lostanza - passing/return struct by value in C

[callendorph]

In C - you can pass or return a defined struct by value from a function. For example - see the GSL complex math library.

I can't seem to work out how to do this in lostanza. It seems like lostanza is focused only on pointers. Do you have an example of using structures as values?

[OlegPliss]

This is a working code from current GC implementation passing and returning small structures by value:
`lostanza deftype LiveRange :
live:ptr
dead:ptr

;Write the live range r into the heap address at the given break location.
;
;Live ranges are stored directly in the heap memory, in the space
;currently occupied by dead objects: the breaks.
;
;The encoding varies depending upon the size of the break.
;The minimum size of a break is 8 bytes long, but the live range struct is
;in general 16 bytes long.
;
;Encoding:
;Case break >= 16 bytes: then just directly store the live range.
;Case break == 8 bytes: only store the 'dead' field, tagged with 1.
;
;Since heap addresses are always 8-byte aligned, the tagged 1 will tell us
;which encoding we are using.
lostanza defn write-live-range (location:ptr, r:LiveRange) -> ref :
;Case: break == 8 bytes.
if addr(location[1]) == r.live :
;Store the tagged 'dead' field.
location[0] = (r.dead as long) | 1
;Case: break >= 16 bytes.
else :
;Store the live range directly.
location[0] = r.live as long
location[1] = r.dead as long
;No meaningful return value
return false

;Read the live range at the given break location.
lostanza defn read-live-range (location:ptr) -> LiveRange :
;Read the first word at the location.
val location0 = location[0]
;Test the tag bit to determine which encoding we are using.
if location0 & 1L :
;Case break == 8 bytes.
val live = addr(location[1]) ;Computed from size of break.
val dead = (location0 - 1) as ptr ;Remove tag bit.
return LiveRange{live, dead}
else :
;Case break >= 16 bytes.
val live = location0 as ptr
val dead = location[1] as ptr
return LiveRange{live, dead}
`

[callendorph]

Sorry - I should have been more specific with what I am trying to do. What I want to do is wrap GSL's complex value implementation. It passes a struct gsl_complex by value to and from functions in C.

The gsl_complex is a struct containing 2 doubles (128 bits). From what I can tell of call-c's calling convention - it can only assign to a value of a primitive type or a pointer (or at least that is what I have gathered from this document). Maybe I'm wrong on that account ?

It is not clear to me how I can use the LiveRange code here to make a call-c invokation of say gsl_complex_polar and get the resulting value as a lostanza deftype.

Below has gotten me the farthest towards my goal:

public lostanza deftype GComplex :
  x: double
  y: double

extern gsl_complex_polar : (double, double) -> GComplex

lostanza var r:GComplex

public lostanza defn GComplex (rad:ref<Double>, th:ref<Double>) -> ref<GComplex> :
  r = call-c gsl_complex_polar(rad.value, th.value)
  return new GComplex{r.x, r.y}

This sadly fails with FATAL ERROR: More than a single C return value!

Is there another way to do this ?

[CuppoJava]

Hi Carl,

What you are trying to do is reasonable, and is the way that it should (and will ultimately) be done. You're hitting a part of the compiler that is not-yet-implemented today: the C calling convention for structs.

Until that is finished, you have to write an additional C-wrapper that changes how gsl_complex_polar returns its results in order to connect to it.

Our apologies. It's on the near-term roadmap however!