WIP

crates/compiler/alias_analysis/src/lib.rs

@@ -1422,6 +1422,38 @@ fn expr_spec<'a>(
                 builder.add_get_tuple_field(block, variant_id, index)
             }
         },
+        UnionFieldPtrAtIndex {
+            index,
+            tag_id,
+            structure,
+            union_layout,
+        } => {
+            let index = (*index) as u32;
+            let tag_value_id = env.symbols[structure];
+
+            let type_name_bytes = recursive_tag_union_name_bytes(union_layout).as_bytes();
+            let type_name = TypeName(&type_name_bytes);
+
+            // unwrap the named wrapper
+            let union_id = builder.add_unwrap_named(block, MOD_APP, type_name, tag_value_id)?;
+
+            // now we have a tuple (cell, union { ... }); decompose
+            let heap_cell = builder.add_get_tuple_field(block, union_id, TAG_CELL_INDEX)?;
+            let union_data = builder.add_get_tuple_field(block, union_id, TAG_DATA_INDEX)?;
+
+            // we're reading from this value, so touch the heap cell
+            builder.add_touch(block, heap_cell)?;
+
+            // next, unwrap the union at the tag id that we've got
+            let variant_id = builder.add_unwrap_union(block, union_data, *tag_id as u32)?;
+
+            let value = builder.add_get_tuple_field(block, variant_id, index)?;
+
+            // construct the box. Here the heap_cell of the tag is re-used, I'm hoping that that
+            // conveys to morphic that we're borrowing into the existing tag?!
+            builder.add_make_tuple(block, &[heap_cell, value])
+        }
+
         StructAtIndex {
             index, structure, ..
         } => {

crates/compiler/can/src/builtins.rs

@@ -85,7 +85,9 @@ macro_rules! map_symbol_to_lowlevel_and_arity {
                 // these are used internally and not tied to a symbol
                 LowLevel::Hash => unimplemented!(),
                 LowLevel::PtrCast => unimplemented!(),
-                LowLevel::PtrWrite => unimplemented!(),
+                LowLevel::PtrStore => unimplemented!(),
+                LowLevel::PtrLoad => unimplemented!(),
+                LowLevel::PtrToZeroed => unimplemented!(),
                 LowLevel::RefCountIncRcPtr => unimplemented!(),
                 LowLevel::RefCountDecRcPtr=> unimplemented!(),
                 LowLevel::RefCountIncDataPtr => unimplemented!(),

crates/compiler/debug_flags/src/lib.rs

@@ -135,6 +135,10 @@ flags! {
     /// instructions.
     ROC_PRINT_IR_AFTER_REFCOUNT
 
+    /// Writes a pretty-printed mono IR to stderr after the tail recursion (modulo cons)
+    /// has been applied.
+    ROC_PRINT_IR_AFTER_TRMC
+
     /// Writes a pretty-printed mono IR to stderr after performing dropspecialization.
     /// Which inlines drop functions to remove pairs of alloc/dealloc instructions of its children.
     ROC_PRINT_IR_AFTER_DROP_SPECIALIZATION

crates/compiler/gen_dev/src/lib.rs

@@ -163,6 +163,9 @@ impl<'a> LastSeenMap<'a> {
                     Expr::UnionAtIndex { structure, .. } => {
                         self.set_last_seen(*structure, stmt);
                     }
+                    Expr::UnionFieldPtrAtIndex { structure, .. } => {
+                        self.set_last_seen(*structure, stmt);
+                    }
                     Expr::Array { elems, .. } => {
                         for elem in *elems {
                             if let ListLiteralElement::Symbol(sym) = elem {
@@ -794,6 +797,14 @@ trait Backend<'a> {
             } => {
                 self.load_union_at_index(sym, structure, *tag_id, *index, union_layout);
             }
+            Expr::UnionFieldPtrAtIndex {
+                structure,
+                tag_id,
+                union_layout,
+                index,
+            } => {
+                todo!();
+            }
             Expr::GetTagId {
                 structure,
                 union_layout,
@@ -1581,7 +1592,7 @@ trait Backend<'a> {
 
                 self.build_ptr_cast(sym, &args[0])
             }
-            LowLevel::PtrWrite => {
+            LowLevel::PtrStore => {
                 let element_layout = match self.interner().get_repr(*ret_layout) {
                     LayoutRepr::Boxed(boxed) => boxed,
                     _ => unreachable!("cannot write to {:?}", self.interner().dbg(*ret_layout)),
@@ -1589,6 +1600,10 @@ trait Backend<'a> {
 
                 self.build_ptr_write(*sym, args[0], args[1], element_layout);
             }
+            LowLevel::PtrLoad => {
+                //
+                todo!()
+            }
             LowLevel::RefCountDecRcPtr => self.build_fn_call(
                 sym,
                 bitcode::UTILS_DECREF_RC_PTR.to_string(),

