[llvm-commits] RFC: initial union syntax support

[Duncan Sands]

Hi,

> The whole point of this exercise is to remove bitcasts.  In fact,
> though I am not suggesting it, you could do away with bitcasts
> completely given unions.

you can also do away with unions given bitcasts.  For example,
here is how you can define a union of two types in a platform
independent way.  It alloca's enough memory for both types
without using any platform specific information.

define void @f() {
        %tmp1 = getelementptr %T1* null, i32 1          ; <%T1*> [#uses=1]
        %size1 = ptrtoint %T1* %tmp1 to i32             ; <i32> [#uses=2]
; the size of type %T1
        %tmp2 = getelementptr %T2* null, i32 1          ; <%T2*> [#uses=1]
        %size2 = ptrtoint %T2* %tmp2 to i32             ; <i32> [#uses=2]
; the size of type %T2
        %gt = icmp ugt i32 %size1, %size2               ; <i1> [#uses=1]
        br i1 %gt, label %use1, label %use2

use1:           ; preds = %0
        br label %allocate

use2:           ; preds = %0
        br label %allocate

allocate:               ; preds = %use2, %use1
        %size = phi i32 [ %size1, %use1 ], [ %size2, %use2 ]            ; <i32> [#uses=1]
; the larger of the two sizes
        %union = alloca i8, i32 %size           ; <i8*> [#uses=2]
; a local variable holding the "union"
        %as_T1 = bitcast i8* %union to %T1*             ; <%T1*> [#uses=0]
; the union viewed as a variable of type %T1
        %as_T2 = bitcast i8* %union to %T2*             ; <%T2*> [#uses=0]
; the union viewed as a variable of type %T2
; do stuff with it here
        ret void
}