Are there any performance differences between representing a number using a (4 byte) `int` and a 4 element unsigned char array?

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Assuming an int in C++ is represented by 4 bytes, and an unsigned char is represented by 1 byte, you could represent an int with an array of unsigned char with 4 elements right?

My question is, are there any performance downsides to representing a number with an array of unsigned char? Like if you wanted to add two numbers together would it be just as fast to do int + int compared to adding each element in the array and dealing with carries manually?

This is just me trying to experiment and to practice working with bytes rather than some practical application.

memory byte arrays c++
2 Answers

There will be many performance downsides on any kind of manipulation using the 4-byte array. For example, take simple addition: almost any CPU these days will have a single instruction that adds two 32-bit integers, in one (maybe two) CPU cycle(s). To emulate that with your 4-byte array, you would need at least 4 separate CPU instructions.

Further, many CPUs actually work faster with 32- or 64-bit data than they do with 8-bit data - because their internal registers are optimized for 32- and 64-bit operands.

user_108710730

Let's scale your question up. Is there any performance difference between single addition of two 16 byte variables compared to four separate additions of 4 byte variables? And here comes the concept of vector registers and vector instructions (MMX, SSE, AVX). It's pretty much the same story, SIMD is always faster, because there is literally less instructions to execute and the whole operation is done by dedicated hardware. On top of that, in your question you also have to take into account that modern CPUs don't work with 1 byte variables, instead they still process 32 or 64 bits at once anyway. So effectively you will do 4 individual additions using 4 byte registers, only to use single lower byte each time and then manually handle carry bit. Yeah, that will be very slow.

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