crates/compiler/gen_llvm/src/llvm/build.rs

@@ -23,7 +23,7 @@ use inkwell::passes::{PassManager, PassManagerBuilder};
 use inkwell::types::{
     AnyType, BasicMetadataTypeEnum, BasicType, BasicTypeEnum, FunctionType, IntType, StructType,
 };
-use inkwell::values::BasicValueEnum::{self};
+use inkwell::values::BasicValueEnum;
 use inkwell::values::{
     BasicMetadataValueEnum, CallSiteValue, FunctionValue, InstructionValue, IntValue, PointerValue,
     StructValue,
@@ -1379,12 +1379,13 @@ pub(crate) fn build_exp_expr<'a, 'ctx>(
                         layout_interner.get_repr(layout),
                     );
 
-                    lookup_at_index_ptr2(
+                    lookup_at_index_ptr(
                         env,
                         layout_interner,
                         field_layouts,
                         *index as usize,
                         ptr,
+                        None,
                         target_loaded_type,
                     )
                 }
@@ -1404,7 +1405,7 @@ pub(crate) fn build_exp_expr<'a, 'ctx>(
                         field_layouts,
                         *index as usize,
                         argument.into_pointer_value(),
-                        struct_type.into_struct_type(),
+                        Some(struct_type.into_struct_type()),
                         target_loaded_type,
                     )
                 }
@@ -1430,12 +1431,13 @@ pub(crate) fn build_exp_expr<'a, 'ctx>(
                         layout_interner.get_repr(layout),
                     );
 
-                    lookup_at_index_ptr2(
+                    lookup_at_index_ptr(
                         env,
                         layout_interner,
                         field_layouts,
                         *index as usize,
                         ptr,
+                        None,
                         target_loaded_type,
                     )
                 }
@@ -1463,13 +1465,117 @@ pub(crate) fn build_exp_expr<'a, 'ctx>(
                         // the tag id is not stored
                         *index as usize,
                         argument.into_pointer_value(),
-                        struct_type.into_struct_type(),
+                        Some(struct_type.into_struct_type()),
                         target_loaded_type,
                     )
                 }
             }
         }
 
+        UnionFieldPtrAtIndex {
+            tag_id,
+            structure,
+            index,
+            union_layout,
+        } => {
+            // cast the argument bytes into the desired shape for this tag
+            let (argument, structure_layout) = scope.load_symbol_and_layout(structure);
+            let ret_repr = layout_interner.get_repr(layout);
+
+            let pointer_value = match union_layout {
+                UnionLayout::NonRecursive(_) => unreachable!(),
+                UnionLayout::Recursive(tag_layouts) => {
+                    debug_assert!(argument.is_pointer_value());
+
+                    let field_layouts = tag_layouts[*tag_id as usize];
+
+                    let ptr = tag_pointer_clear_tag_id(env, argument.into_pointer_value());
+                    let target_loaded_type = basic_type_from_layout(env, layout_interner, ret_repr);
+
+                    union_field_at_index(
+                        env,
+                        layout_interner,
+                        field_layouts,
+                        None,
+                        *index as usize,
+                        ptr,
+                        target_loaded_type,
+                    )
+                }
+                UnionLayout::NonNullableUnwrapped(field_layouts) => {
+                    let struct_layout = LayoutRepr::struct_(field_layouts);
+
+                    let struct_type = basic_type_from_layout(env, layout_interner, struct_layout);
+                    let target_loaded_type = basic_type_from_layout(env, layout_interner, ret_repr);
+
+                    union_field_at_index(
+                        env,
+                        layout_interner,
+                        field_layouts,
+                        Some(struct_type.into_struct_type()),
+                        *index as usize,
+                        argument.into_pointer_value(),
+                        target_loaded_type,
+                    )
+                }
+                UnionLayout::NullableWrapped {
+                    nullable_id,
+                    other_tags,
+                } => {
+                    debug_assert!(argument.is_pointer_value());
+                    debug_assert_ne!(*tag_id, *nullable_id);
+
+                    let tag_index = if *tag_id < *nullable_id {
+                        *tag_id
+                    } else {
+                        tag_id - 1
+                    };
+
+                    let field_layouts = other_tags[tag_index as usize];
+
+                    let ptr = tag_pointer_clear_tag_id(env, argument.into_pointer_value());
+                    let target_loaded_type = basic_type_from_layout(env, layout_interner, ret_repr);
+
+                    union_field_at_index(
+                        env,
+                        layout_interner,
+                        field_layouts,
+                        None,
+                        *index as usize,
+                        ptr,
+                        target_loaded_type,
+                    )
+                    .into()
+                }
+                UnionLayout::NullableUnwrapped {
+                    nullable_id,
+                    other_fields,
+                } => {
+                    debug_assert!(argument.is_pointer_value());
+                    debug_assert_ne!(*tag_id != 0, *nullable_id);
+
+                    let field_layouts = other_fields;
+                    let struct_layout = LayoutRepr::struct_(field_layouts);
+
+                    let struct_type = basic_type_from_layout(env, layout_interner, struct_layout);
+                    let target_loaded_type = basic_type_from_layout(env, layout_interner, ret_repr);
+
+                    union_field_at_index(
+                        env,
+                        layout_interner,
+                        field_layouts,
+                        Some(struct_type.into_struct_type()),
+                        // the tag id is not stored
+                        *index as usize,
+                        argument.into_pointer_value(),
+                        target_loaded_type,
+                    )
+                }
+            };
+
+            pointer_value.into()
+        }
+
         GetTagId {
             structure,
             union_layout,
@@ -2025,21 +2131,20 @@ fn lookup_at_index_ptr<'a, 'ctx>(
     field_layouts: &[InLayout<'a>],
     index: usize,
     value: PointerValue<'ctx>,
-    struct_type: StructType<'ctx>,
+    struct_type: Option<StructType<'ctx>>,
     target_loaded_type: BasicTypeEnum<'ctx>,
 ) -> BasicValueEnum<'ctx> {
     let builder = env.builder;
 
