inc/dec for recursive unions; first pass

compiler/gen/src/llvm/build.rs

@@ -22,7 +22,7 @@ use inkwell::values::BasicValueEnum::{self, *};
 use inkwell::values::{BasicValue, FloatValue, FunctionValue, IntValue, PointerValue, StructValue};
 use inkwell::AddressSpace;
 use inkwell::{IntPredicate, OptimizationLevel};
-use roc_collections::all::{ImMap, MutSet};
+use roc_collections::all::{ImMap, MutMap, MutSet};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::{Interns, Symbol};
 use roc_mono::ir::JoinPointId;
@@ -79,6 +79,11 @@ impl<'a, 'ctx> Scope<'a, 'ctx> {
     */
 }
 
+pub struct RcFunctions<'ctx> {
+    inc: FunctionValue<'ctx>,
+    dec: FunctionValue<'ctx>,
+}
+
 pub struct Env<'a, 'ctx, 'env> {
     pub arena: &'a Bump,
     pub context: &'ctx Context,
@@ -88,6 +93,7 @@ pub struct Env<'a, 'ctx, 'env> {
     pub ptr_bytes: u32,
     pub leak: bool,
     pub exposed_to_host: MutSet<Symbol>,
+    pub rc_functions: MutMap<Layout<'a>, RcFunctions<'ctx>>,
 }
 
 impl<'a, 'ctx, 'env> Env<'a, 'ctx, 'env> {
@@ -358,7 +364,7 @@ pub fn build_exp_expr<'a, 'ctx, 'env>(
                 // If this is an external-facing function, use the C calling convention.
                 call.set_call_convention(C_CALL_CONV);
             } else {
-                // If it's an internal-only function, use the fast calling conention.
+                // If it's an internal-only function, use the fast calling convention.
                 call.set_call_convention(FAST_CALL_CONV);
             }
 
@@ -571,6 +577,10 @@ pub fn build_exp_expr<'a, 'ctx, 'env>(
             )
         }
         Tag { .. } => unreachable!("tags should have a union layout"),
+
+        Reset(_) => todo!(),
+        Reuse { .. } => todo!(),
+
         AccessAtIndex {
             index,
             structure,
@@ -684,6 +694,8 @@ pub fn allocate_with_refcount<'a, 'ctx, 'env>(
     let len_type = env.ptr_int();
     // bytes per element
     let bytes_len = len_type.const_int(value_bytes, false);
+
+    // TODO fix offset
     let offset = (env.ptr_bytes as u64).max(value_bytes);
 
     let ptr = {
@@ -1022,6 +1034,11 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             let (value, layout) = load_symbol_and_layout(env, scope, symbol);
             let layout = layout.clone();
 
+            if layout.contains_refcounted() {
+                increment_refcount_layout(env, parent, layout_ids, value, &layout);
+            }
+
+            /*
             match layout {
                 Layout::Builtin(Builtin::List(MemoryMode::Refcounted, _)) => {
                     increment_refcount_list(env, parent, value.into_struct_value());
@@ -1029,13 +1046,16 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                 }
                 _ => build_exp_stmt(env, layout_ids, scope, parent, cont),
             }
+            */
+
+            build_exp_stmt(env, layout_ids, scope, parent, cont)
         }
         Dec(symbol, cont) => {
             let (value, layout) = load_symbol_and_layout(env, scope, symbol);
             let layout = layout.clone();
 
             if layout.contains_refcounted() {
-                decrement_refcount_layout(env, parent, value, &layout);
+                decrement_refcount_layout(env, parent, layout_ids, value, &layout);
             }
 
             build_exp_stmt(env, layout_ids, scope, parent, cont)
@@ -1058,31 +1078,57 @@ fn refcount_is_one_comparison<'ctx>(
     )
 }
 
