[LLVMdev] [PATCH] - Union types, attempt 2

Dustin Laurence dllaurence at dslextreme.com
Fri Jan 15 13:51:36 PST 2010

On 01/15/2010 11:37 AM, Talin wrote:

>     Yes, that's closer to the frontend semantics: the variants of a
>     union type don't have any natural ordering, so list semantics could
>     cause problems.

I agree.  I probably shouldn't even comment, as I know so little about 
LLVM.  But I've hand-written a couple kLOC of IR now and am starting to 
get a feel for the syntax, so I'll just say what "feels" right based on 
that and leave it to others to decide if I've absorbed enough to make 
any kind of sense.

Just imagining myself using such a language extension, I really would 
not want an ordering imposed where no natural one exists.  Indices feel 
very wrong.  Isn't a union basically just a convenient alternate 
interface to the various other conversion operators like bitcast, 
inttoptr, trunc, zext, and the rest?  (In fact that's how I manipulate 
my expressions, the three-bit tag in the low-order bits tell me how to 
treat the high-order bits.)  The "index" doesn't (generally) represent 
any kind of offset, but rather an interpretation of the bits, and none 
of the offset arithmetic implied by getelementptr or physical register 
choice implied by extractvalue will occur (except perhaps to satisfy 
alignment constraints, but that would be architecture dependent and I 
assume should therefore be invisible).  Correct?

If that argument is persuasive, then the following seems a bit more 
consistent with the existing syntax:

     ; Manipulation of a union register variable
     %myUnion = unioncast i32, %myValue to union {i32, float}
     %fieldValue = unioncast union {i32, float} %myUnion to i32
     ; %fieldValue == %myValue

This specialized union cast fits the pattern of having specialized cast 
operations between value and pointer as opposed to two values or two 

That's enough, as you could require that unions be loaded and stored as 
unions and then elements extracted.  But if you want to make it a bit 
less syntactically noisy, and also allow the same flexibility that 
getelementptr would allow in accessing a single member through a 
pointer, you could allow

     ; Load/store of one particular union field
     store i32 %myValue, union {i32, float}* %myUnionPtr
     %fieldValue = load union {i32, float}* %myUnionPtr as i32
     ; %fieldValue == %myValue

Where I've added a preposition 'as' to the load instruction by analogy 
with what the cast operators do with 'to'.

I don't know that I'd argue the point much, but offhand it "feels" 
consistent with the rest of the syntax to have a specialized 'unioncast' 
operator analogous with the other specialized conversions, but overload 
load/store as I illustrated so that pointers to unions are conceptually 
just funny kinds of pointers to their fields (which they are).  So in 
that vein, if you want a pointer to one of the alternatives in the union 
you'd just cast one pointer to another; to avoid alignment adjustments 
on what is supposed to be a no-op that cast probably shouldn't be 
bitcast.  So what about

     %intPtr = unioncast union {i32, float}* %myUnionPtr to i32*
     %newUnionPtr = unioncast i32* %intPtr to union {i32, float}*
     ; %newUnionPtr == %myUnionPtr

I'm not necessarily advocating overloading one keyword ('unioncast') 
that way, though I note that it should always be unambiguous based on 
whether the operands are values or pointers (LLVM seems to have a strong 
notion of what is and is not a pointer, so this makes some kind of 
conceptual sense to me).  Whether it's OK to create two new keywords is 
perhaps too fine a detail for me to have a good sense of.  What would 
matter to me is not imposing order on unordered interpretations.


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