-    let ptr = env.builder.build_pointer_cast(
+    let elem_ptr = union_field_at_index_help(
+        env,
+        layout_interner,
+        field_layouts,
+        struct_type,
+        index,
         value,
-        struct_type.ptr_type(AddressSpace::default()),
-        "cast_lookup_at_index_ptr",
     );
 
-    let elem_ptr = builder
-        .new_build_struct_gep(struct_type, ptr, index as u32, "at_index_struct_gep")
-        .unwrap();
-
     let field_layout = field_layouts[index];
     let result = load_roc_value(
         env,
@@ -2054,19 +2159,23 @@ fn lookup_at_index_ptr<'a, 'ctx>(
     cast_if_necessary_for_opaque_recursive_pointers(env.builder, result, target_loaded_type)
 }
 
-fn lookup_at_index_ptr2<'a, 'ctx>(
+fn union_field_at_index_help<'a, 'ctx>(
     env: &Env<'a, 'ctx, '_>,
     layout_interner: &STLayoutInterner<'a>,
     field_layouts: &'a [InLayout<'a>],
+    opt_struct_type: Option<StructType<'ctx>>,
     index: usize,
     value: PointerValue<'ctx>,
-    target_loaded_type: BasicTypeEnum<'ctx>,
+) -> PointerValue<'ctx> {
-) -> BasicValueEnum<'ctx> {
     let builder = env.builder;
 
+    let struct_type = match opt_struct_type {
+        Some(st) => st,
+        None => {
             let struct_layout = LayoutRepr::struct_(field_layouts);
-    let struct_type =
+            basic_type_from_layout(env, layout_interner, struct_layout).into_struct_type()
-        basic_type_from_layout(env, layout_interner, struct_layout).into_struct_type();
+        }
+    };
 
     let data_ptr = env.builder.build_pointer_cast(
         value,
@@ -2074,27 +2183,40 @@ fn lookup_at_index_ptr2<'a, 'ctx>(
         "cast_lookup_at_index_ptr",
     );
 
-    let elem_ptr = builder
+    builder
         .new_build_struct_gep(
             struct_type,
             data_ptr,
             index as u32,
             "at_index_struct_gep_data",
         )
-        .unwrap();
+        .unwrap()
+}
 
-    let field_layout = field_layouts[index];
+fn union_field_at_index<'a, 'ctx>(
-    let result = load_roc_value(
+    env: &Env<'a, 'ctx, '_>,
+    layout_interner: &STLayoutInterner<'a>,
+    field_layouts: &'a [InLayout<'a>],
+    opt_struct_type: Option<StructType<'ctx>>,
+    index: usize,
+    value: PointerValue<'ctx>,
+    target_loaded_type: BasicTypeEnum<'ctx>,
+) -> PointerValue<'ctx> {
+    let result = union_field_at_index_help(
         env,
         layout_interner,
-        layout_interner.get_repr(field_layout),
+        field_layouts,
-        elem_ptr,
+        opt_struct_type,
-        "load_at_index_ptr",
+        index,
+        value,
     );
 