-#[allow(dead_code)]
 fn list_get_refcount_ptr<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
+    layout: &Layout<'a>,
     list_wrapper: StructValue<'ctx>,
 ) -> PointerValue<'ctx> {
-    let builder = env.builder;
-    let ctx = env.context;
-
-    // pointer to usize
-    let ptr_bytes = env.ptr_bytes;
-    let int_type = ptr_int(ctx, ptr_bytes);
-
     // fetch the pointer to the array data, as an integer
-    let ptr_as_int = builder
+    let ptr_as_int = env
+        .builder
         .build_extract_value(list_wrapper, Builtin::WRAPPER_PTR, "read_list_ptr")
         .unwrap()
         .into_int_value();
 
-    // subtract ptr_size, to access the refcount
+    get_refcount_ptr_help(env, layout, ptr_as_int)
+}
+
+fn get_refcount_ptr<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout: &Layout<'a>,
+    ptr: PointerValue<'ctx>,
+) -> PointerValue<'ctx> {
+    let ptr_as_int =
+        cast_basic_basic(env.builder, ptr.into(), env.context.i64_type().into()).into_int_value();
+
+    get_refcount_ptr_help(env, layout, ptr_as_int)
+}
+
+fn get_refcount_ptr_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout: &Layout<'a>,
+    ptr_as_int: IntValue<'ctx>,
+) -> PointerValue<'ctx> {
+    let builder = env.builder;
+    let ctx = env.context;
+
+    let value_bytes = layout.stack_size(env.ptr_bytes) as u64;
+    let offset = match dbg!(layout) {
+        Layout::Builtin(Builtin::List(_, _)) => env.ptr_bytes as u64,
+        _ => (env.ptr_bytes as u64).max(value_bytes),
+    };
+
+    // subtract offset, to access the refcount
     let refcount_ptr = builder.build_int_sub(
         ptr_as_int,
-        ctx.i64_type().const_int(env.ptr_bytes as u64, false),
+        ctx.i64_type().const_int(offset, false),
         "make_refcount_ptr",
     );
 
+    // pointer to usize
+    let ptr_bytes = env.ptr_bytes;
+    let int_type = ptr_int(ctx, ptr_bytes);
+
     builder.build_int_to_ptr(
         refcount_ptr,
         int_type.ptr_type(AddressSpace::Generic),
@@ -1093,13 +1139,14 @@ fn list_get_refcount_ptr<'a, 'ctx, 'env>(
 fn decrement_refcount_layout<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
+    layout_ids: &mut LayoutIds<'a>,
     value: BasicValueEnum<'ctx>,
     layout: &Layout<'a>,
 ) {
     use Layout::*;
 
     match layout {
-        Builtin(builtin) => decrement_refcount_builtin(env, parent, value, builtin),
+        Builtin(builtin) => decrement_refcount_builtin(env, parent, value, layout, builtin),
         Struct(layouts) => {
             let wrapper_struct = value.into_struct_value();
 
@@ -1110,14 +1157,12 @@ fn decrement_refcount_layout<'a, 'ctx, 'env>(
                         .build_extract_value(wrapper_struct, i as u32, "decrement_struct_field")
                         .unwrap();
 
-                    decrement_refcount_layout(env, parent, field_ptr, field_layout)
+                    decrement_refcount_layout(env, parent, layout_ids, field_ptr, field_layout)
                 }
             }
         }
-        RecursiveUnion(_) => {
-            println!("TODO implement decrement layout of recursive tag union");
-        }
         RecursivePointer => todo!("TODO implement decrement layout of recursive tag union"),
+
         Union(tags) => {
             debug_assert!(!tags.is_empty());
             let wrapper_struct = value.into_struct_value();
@@ -1145,7 +1190,7 @@ fn decrement_refcount_layout<'a, 'ctx, 'env>(
                             .build_extract_value(wrapper_struct, i as u32, "decrement_struct_field")
                             .unwrap();
 
