[NEON] Shift left requires constant integer

[lawben]

When downloading the current tar.gz from v0.1.9-rc5 (and probably ones before that but I didn't check), I cannot compile the NEON stuff on Mac with LLVM/Clang 18. I get the following error multiple times.

cmake-build-release/_deps/tsl_gen-src/generate_tsl_neon-asimd/include/generated/definitions/binary/binary_neon.hpp:1117:104: error: argument to '__builtin_neon_vshlq_n_v' must be a constant integer
 1117 |                 return __extension__ ({ uint8x16_t __ret; uint8x16_t __s0 = data; __ret = (uint8x16_t) __builtin_neon_vshlq_n_v((int8x16_t)__s0, shift, 48); __ret; });

which is the expanded macro for:

[[nodiscard]] 
TSL_FORCE_INLINE 
static typename Vec::register_type apply(
    const typename Vec::register_type data, const unsigned int shift
) {
    return vshlq_n_u8(data, shift);  // <-- this is a macro in Clang
}

When looking at the NEON specs, this error is correct, as vshlq_n_u8 requires a const int as the second argument. I'm not sure how this should be handled, but probably this needs a vdup_n_* for the runtime value before the shift.

[JPietrzykTUD]

The most straightforward way (yet, probably not very efficient) would be to fallback to vshlq_u8:

/*...*/
return vshlq_u8(data, vdupq_n_u8(shift));

Another approach would be to change the interface and pass the shift value as an integral constant.

[lawben]

FYI: This also hold for vshrq_n_* (right shift). The common fix for this is to use a left shift with negative values.

[JPietrzykTUD]

This should be fixed with #97.

[alexKrauseTUD]

Thank you for pointing out the issue! As for the common fix: AFAIK left-shift using negative values is undefined behavior and not standard conforming.

• Stated at cppreference/Bitwise shift operators: "If the value of rhs is negative or is not less than the number of bits in lhs, the behavior is undefined."
• Stated in the C standard "The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined."

I would thus not include the workaround and rather rely on the vdup for now until we come up with something better.

[lawben]

That may be true fur C++ as a language, but specifically in NEON, this is well-defined and documented:

If the signed integer value is negative, it results in a right shift.