mem zero rather than store to a union where the variant is of size zero

src/llvm_backend.hpp

@@ -550,6 +550,7 @@ gb_internal LLVMValueRef lb_call_intrinsic(lbProcedure *p, const char *name, LLV
 gb_internal void lb_mem_copy_overlapping(lbProcedure *p, lbValue dst, lbValue src, lbValue len, bool is_volatile=false);
 gb_internal void lb_mem_copy_non_overlapping(lbProcedure *p, lbValue dst, lbValue src, lbValue len, bool is_volatile=false);
 gb_internal LLVMValueRef lb_mem_zero_ptr_internal(lbProcedure *p, LLVMValueRef ptr, LLVMValueRef len, unsigned alignment, bool is_volatile);
+gb_internal LLVMValueRef lb_mem_zero_ptr_internal(lbProcedure *p, LLVMValueRef ptr, usize len, unsigned alignment, bool is_volatile);
 
 gb_internal gb_inline i64 lb_max_zero_init_size(void) {
 	return cast(i64)(4*build_context.int_size);

src/llvm_backend_general.cpp

@@ -1332,6 +1332,8 @@ gb_internal void lb_emit_store_union_variant(lbProcedure *p, lbValue parent, lbV
 	Type *pt = base_type(type_deref(parent.type));
 	GB_ASSERT(pt->kind == Type_Union);
 	if (pt->Union.kind == UnionType_shared_nil) {
+		GB_ASSERT(type_size_of(variant_type));
+
 		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");
@@ -1353,9 +1355,13 @@ gb_internal void lb_emit_store_union_variant(lbProcedure *p, lbValue parent, lbV
 
 
 	} else {
-		lbValue underlying = lb_emit_conv(p, parent, alloc_type_pointer(variant_type));
-
-		lb_emit_store(p, underlying, variant);
+		if (type_size_of(variant_type) == 0) {
+			unsigned alignment = 1;
+			lb_mem_zero_ptr_internal(p, parent.value, pt->Union.variant_block_size, alignment, false);
+		} 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);
 	}
 }

src/llvm_backend_utility.cpp

@@ -57,6 +57,10 @@ gb_internal lbValue lb_correct_endianness(lbProcedure *p, lbValue value) {
 	return value;
 }
 
+gb_internal LLVMValueRef lb_mem_zero_ptr_internal(lbProcedure *p, LLVMValueRef ptr, usize len, unsigned alignment, bool is_volatile) {
+	return lb_mem_zero_ptr_internal(p, ptr, LLVMConstInt(lb_type(p->module, t_uint), len, false), alignment, is_volatile);
+}
+
 gb_internal LLVMValueRef lb_mem_zero_ptr_internal(lbProcedure *p, LLVMValueRef ptr, LLVMValueRef len, unsigned alignment, bool is_volatile) {
 	bool is_inlinable = false;