`union #shared_nil`

This adds a feature to `union` which requires all the variants to have a `nil` value and on assign to the union, checks whether that value is `nil` or not. If the value is `nil`, the union will be `nil` (thus sharing the `nil` value)

1 files changed, 28 insertions, 3 deletions

diff --git a/src/llvm_backend_general.cpp b/src/llvm_backend_general.cpp
index dd1555193..f6dd97966 100644
--- a/src/llvm_backend_general.cpp
+++ b/src/llvm_backend_general.cpp
@@ -1176,10 +1176,35 @@ void lb_emit_store_union_variant_tag(lbProcedure *p, lbValue parent, Type *varia
 }
 
 void lb_emit_store_union_variant(lbProcedure *p, lbValue parent, lbValue variant, Type *variant_type) {
-	lbValue underlying = lb_emit_conv(p, parent, alloc_type_pointer(variant_type));
+	Type *pt = base_type(type_deref(parent.type));
+	GB_ASSERT(pt->kind == Type_Union);
+	if (pt->Union.kind == UnionType_shared_nil) {
+		lbBlock *if_nil     = lb_create_block(p, "shared_nil.if_nil");
+		lbBlock *if_not_nil = lb_create_block(p, "shared_nil.if_not_nil");
+		lbBlock *done       = lb_create_block(p, "shared_nil.done");
+
+		lbValue cond_is_nil = lb_emit_comp_against_nil(p, Token_CmpEq, variant);
+		lb_emit_if(p, cond_is_nil, if_nil, if_not_nil);
+
+		lb_start_block(p, if_nil);
+		lb_emit_store(p, parent, lb_const_nil(p->module, type_deref(parent.type)));
+		lb_emit_jump(p, done);
+
+		lb_start_block(p, if_not_nil);
+		lbValue underlying = lb_emit_conv(p, parent, alloc_type_pointer(variant_type));
+		lb_emit_store(p, underlying, variant);
+		lb_emit_store_union_variant_tag(p, parent, variant_type);
+		lb_emit_jump(p, done);
+
+		lb_start_block(p, done);
 
-	lb_emit_store(p, underlying, variant);
-	lb_emit_store_union_variant_tag(p, parent, variant_type);
+
+	} else {
+		lbValue underlying = lb_emit_conv(p, parent, alloc_type_pointer(variant_type));
+
+		lb_emit_store(p, underlying, variant);
+		lb_emit_store_union_variant_tag(p, parent, variant_type);
+	}
 }