-                        decrement_refcount_layout(env, parent, field_ptr, field_layout)
+                        decrement_refcount_layout(env, parent, layout_ids, field_ptr, field_layout)
                     }
                 }
 
@@ -1161,6 +1206,10 @@ fn decrement_refcount_layout<'a, 'ctx, 'env>(
             env.builder.position_at_end(merge_block);
         }
 
+        RecursiveUnion(tags) => {
+            build_dec_union(env, layout_ids, tags, value);
+        }
+
         FunctionPointer(_, _) | Pointer(_) => {}
     }
 }
@@ -1170,6 +1219,7 @@ fn decrement_refcount_builtin<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
     value: BasicValueEnum<'ctx>,
+    layout: &Layout<'a>,
     builtin: &Builtin<'a>,
 ) {
     use Builtin::*;
@@ -1180,7 +1230,7 @@ fn decrement_refcount_builtin<'a, 'ctx, 'env>(
                 // TODO decrement all values
             }
             let wrapper_struct = value.into_struct_value();
-            decrement_refcount_list(env, parent, wrapper_struct);
+            decrement_refcount_list(env, parent, layout, wrapper_struct);
         }
         List(MemoryMode::Unique, _element_layout) => {
             // do nothing
@@ -1189,16 +1239,71 @@ fn decrement_refcount_builtin<'a, 'ctx, 'env>(
             if element_layout.contains_refcounted() {
                 // TODO decrement all values
             }
-            let wrapper_struct = value.into_struct_value();
-            decrement_refcount_list(env, parent, wrapper_struct);
+            todo!();
         }
         Map(key_layout, value_layout) => {
             if key_layout.contains_refcounted() || value_layout.contains_refcounted() {
                 // TODO decrement all values
             }
 
+            todo!();
+        }
+        _ => {}
+    }
+}
+
+fn increment_refcount_layout<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    parent: FunctionValue<'ctx>,
+    layout_ids: &mut LayoutIds<'a>,
+    value: BasicValueEnum<'ctx>,
+    layout: &Layout<'a>,
+) {
+    use Layout::*;
+
+    match layout {
+        Builtin(builtin) => increment_refcount_builtin(env, parent, value, layout, builtin),
+
+        RecursiveUnion(tags) => {
+            build_inc_union(env, layout_ids, tags, value);
+        }
+        _ => {}
+    }
+}
+
+#[inline(always)]
+fn increment_refcount_builtin<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    parent: FunctionValue<'ctx>,
+    value: BasicValueEnum<'ctx>,
+    layout: &Layout<'a>,
+    builtin: &Builtin<'a>,
+) {
+    use Builtin::*;
+
+    match builtin {
+        List(MemoryMode::Refcounted, element_layout) => {
+            if element_layout.contains_refcounted() {
+                // TODO decrement all values
+            }
             let wrapper_struct = value.into_struct_value();
-            decrement_refcount_list(env, parent, wrapper_struct);
+            increment_refcount_list(env, parent, layout, wrapper_struct);
+        }
+        List(MemoryMode::Unique, _element_layout) => {
+            // do nothing
+        }
+        Set(element_layout) => {
+            if element_layout.contains_refcounted() {
+                // TODO decrement all values
+            }
+            todo!();
+        }
+        Map(key_layout, value_layout) => {
+            if key_layout.contains_refcounted() || value_layout.contains_refcounted() {
+                // TODO decrement all values
+            }
+
+            todo!();
         }
         _ => {}
     }
@@ -1207,6 +1312,7 @@ fn decrement_refcount_builtin<'a, 'ctx, 'env>(
 fn increment_refcount_list<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
+    layout: &Layout<'a>,
     original_wrapper: StructValue<'ctx>,
 ) {
     let builder = env.builder;
@@ -1229,7 +1335,7 @@ fn increment_refcount_list<'a, 'ctx, 'env>(
 
     builder.position_at_end(increment_block);
 
