[go-nuts] Multiple assignment question

Yes, a, b = b, a uses a temp variable

Technically I think it evaluates and stores in registers before assigning.
It's not temporary "variables" as such.

Temporary value, then.

The exchange operation

x, y = y, x

certainly suggests

t := x
x = y
y = t

...but a compiler, with no spare registers, might choose to code it as

x ^= y
y ^= x
x ^= y

where the values x and y are treated as if they are 8, 16, 32, or 64 bit
ints no matter what their actual types might be.

I don't think this is the case, but I honestly don't know the compiler well
enough to say. I'm getting my info from here:
https://code.google.com/p/go/issues/detail?id=3126#c5

No, sorry --- the compiler's don't do that. Just saying that it's always a
possibility.

Oh, right, certainly. Missed "might choose", sorry.

While not relevant to current go compilers, most/many VLIW machines
naturally support multiple assignment in the hardware. The actual
instruction for x,y = y, x is generally one which simply executes x = y and
y = x concurrently, but with the semantic rule that all the right hands are
evaluated first and in parallel, and then all the right hands are executed
in parallel. In the processor pipeline, all the source registers are read
in parallel, an operation (a null op in this case) is performed in the next
stage, and then in the next stage the values are all written in parallel.

stack or even heap because of free register exhaustion, using the xor-trick
can be the last resort to avoid allocation and book-keeping. Besides that,
I would assume a plain assignment to a register instead of three XORs to be
more effective in terms of OPS.