     // A recursive pointer in the loaded structure is stored as a `i64*`, but the loaded layout
     // might want a more precise structure. As such, cast it to the refined type if needed.
-    cast_if_necessary_for_opaque_recursive_pointers(env.builder, result, target_loaded_type)
+    let from_value: BasicValueEnum = result.into();
+    let to_type: BasicTypeEnum = target_loaded_type;
+    cast_if_necessary_for_opaque_recursive_pointers(env.builder, from_value, to_type)
+        .into_pointer_value()
 }
 
 pub fn reserve_with_refcount<'a, 'ctx>(
@@ -3071,7 +3193,7 @@ pub fn cast_if_necessary_for_opaque_recursive_pointers<'ctx>(
     to_type: BasicTypeEnum<'ctx>,
 ) -> BasicValueEnum<'ctx> {
     if from_value.get_type() != to_type
-        // Only perform the cast if the target types are transumatble.
+        // Only perform the cast if the target types are transmutable.
         && equivalent_type_constructors(&from_value.get_type(), &to_type)
     {
         complex_bitcast(

crates/compiler/gen_llvm/src/llvm/lowlevel.rs

@@ -1304,8 +1304,28 @@ pub(crate) fn run_low_level<'a, 'ctx>(
                 .into()
         }
 
-        PtrStore | PtrLoad | PtrToZeroed | RefCountIncRcPtr | RefCountDecRcPtr
+        PtrStore => {
-        | RefCountIncDataPtr | RefCountDecDataPtr => {
+            arguments!(ptr, value);
+
+            env.builder.build_store(ptr.into_pointer_value(), value);
+
+            // ptr
+            env.context.struct_type(&[], false).const_zero().into()
+        }
+
+        PtrLoad => {
+            arguments!(ptr);
+
+            let ret_repr = layout_interner.get_repr(layout);
+            let element_type = basic_type_from_layout(env, layout_interner, ret_repr);
+
+            env.builder
+                .new_build_load(element_type, ptr.into_pointer_value(), "ptr_load")
+        }
+
+        PtrToZeroed => todo!(),
+
+        RefCountIncRcPtr | RefCountDecRcPtr | RefCountIncDataPtr | RefCountDecDataPtr => {
             unreachable!("Not used in LLVM backend: {:?}", op);
         }
 

crates/compiler/gen_wasm/src/backend.rs

@@ -1079,6 +1079,13 @@ impl<'a, 'r> WasmBackend<'a, 'r> {
                 index,
             } => self.expr_union_at_index(*structure, *tag_id, union_layout, *index, sym),
 
+            Expr::UnionFieldPtrAtIndex {
+                structure,
+                tag_id,
+                union_layout,
+                index,
+            } => todo!(),
+
             Expr::ExprBox { symbol: arg_sym } => self.expr_box(sym, *arg_sym, layout, storage),
 
             Expr::ExprUnbox { symbol: arg_sym } => self.expr_unbox(sym, *arg_sym),

crates/compiler/load_internal/src/file.rs

@@ -16,7 +16,7 @@ use roc_can::module::{
 };
 use roc_collections::{default_hasher, BumpMap, MutMap, MutSet, VecMap, VecSet};
 use roc_constrain::module::constrain_module;
-use roc_debug_flags::dbg_do;
+use roc_debug_flags::{dbg_do, ROC_PRINT_IR_AFTER_TRMC};
 #[cfg(debug_assertions)]
 use roc_debug_flags::{
     ROC_CHECK_MONO_IR, ROC_PRINT_IR_AFTER_DROP_SPECIALIZATION, ROC_PRINT_IR_AFTER_REFCOUNT,
@@ -3104,6 +3104,16 @@ fn update<'a>(
 
                     let ident_ids = state.constrained_ident_ids.get_mut(&module_id).unwrap();
 
+                    roc_mono::tail_recursion::apply_trmc(
+                        arena,
+                        &mut layout_interner,
+                        module_id,
+                        ident_ids,
+                        &mut state.procedures,
+                    );
+
+                    debug_print_ir!(state, &layout_interner, ROC_PRINT_IR_AFTER_TRMC);
+
                     inc_dec::insert_inc_dec_operations(
                         arena,
                         &layout_interner,

crates/compiler/module/src/low_level.rs

@@ -230,7 +230,9 @@ macro_rules! map_symbol_to_lowlevel {
                 // these are used internally and not tied to a symbol
                 LowLevel::Hash => unimplemented!(),
                 LowLevel::PtrCast => unimplemented!(),
-                LowLevel::PtrWrite => unimplemented!(),
+                LowLevel::PtrStore => unimplemented!(),
+                LowLevel::PtrLoad => unimplemented!(),
+                LowLevel::PtrToZeroed => unimplemented!(),
                 LowLevel::RefCountIncRcPtr => unimplemented!(),
                 LowLevel::RefCountDecRcPtr=> unimplemented!(),
                 LowLevel::RefCountIncDataPtr => unimplemented!(),

crates/compiler/mono/src/borrow.rs

@@ -741,6 +741,14 @@ impl<'a> BorrowInfState<'a> {
                 self.if_is_owned_then_own(z, *x);
             }
 