-    let refcount_ptr = list_get_refcount_ptr(env, original_wrapper);
+    let refcount_ptr = list_get_refcount_ptr(env, layout, original_wrapper);
 
     let refcount = env
         .builder
@@ -1253,6 +1359,7 @@ fn increment_refcount_list<'a, 'ctx, 'env>(
 fn decrement_refcount_list<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
+    layout: &Layout<'a>,
     original_wrapper: StructValue<'ctx>,
 ) {
     let builder = env.builder;
@@ -1282,7 +1389,7 @@ fn decrement_refcount_list<'a, 'ctx, 'env>(
     let then_block = ctx.append_basic_block(parent, "then");
     let else_block = ctx.append_basic_block(parent, "else");
 
-    let refcount_ptr = list_get_refcount_ptr(env, original_wrapper);
+    let refcount_ptr = list_get_refcount_ptr(env, layout, original_wrapper);
 
     let refcount = env
         .builder
@@ -1326,6 +1433,65 @@ fn decrement_refcount_list<'a, 'ctx, 'env>(
     builder.position_at_end(cont_block);
 }
 
+fn decrement_refcount_ptr<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    parent: FunctionValue<'ctx>,
+    layout: &Layout<'a>,
+    field_ptr: PointerValue<'ctx>,
+) {
+    let builder = env.builder;
+    let ctx = env.context;
+
+    // the block we'll always jump to when we're done
+    let cont_block = ctx.append_basic_block(parent, "after_decrement_block");
+
+    let refcount_ptr = get_refcount_ptr(env, layout, field_ptr);
+
+    let refcount = env
+        .builder
+        .build_load(refcount_ptr, "get_refcount")
+        .into_int_value();
+
+    let comparison = refcount_is_one_comparison(builder, env.context, refcount);
+
+    // build blocks
+    let then_block = ctx.append_basic_block(parent, "then");
+    let else_block = ctx.append_basic_block(parent, "else");
+
+    // TODO what would be most optimial for the branch predictor
+    //
+    // are most refcounts 1 most of the time? or not?
+    builder.build_conditional_branch(comparison, then_block, else_block);
+
+    // build then block
+    {
+        builder.position_at_end(then_block);
+        if !env.leak {
+            builder.build_free(refcount_ptr);
+        }
+        builder.build_unconditional_branch(cont_block);
+    }
+
+    // build else block
+    {
+        builder.position_at_end(else_block);
+        // our refcount 0 is actually usize::MAX, so decrementing the refcount means incrementing this value.
+        let decremented = env.builder.build_int_add(
+            ctx.i64_type().const_int(1 as u64, false),
+            refcount,
+            "decremented_refcount",
+        );
+
+        // Mutate the new array in-place to change the element.
+        builder.build_store(refcount_ptr, decremented);
+
+        builder.build_unconditional_branch(cont_block);
+    }
+
+    // emit merge block
+    builder.position_at_end(cont_block);
+}
+
 pub fn load_symbol<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     scope: &Scope<'a, 'ctx>,
@@ -2004,7 +2170,8 @@ fn run_low_level<'a, 'ctx, 'env>(
                     let ret_type =
                         basic_type_from_layout(env.arena, ctx, list_layout, env.ptr_bytes);
 
-                    let refcount_ptr = list_get_refcount_ptr(env, list_symbol.into_struct_value());
+                    let refcount_ptr =
+                        list_get_refcount_ptr(env, list_layout, list_symbol.into_struct_value());
 
