[LLVMdev] Status of first-class aggregate types

[John McCall]

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?

I can't answer this or most of your other questions, but:

> Another thing I noticed recently was that although we have insert/extract value, there's no 'constructvalue' instruction. I'm thinking of something that would take a structure type + a list of individual field values and returns a first class aggregate. Right now, if you want to construct a new aggregate, it seems that you either have to alloca it, use GEP to fill in the fields, and then load it - or you have to alloca it, load the uninitialized struct, and then use successive insertvalue instructions to initialize it. At least, I can't think of another way to do it. (Of course, you can get SSA struct values from other places, but I'm talking about creating a fresh value.)

The way to do this is to start with undef and repeatedly insert into it.  For example:
  %0 = insertvalue undef {i1,i8*}, i32 0, i1 0
  %1 = insertvalue {i1,i8*} %0, i32 1, i8* %pointer

Arguably there should be a helper method for this on IRBuilder.

John.
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