+            UnionFieldPtrAtIndex { structure: x, .. } => {
+                // if the structure (record/tag/array) is owned, the extracted value is
+                self.if_is_owned_then_own(*x, z);
+
+                // if the extracted value is owned, the structure must be too
+                self.if_is_owned_then_own(z, *x);
+            }
+
             GetTagId { structure: x, .. } => {
                 // if the structure (record/tag/array) is owned, the extracted value is
                 self.if_is_owned_then_own(*x, z);
@@ -1035,7 +1043,9 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[Ownership] {
             unreachable!("These lowlevel operations are turned into mono Expr's")
         }
 
-        PtrWrite => arena.alloc_slice_copy(&[irrelevant, irrelevant]),
+        PtrStore => arena.alloc_slice_copy(&[owned, borrowed]),
+        PtrLoad => arena.alloc_slice_copy(&[owned]),
+        PtrToZeroed => arena.alloc_slice_copy(&[owned]),
 
         PtrCast | RefCountIncRcPtr | RefCountDecRcPtr | RefCountIncDataPtr | RefCountDecDataPtr
         | RefCountIsUnique => {

crates/compiler/mono/src/debug/checker.rs

@@ -429,6 +429,15 @@ impl<'a, 'r> Ctx<'a, 'r> {
             } => self.with_sym_layout(structure, |ctx, _def_line, layout| {
                 ctx.check_union_at_index(structure, layout, union_layout, tag_id, index)
             }),
+            &Expr::UnionFieldPtrAtIndex {
+                structure,
+                tag_id,
+                union_layout,
+                index,
+            } => self.with_sym_layout(structure, |ctx, _def_line, layout| {
+                // TODO: I suspect this will fail because the output layout has an extra Box layer?
+                ctx.check_union_at_index(structure, layout, union_layout, tag_id, index)
+            }),
             Expr::Array { elem_layout, elems } => {
                 for elem in elems.iter() {
                     match elem {

crates/compiler/mono/src/drop_specialization.rs

@@ -211,7 +211,20 @@ fn specialize_drops_stmt<'a, 'i>(
                     // TODO perhaps we need the union_layout later as well? if so, create a new function/map to store it.
                     environment.add_union_child(*structure, *binding, *tag_id, *index);
                     // Generated code might know the tag of the union without switching on it.
-                    // So if we unionAtIndex, we must know the tag and we can use it to specialize the drop.
+                    // So if we UnionAtIndex, we must know the tag and we can use it to specialize the drop.
+                    environment.symbol_tag.insert(*structure, *tag_id);
+                    alloc_let_with_continuation!(environment)
+                }
+                Expr::UnionFieldPtrAtIndex {
+                    structure,
+                    tag_id,
+                    union_layout: _,
+                    index,
+                } => {
+                    // TODO perhaps we need the union_layout later as well? if so, create a new function/map to store it.
+                    environment.add_union_child(*structure, *binding, *tag_id, *index);
+                    // Generated code might know the tag of the union without switching on it.
+                    // So if we UnionFieldPtrAtIndex, we must know the tag and we can use it to specialize the drop.
                     environment.symbol_tag.insert(*structure, *tag_id);
                     alloc_let_with_continuation!(environment)
                 }
@@ -1666,8 +1679,13 @@ fn low_level_no_rc(lowlevel: &LowLevel) -> RC {
             unreachable!("These lowlevel operations are turned into mono Expr's")
         }
 
-        PtrCast | PtrWrite | RefCountIncRcPtr | RefCountDecRcPtr | RefCountIncDataPtr
+        // only inserted for internal purposes. RC should not touch it
-        | RefCountDecDataPtr | RefCountIsUnique => {
+        PtrStore => RC::NoRc,
+        PtrLoad => RC::NoRc,
+        PtrToZeroed => RC::NoRc,
+
+        PtrCast | RefCountIncRcPtr | RefCountDecRcPtr | RefCountIncDataPtr | RefCountDecDataPtr
+        | RefCountIsUnique => {
             unreachable!("Only inserted *after* borrow checking: {:?}", lowlevel);
         }
     }