                     let refcount = env
                         .builder
@@ -2403,3 +2570,385 @@ fn build_float_unary_op<'a, 'ctx, 'env>(
         }
     }
 }
+
+pub fn build_inc_dec_header<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout: &Layout<'a>,
+    fn_name: String,
+) -> FunctionValue<'ctx> {
+    let arena = env.arena;
+    let context = &env.context;
+
+    let arg_type = basic_type_from_layout(arena, env.context, &layout, env.ptr_bytes);
+
+    let fn_type = context.void_type().fn_type(&[arg_type], false);
+
+    let fn_val = env
+        .module
+        .add_function(fn_name.as_str(), fn_type, Some(Linkage::Private));
+
+    // If it's an internal-only function, it should use the fast calling convention.
+    fn_val.set_call_conventions(FAST_CALL_CONV);
+
+    fn_val
+}
+
+pub fn build_dec_union<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    fields: &'a [&'a [Layout<'a>]],
+    value: BasicValueEnum<'ctx>,
+) {
+    let layout = Layout::Union(fields);
+
+    let block = env.builder.get_insert_block().expect("to be in a function");
+
+    let symbol = Symbol::DEC;
+    let fn_name = layout_ids
+        .get(symbol, &layout)
+        .to_symbol_string(symbol, &env.interns);
+
+    let function = match env.module.get_function(fn_name.as_str()) {
+        Some(function_value) => function_value,
+        None => {
+            let function_value = build_inc_dec_header(env, &layout, fn_name);
+
+            build_dec_union_help(env, layout_ids, fields, function_value);
+
+            function_value
+        }
+    };
+
+    env.builder.position_at_end(block);
+    let call = env
+        .builder
+        .build_call(function, &[value], "decrement_union");
+
+    call.set_call_convention(FAST_CALL_CONV);
+}
+
+pub fn build_dec_union_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    tags: &[&[Layout<'a>]],
+    fn_val: FunctionValue<'ctx>,
+) {
+    debug_assert!(!tags.is_empty());
+
+    use inkwell::types::BasicType;
+
+    let context = &env.context;
+    let builder = env.builder;
+
+    // Add a basic block for the entry point
+    let entry = context.append_basic_block(fn_val, "entry");
+
+    builder.position_at_end(entry);
+
+    let mut scope = Scope::default();
+
+    // Add args to scope
+    let arg_symbol = Symbol::ARG_1;
+    let layout = Layout::Union(tags);
+    let arg_val = fn_val.get_param_iter().next().unwrap();
+
+    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+
+    let alloca = create_entry_block_alloca(
+        env,
+        fn_val,
+        arg_val.get_type(),
+        arg_symbol.ident_string(&env.interns),
+    );
+
+    builder.build_store(alloca, arg_val);
+
+    scope.insert(arg_symbol, (layout.clone(), alloca));
+
+    let parent = fn_val;
+
+    let layout = Layout::RecursiveUnion(tags);
+    let before_block = env.builder.get_insert_block().expect("to be in a function");
+
+    let wrapper_struct = arg_val.into_struct_value();
+
+    // let tag_id_u8 = cast_basic_basic(env.builder, tag_id.into(), env.context.i8_type().into());
+
+    // next, make a jump table for all possible values of the tag_id
+    let mut cases = Vec::with_capacity_in(tags.len(), env.arena);
+
+    let merge_block = env.context.append_basic_block(parent, "decrement_merge");
+
+    for (tag_id, field_layouts) in tags.iter().enumerate() {
+        let block = env.context.append_basic_block(parent, "tag_id_decrement");
+        env.builder.position_at_end(block);
+
+        let wrapper_type = basic_type_from_layout(
+            env.arena,
+            env.context,
+            &Layout::Struct(field_layouts),
+            env.ptr_bytes,
+        );
+
+        let wrapper_struct =
+            cast_struct_struct(env.builder, wrapper_struct, wrapper_type.into_struct_type());
+
+        for (i, field_layout) in field_layouts.iter().enumerate() {
+            if let Layout::RecursivePointer = field_layout {
+                // a *i64 pointer to the recursive data
+                // we need to cast this pointer to the appropriate type
+                let field_ptr = env
+                    .builder
+                    .build_extract_value(wrapper_struct, i as u32, "decrement_struct_field")
+                    .unwrap();
