NEON 内在，使用 vshr_n_u32 时编译错误 "argument must be a constant"

Question

我在使用 NEON 内部 vshr_n_u32 时遇到编译错误 "argument must be a constant"。函数原型为：

__extension__ static __inline uint32x2_t __attribute__ ((__always_inline__))
vshr_n_u32 (uint32x2_t __a, const int __b)
{
  return (uint32x2_t)__builtin_neon_vshr_nv2si ((int32x2_t) __a, __b, 0);
}

这是我的函数：

uint32x2_t shift_func(int index)
{
    int shift_bit[] = {2, 4, 5, 6, 7, 8, 9, 10};
    int n_val = shift_bit[index];

    uint32x2_t src_reg = {16, 32};
    return vshr_n_u32(src_reg, n_val);
}

n_val 的值只能在运行时知道。但是根据编译错误，似乎 n_val 的值应该在编译时就知道了。虽然__b的类型是const int，但我觉得输入一个int变量应该是对的

如何消除编译错误？或者如何在这个函数中使用vshr_n_u32？

Answer 1

vshr_n_u32 has to be a compile-time literal constant, as the shift value is encoded as part of the ARM instruction itself. Since you only have a few possible shift values then you might want to use a switch statement to handle each case. Or you could just use vshl_u32 中的偏移为负偏移（您传递第二个包含偏移值的 int32x2_t 参数）。

NEON 内在，使用 vshr_n_u32 时编译错误 "argument must be a constant"

NEON intrinsic, compile error "argument must be a constant" when using vshr_n_u32

c

arm

simd

neon