crates/compiler/mono/src/inc_dec.rs

@@ -345,17 +345,16 @@ impl<'v> RefcountEnvironment<'v> {
         // A groupby or something similar would be nice here.
         let mut symbol_usage = MutMap::default();
         for symbol in symbols {
-            match {
+            match self.symbols_rc_types.get(&symbol) {
-                self.symbols_rc_types
-                    .get(&symbol)
-                    .expect("Expected symbol to be in the map")
-            } {
                 // If the symbol is reference counted, we need to increment the usage count.
-                VarRcType::ReferenceCounted => {
+                Some(VarRcType::ReferenceCounted) => {
                     *symbol_usage.entry(symbol).or_default() += 1;
                 }
                 // If the symbol is not reference counted, we don't need to do anything.
-                VarRcType::NotReferenceCounted => continue,
+                Some(VarRcType::NotReferenceCounted) => continue,
+                None => {
+                    internal_error!("symbol {symbol:?} does not have an rc type")
+                }
             }
         }
         symbol_usage
@@ -891,6 +890,7 @@ fn insert_refcount_operations_binding<'a>(
         Expr::GetTagId { structure, .. }
         | Expr::StructAtIndex { structure, .. }
         | Expr::UnionAtIndex { structure, .. }
+        | Expr::UnionFieldPtrAtIndex { structure, .. }
         | Expr::ExprUnbox { symbol: structure } => {
             // All structures are alive at this point and don't have to be copied in order to take an index out/get tag id/copy values to the stack.
             // But we do want to make sure to decrement this item if it is the last reference.
@@ -904,6 +904,7 @@ fn insert_refcount_operations_binding<'a>(
                 match expr {
                     Expr::StructAtIndex { .. }
                     | Expr::UnionAtIndex { .. }
+                    | Expr::UnionFieldPtrAtIndex { .. }
                     | Expr::ExprUnbox { .. } => insert_inc_stmt(arena, *binding, 1, new_stmt),
                     // No usage of an element of a reference counted symbol. No need to increment.
                     Expr::GetTagId { .. } => new_stmt,

crates/compiler/mono/src/ir.rs

@@ -403,41 +403,6 @@ impl<'a> Proc<'a> {
         w.push(b'\n');
         String::from_utf8(w).unwrap()
     }
-
-    fn make_tail_recursive<I>(&mut self, interner: &mut I, env: &mut Env<'a, '_>)
-    where
-        I: LayoutInterner<'a>,
-    {
-        let mut args = Vec::with_capacity_in(self.args.len(), env.arena);
-        let mut proc_args = Vec::with_capacity_in(self.args.len(), env.arena);
-
-        for (layout, symbol) in self.args {
-            let new = env.unique_symbol();
-            args.push((*layout, *symbol, new));
-            proc_args.push((*layout, new));
-        }
-
-        use self::SelfRecursive::*;
-        if let SelfRecursive(id) = self.is_self_recursive {
-            if crate::tail_recursion::is_trmc_candidate(interner, self) {
-                *self = crate::tail_recursion::TrmcEnv::init(env, interner, self);
-            } else {
-                let transformed = crate::tail_recursion::make_tail_recursive(
-                    env.arena,
-                    id,
-                    self.name,
-                    self.body.clone(),
-                    args.into_bump_slice(),
-                    self.ret_layout,
-                );
-
-                if let Some(with_tco) = transformed {
-                    self.body = with_tco;
-                    self.args = proc_args.into_bump_slice();
-                }
-            }
-        }
-    }
 }
 
 /// A host-exposed function must be specialized; it's a seed for subsequent specializations
@@ -1032,7 +997,7 @@ impl<'a> Procs<'a> {
             MutMap::with_capacity_and_hasher(self.specialized.len(), default_hasher());
 
         for (symbol, layout, mut proc) in self.specialized.into_iter_assert_done() {
-            proc.make_tail_recursive(&mut layout_cache.interner, env);
+            // proc.make_tail_recursive(&mut layout_cache.interner, env);
 
             let key = (symbol, layout);
             specialized_procs.insert(key, proc);
@@ -1888,6 +1853,12 @@ pub enum Expr<'a> {
         union_layout: UnionLayout<'a>,
         index: u64,
     },
+    UnionFieldPtrAtIndex {
+        structure: Symbol,
+        tag_id: TagIdIntType,
+        union_layout: UnionLayout<'a>,
+        index: u64,
+    },
 
     Array {
         elem_layout: InLayout<'a>,
@@ -2105,6 +2076,19 @@ impl<'a> Expr<'a> {
                 ..
             } => text!(alloc, "UnionAtIndex (Id {}) (Index {}) ", tag_id, index)
                 .append(symbol_to_doc(alloc, *structure, pretty)),
+
+            UnionFieldPtrAtIndex {
+                tag_id,
+                structure,
+                index,
+                ..
+            } => text!(
+                alloc,
+                "UnionFieldPtrAtIndex (Id {}) (Index {}) ",
+                tag_id,
+                index
+            )
+            .append(symbol_to_doc(alloc, *structure, pretty)),
         }
     }
 