+
+                // recursively decrement
+                let union_type = block_of_memory(env.context, &layout, env.ptr_bytes);
+                let recursive_field_ptr = cast_basic_basic(
+                    env.builder,
+                    field_ptr,
+                    union_type.ptr_type(AddressSpace::Generic).into(),
+                )
+                .into_pointer_value();
+
+                let recursive_field = env
+                    .builder
+                    .build_load(recursive_field_ptr, "load_recursive_field");
+
+                // recursively decrement the field
+                let call =
+                    env.builder
+                        .build_call(fn_val, &[recursive_field], "recursive_tag_decrement");
+
+                // Because it's an internal-only function, use the fast calling convention.
+                call.set_call_convention(FAST_CALL_CONV);
+
+                // first decrement the pointer itself
+                // This is so the recursive decrement may optimize into a tail call
+                decrement_refcount_ptr(env, parent, &layout, field_ptr.into_pointer_value());
+            } else if field_layout.contains_refcounted() {
+                let field_ptr = env
+                    .builder
+                    .build_extract_value(wrapper_struct, i as u32, "decrement_struct_field")
+                    .unwrap();
+
+                decrement_refcount_layout(env, parent, layout_ids, field_ptr, field_layout);
+            }
+        }
+
+        env.builder.build_unconditional_branch(merge_block);
+
+        cases.push((
+            env.context.i64_type().const_int(tag_id as u64, false),
+            block,
+        ));
+    }
+
+    cases.reverse();
+
+    let (_, default_block) = cases.pop().unwrap();
+
+    env.builder.position_at_end(before_block);
+
+    // read the tag_id
+    let current_tag_id = {
+        // the first element of the wrapping struct is an array of i64
+        let first_array = env
+            .builder
+            .build_extract_value(wrapper_struct, 0, "read_tag_id")
+            .unwrap()
+            .into_array_value();
+
+        env.builder
+            .build_extract_value(first_array, 0, "read_tag_id_2")
+            .unwrap()
+            .into_int_value()
+    };
+
+    // switch on it
+    env.builder
+        .build_switch(current_tag_id, default_block, &cases);
+
+    env.builder.position_at_end(merge_block);
+
+    // this function returns void
+    builder.build_return(None);
+}
+
+pub fn build_inc_union<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    fields: &'a [&'a [Layout<'a>]],
+    value: BasicValueEnum<'ctx>,
+) {
+    let layout = Layout::Union(fields);
+
+    let block = env.builder.get_insert_block().expect("to be in a function");
+
+    let symbol = Symbol::INC;
+    let fn_name = layout_ids
+        .get(symbol, &layout)
+        .to_symbol_string(symbol, &env.interns);
+
+    let function = match env.module.get_function(fn_name.as_str()) {
+        Some(function_value) => function_value,
+        None => {
+            let function_value = build_inc_dec_header(env, &layout, fn_name);
+
+            build_inc_union_help(env, layout_ids, fields, function_value);
+
+            function_value
+        }
+    };
+
+    env.builder.position_at_end(block);
+    let call = env
+        .builder
+        .build_call(function, &[value], "increment_union");
+
+    call.set_call_convention(FAST_CALL_CONV);
+}
+
+pub fn build_inc_union_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    tags: &[&[Layout<'a>]],
+    fn_val: FunctionValue<'ctx>,
+) {
+    debug_assert!(!tags.is_empty());
+
+    use inkwell::types::BasicType;
+
+    let context = &env.context;
+    let builder = env.builder;
+
+    // Add a basic block for the entry point
+    let entry = context.append_basic_block(fn_val, "entry");
+
+    builder.position_at_end(entry);
+
+    let mut scope = Scope::default();
+
+    // Add args to scope
+    let arg_symbol = Symbol::ARG_1;
+    let layout = Layout::Union(tags);
+    let arg_val = fn_val.get_param_iter().next().unwrap();
+
+    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+
+    let alloca = create_entry_block_alloca(
