaaaaaaaaaaaaa, 0xcccccccccccccccc, 0xf0f0f0f0f0f0f0f0,
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   31 |                         0xff00ff00ff00ff00, 0xffff0000ffff0000, 0xffffffff00000000),
      |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   32 |           1 << (i - 1),
      |           ~~~~~~~~~~~~~     
   33 |           value));
      |           ~~~~~~~

最佳答案

select实际上并不调用自身。它弹出它收到的类型列表的前面，然后调用 select<Type, ...> 的另一个特化 .尾随参数包不同。由于该“递归”本质上是一组有限的嵌套函数调用(不同的函数)，GCC 可以直接看穿它，而不管运行时参数如何。

但是reflect_mask_helper_0会无限期地使用相同的模板参数调用自身。 GCC 无法判断此运行时递归在运行时将进行多深。回想一下 constexpr函数仍然是必须在运行时调用的常规函数​​。

关于c++ - 递归函数的内联，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/57765645/