digitalmars.D - Multiple return values...
Manu <turkeyman gmail.com> writes:
I find myself really wishing for proper multiple return values almost every day, particularly when I work with maths heavy code, and especially for efficiently returning error codes in functions I'd rather not throw from. Many maths-y functions return some sort of pair; intersections return (ray, t) or something of that type. I'm finding HEAPS of SIMD functions want to return pairs (unpacks in particular): int4 (low, hight) = unpack(someShort8); Currently I have to duplicate everyting: int4 low = unpackLow(someShort8); int4 high = unpackHigh(someShort8); I'm getting really sick of that, it feels so... last millennium. The point of 'proper' multiple return values is to return each value in registers, in its own register type, using exactly the same register assignment pattern as when passing args TO functions. I don't think this causes any side effects to the ABI, since the arg registers are already volatile across function calls in the first place. It just means that the returned-to function can find its return values already conveniently in an appropriate register, avoiding memory access. People argue I should return a tuple, but this isn't really the same, it has hidden implications that complicate the optimisation potential. For instance, tuples have an implicit structure/memory layout which can't be ignored, whereas return values don't have memory allocated, ie, you can't take the address of a return value without first assigning it to some local syntactically. The implementation of efficient tuple return values would be much more complicated I would imagine too, and the rules are less clear; I can't intuitively presume what behaviour returning a tuple of different things should actually have in terms of register assignment. I also know precisely how multiple return values should work, because it is exactly the same as passing arguments to the function, but in reverse. ... just saying :)
Mar 08 2012
"a" <a a.com> writes:
I'm finding HEAPS of SIMD functions want to return pairs (unpacks in particular): int4 (low, hight) = unpack(someShort8); Currently I have to duplicate everyting: int4 low = unpackLow(someShort8); int4 high = unpackHigh(someShort8); I'm getting really sick of that, it feels so... last millennium.
It can also be realy inefficient. For example ARM NEON has vzip instruction that is used like this: vzip.32 q0, q1 This will interleave elements of vectors in q0 and q1 in one instruction.
Mar 09 2012