@@ -7678,6 +7662,21 @@ fn substitute_in_expr<'a>(
             }),
             None => None,
         },
+
+        UnionFieldPtrAtIndex {
+            structure,
+            tag_id,
+            index,
+            union_layout,
+        } => match substitute(subs, *structure) {
+            Some(structure) => Some(UnionFieldPtrAtIndex {
+                structure,
+                tag_id: *tag_id,
+                index: *index,
+                union_layout: *union_layout,
+            }),
+            None => None,
+        },
     }
 }
 

crates/compiler/mono/src/tail_recursion.rs

@@ -1,11 +1,88 @@
 #![allow(clippy::manual_map)]
 
 use crate::borrow::Ownership;
-use crate::ir::{Call, CallType, Env, Expr, JoinPointId, Param, Proc, SelfRecursive, Stmt};
+use crate::ir::{
-use crate::layout::{InLayout, LambdaName, LayoutInterner, LayoutRepr, TagIdIntType, UnionLayout};
+    Call, CallType, Expr, JoinPointId, Param, Proc, ProcLayout, SelfRecursive, Stmt, UpdateModeId,
+};
+use crate::layout::{
+    InLayout, LambdaName, Layout, LayoutInterner, LayoutRepr, STLayoutInterner, TagIdIntType,
+    UnionLayout,
+};
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
-use roc_module::symbol::Symbol;
+use roc_collections::MutMap;
+use roc_module::low_level::LowLevel;
+use roc_module::symbol::{IdentIds, ModuleId, Symbol};
+
+pub struct Env<'a, 'i> {
+    arena: &'a Bump,
+    home: ModuleId,
+    interner: &'i mut STLayoutInterner<'a>,
+    ident_ids: &'i mut IdentIds,
+}
+
+impl<'a, 'i> Env<'a, 'i> {
+    pub fn unique_symbol(&mut self) -> Symbol {
+        let ident_id = self.ident_ids.gen_unique();
+
+        Symbol::new(self.home, ident_id)
+    }
+
+    pub fn named_unique_symbol(&mut self, name: &str) -> Symbol {
+        let ident_id = self.ident_ids.add_str(name);
+        Symbol::new(self.home, ident_id)
+    }
+}
+
+pub fn apply_trmc<'a, 'i>(
+    arena: &'a Bump,
+    interner: &'i mut STLayoutInterner<'a>,
+    home: ModuleId,
+    ident_ids: &'i mut IdentIds,
+    procs: &mut MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
+) {
+    let mut env = Env {
+        arena,
+        interner,
+        home,
+        ident_ids,
+    };
+
+    let env = &mut env;
+
+    for (_, proc) in procs {
+        use self::SelfRecursive::*;
+        if let SelfRecursive(id) = proc.is_self_recursive {
+            if crate::tail_recursion::is_trmc_candidate(env.interner, proc) {
+                let new_proc = crate::tail_recursion::TrmcEnv::init(env, proc);
+                *proc = new_proc;
+            } else {
+                let mut args = Vec::with_capacity_in(proc.args.len(), arena);
+                let mut proc_args = Vec::with_capacity_in(proc.args.len(), arena);
+
+                for (layout, symbol) in proc.args {
+                    let new = env.unique_symbol();
+                    args.push((*layout, *symbol, new));
+                    proc_args.push((*layout, new));
+                }
+
+                let transformed = crate::tail_recursion::make_tail_recursive(
+                    arena,
+                    id,
+                    proc.name,
+                    proc.body.clone(),
+                    args.into_bump_slice(),
+                    proc.ret_layout,
+                );
+
+                if let Some(with_tco) = transformed {
+                    proc.body = with_tco;
+                    proc.args = proc_args.into_bump_slice();
+                }
+            }
+        }
+    }
+}
 
 /// Make tail calls into loops (using join points)
 ///
@@ -325,7 +402,7 @@ fn insert_jumps<'a>(
     }
 }
 
-pub(crate) fn is_trmc_candidate<'a, I>(interner: &I, proc: &Proc<'a>) -> bool
+pub(crate) fn is_trmc_candidate<'a, I>(interner: &'_ I, proc: &'_ Proc<'a>) -> bool
 where
     I: LayoutInterner<'a>,
 {
@@ -338,10 +415,68 @@ where
     }
 
