[LLVMdev] Status of first-class aggregate types

Talin viridia at gmail.com
Sun Dec 20 10:27:36 PST 2009

On Sun, Dec 20, 2009 at 9:50 AM, Chris Lattner <clattner at apple.com> wrote:

> On Dec 20, 2009, at 12:27 AM, Talin wrote:
> What's the current status on support for first-class structs? The last I
> heard was:
>    - Structs which are smaller or equal to two pointers can be passed /
>    returned / loaded / stored by value.
>    - There are plans to expand this in the future to support
>    arbitrary-sized structs as first class values. (Probably via some
>    transformation pass the converts the return value into a hidden parameter.)
> First question - are those statements still true?
> Yes, I think so.  Also support is actually better than that, the sticking
> issue occurs when passing and returning large aggregates.  I think that
> improved recently but am not sure of the status.
> The reason I am asking all of this is that I am starting to work on
> supporting tuple types in my language - and I'm trying to decide what is the
> best way to implement them. Since these are value types, not reference
> types, they need to be copied when passing to or returning from a function.
> The two approaches I am considering is (a) always pass them by reference
> internally, using a hidden parameter for the return value, and then have the
> receiver of the value copy it into an SSA value before use, and (b) for
> types <= 2 pointers, use SSA values only, and then pass larger types as in
> case (a). Of course, if support for large, first-class SSA values was
> finished, then I wouldn't have to care about any of these issues :)
> I'd pass them by value if they are small but by reference if they are
> large.  Passing large tuples by value isn't going to provide a win.
> OK, thanks for that confirmation, now I can proceed ahead with less
trepidation. :)

For large aggregates (well, not huge, but the size of a typical structure or
class), do you recommend representing them as SSA values internally within a
function, or using allocas? That is, even if they are going to be passed by
reference, the question is whether they should also be internally
represented that way.

So for example, if I have say a tuple of (double, double, double)
representing a 3d coordinate which is a value type, and I pass that to a
function, the first thing I want to do is copy it so that mutations don't
affect the caller. I can either load it into an SSA value and keep it there,
or I can create an alloca, load it and then store it to the alloca. What I'd
like to know is whether either approach is likely to produce more efficient
code (it's about the same amount of work for me to implement either way.)

The other question I'd have is whether there's a good target-independent
test (it can be a conservative test) to know whether or not something can be
passed / returned as an SSA value. At the moment I am using a really dodgy
heuristic. Of course, I realize that this would be a much better decision to
make after I've selected a target, but that requires my linker to re-write
function signatures which frankly I am not sure how to do :)


-- Talin
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