+        env,
+        fn_val,
+        arg_val.get_type(),
+        arg_symbol.ident_string(&env.interns),
+    );
+
+    builder.build_store(alloca, arg_val);
+
+    scope.insert(arg_symbol, (layout.clone(), alloca));
+
+    let parent = fn_val;
+
+    let layout = Layout::RecursiveUnion(tags);
+    let before_block = env.builder.get_insert_block().expect("to be in a function");
+
+    let wrapper_struct = arg_val.into_struct_value();
+
+    // read the tag_id
+    let tag_id = {
+        // the first element of the wrapping struct is an array of i64
+        let first_array = env
+            .builder
+            .build_extract_value(wrapper_struct, 0, "read_tag_id")
+            .unwrap()
+            .into_array_value();
+
+        env.builder
+            .build_extract_value(first_array, 0, "read_tag_id_2")
+            .unwrap()
+            .into_int_value()
+    };
+
+    let tag_id_u8 = cast_basic_basic(env.builder, tag_id.into(), env.context.i8_type().into());
+
+    // next, make a jump table for all possible values of the tag_id
+    let mut cases = Vec::with_capacity_in(tags.len(), env.arena);
+
+    let merge_block = env.context.append_basic_block(parent, "decrement_merge");
+
+    for (tag_id, field_layouts) in tags.iter().enumerate() {
+        let block = env.context.append_basic_block(parent, "tag_id_decrement");
+        env.builder.position_at_end(block);
+
+        let wrapper_type = basic_type_from_layout(
+            env.arena,
+            env.context,
+            &Layout::Struct(field_layouts),
+            env.ptr_bytes,
+        );
+
+        let wrapper_struct =
+            cast_struct_struct(env.builder, wrapper_struct, wrapper_type.into_struct_type());
+
+        for (i, field_layout) in field_layouts.iter().enumerate() {
+            if let Layout::RecursivePointer = field_layout {
+                let field_ptr = env
+                    .builder
+                    .build_extract_value(wrapper_struct, i as u32, "decrement_struct_field")
+                    .unwrap()
+                    .into_pointer_value();
+
+                // recursively decrement
+                let recursive_field_ptr_as_int =
+                    env.builder.build_load(field_ptr, "recursive_decrement");
+
+                let union_type = block_of_memory(env.context, &layout, env.ptr_bytes);
+                let recursive_field_ptr = cast_basic_basic(
+                    env.builder,
+                    recursive_field_ptr_as_int,
+                    union_type.ptr_type(AddressSpace::Generic).into(),
+                )
+                .into_pointer_value();
+
+                let recursive_field = env
+                    .builder
+                    .build_load(recursive_field_ptr, "load_recursive_field");
+
+                // first decrement the pointer itself
+                // This is so the recursive decrement may optimize into a tail call
+                //decrement_refcount_ptr(env, parent, wrapper_struct, field_ptr);
+
+                // recursively decrement the field
+                let call =
+                    env.builder
+                        .build_call(fn_val, &[recursive_field], "recursive_tag_increment");
+
+                // Because it's an internal-only function, use the fast calling convention.
+                call.set_call_convention(FAST_CALL_CONV);
+            } else if field_layout.contains_refcounted() {
+                let field_ptr = env
+                    .builder
+                    .build_extract_value(wrapper_struct, i as u32, "increment_struct_field")
+                    .unwrap();
+
+                decrement_refcount_layout(env, parent, layout_ids, field_ptr, field_layout);
+            }
+        }
+
+        env.builder.build_unconditional_branch(merge_block);
+
+        cases.push((env.context.i8_type().const_int(tag_id as u64, false), block));
+    }
+
+    let (_, default_block) = cases.pop().unwrap();
+
+    env.builder.position_at_end(before_block);
+
+    env.builder
+        .build_switch(tag_id_u8.into_int_value(), default_block, &cases);
+
+    env.builder.position_at_end(merge_block);
+
+    // this function returns void
+    builder.build_return(None);
+}