CODEWITHSUNDEEP
carmsimdneon
maofu
maofu

Reputation: 171

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

I got a compile error "argument must be a constant" when using NEON intrinsic vshr_n_u32. The function prototype is:

__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);
}

And here is my function:

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);
}

The value of n_val only can be known at run-time. But according to compile error, it seems that the value of n_val should be known at compile-time. Although the type of __b is const int, I think it should be right to input an int variable.

How to remove the compile error? Or how to use vshr_n_u32 in this function?

Upvotes: 1

Views: 3428

Answers (1)

Paul R
Paul R

Reputation: 212979

The shift in 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 with a negative shift (you pass a second int32x2_t parameter containing the shift values).

Upvotes: 3

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