modulo VS BIT-AND operation

[bigbro-pg]

Hi，
I found the function of index in file _concurrent_queue_base.h using modulo to caculate the index.
Why not use the more efficient opeartion of BIT opeartor like k * phi & (n_queue - 1);

[dnmokhov]

Thank you for submitting this question, @bigbro-pg. The inline function that is used for modulo, modulo_power_of_two, is already implemented with the BIT-AND:

oneTBB/include/oneapi/tbb/detail/_utils.h

Lines 177 to 182 in c456844

	// A function to compute arg modulo divisor where divisor is a power of 2.
	template<typename ArgIntegerType, typename DivisorIntegerType>
	inline ArgIntegerType modulo_power_of_two(ArgIntegerType arg, DivisorIntegerType divisor) {
	__TBB_ASSERT( is_power_of_two(divisor), "Divisor should be a power of two" );
	return arg & (divisor - 1);
	}

[dnmokhov]

And w.r.t. concurrent_queue_rep::index,

oneTBB/include/oneapi/tbb/detail/_concurrent_queue_base.h

Lines 487 to 490 in eac9465

	// Map ticket_type to an array index
	static size_type index( ticket_type k ) {
	return k * phi % n_queue;
	}

modern C++ compilers automatically optimize it to a bit-mask, since n_queue is a compile-time 2ⁿ:

oneTBB/include/oneapi/tbb/detail/_concurrent_queue_base.h

Lines 422 to 423 in eac9465

	// must be power of 2
	static constexpr size_type n_queue = 8;