     // and return a recursive tag union
-    match interner.get_repr(proc.ret_layout) {
+    if !matches!(interner.get_repr(proc.ret_layout), LayoutRepr::Union(union_layout) if union_layout.is_recursive())
-        LayoutRepr::Union(union_layout) => union_layout.is_recursive(),
+    {
-        _ => false,
+        return false;
     }
+
+    has_cons_in_tail_position(&proc.body, proc.name)
+}
+
+fn has_cons_in_tail_position(initial_stmt: &Stmt<'_>, function_name: LambdaName) -> bool {
+    // we are looking for code of the form
+    //
+    // let x = Tag a b c
+    // ret x
+
+    let mut stack = vec![(None, initial_stmt)];
+
+    while let Some((recursive_call, stmt)) = stack.pop() {
+        match stmt {
+            Stmt::Let(symbol, expr, _, next) => {
+                if let Some(cons_info) = TrmcEnv::is_terminal_constructor(stmt) {
+                    // must use the result of a recursive call directly as an argument
+                    if let Some(recursive_call) = recursive_call {
+                        if cons_info.arguments.contains(&recursive_call) {
+                            return true;
+                        }
+                    }
+                }
+
+                let recursive_call = recursive_call
+                    .or_else(|| TrmcEnv::is_recursive_expr(expr, function_name).map(|_| *symbol));
+
+                stack.push((recursive_call, next));
+            }
+            Stmt::Switch {
+                branches,
+                default_branch,
+                ..
+            } => {
+                for (_, _, stmt) in branches.iter() {
+                    stack.push((recursive_call, stmt));
+                }
+                stack.push((recursive_call, default_branch.1));
+            }
+            Stmt::Refcounting(_, next) => {
+                stack.push((recursive_call, next));
+            }
+            Stmt::Expect { remainder, .. }
+            | Stmt::ExpectFx { remainder, .. }
+            | Stmt::Dbg { remainder, .. } => {
+                stack.push((recursive_call, remainder));
+            }
+            Stmt::Join {
+                body, remainder, ..
+            } => {
+                stack.push((recursive_call, body));
+                stack.push((recursive_call, remainder));
+            }
+            Stmt::Ret(_) | Stmt::Jump(_, _) | Stmt::Crash(_, _) => { /* terminal */ }
+        }
+    }
+
+    false
 }
 
 #[derive(Clone)]
@@ -358,6 +493,7 @@ pub(crate) struct TrmcEnv<'a> {
     recursive_call: Option<(Symbol, Call<'a>)>,
 }
 
+#[derive(Debug)]
 struct ConstructorInfo<'a> {
     tag_layout: UnionLayout<'a>,
     tag_id: TagIdIntType,
@@ -365,25 +501,18 @@ struct ConstructorInfo<'a> {
 }
 
 impl<'a> TrmcEnv<'a> {
-    fn is_recursive_expr(&mut self, expr: &Expr<'a>) -> Option<Call<'a>> {
+    #[inline(always)]
-        if let Expr::Call(call) = expr {
+    fn is_terminal_constructor(stmt: &Stmt<'a>) -> Option<ConstructorInfo<'a>> {
-            self.is_recursive_call(call).then_some(call.clone())
-        } else {
-            None
-        }
-    }
-
-    fn is_terminal_constructor(&mut self, stmt: &Stmt<'a>) -> Option<ConstructorInfo<'a>> {
         match stmt {
             Stmt::Let(s1, expr, _layout, Stmt::Ret(s2)) if s1 == s2 => {
-                self.get_contructor_info(expr)
+                Self::get_contructor_info(expr)
             }
 
             _ => None,
         }
     }
 
-    fn get_contructor_info(&mut self, expr: &Expr<'a>) -> Option<ConstructorInfo<'a>> {
+    fn get_contructor_info(expr: &Expr<'a>) -> Option<ConstructorInfo<'a>> {
         if let Expr::Tag {
             tag_layout,
             tag_id,
@@ -402,16 +531,19 @@ impl<'a> TrmcEnv<'a> {
         }
     }
 
-    fn is_recursive_call(&mut self, call: &Call<'a>) -> bool {
+    fn is_recursive_expr(expr: &Expr<'a>, lambda_name: LambdaName<'_>) -> Option<Call<'a>> {
+        if let Expr::Call(call) = expr {
+            Self::is_recursive_call(call, lambda_name).then_some(call.clone())
+        } else {
+            None
+        }
+    }
+
+    fn is_recursive_call(call: &Call<'a>, lambda_name: LambdaName<'_>) -> bool {
         match call.call_type {
-            CallType::ByName {
+            CallType::ByName { name, .. } => {
-                name,
-                ret_layout,
-                arg_layouts,
-                specialization_id,
-            } => {
                 // TODO are there other restrictions?
-                name == self.function_name
+                name == lambda_name
             }
             CallType::Foreign { .. } | CallType::LowLevel { .. } | CallType::HigherOrder(_) => {
                 false
@@ -421,16 +553,16 @@ impl<'a> TrmcEnv<'a> {
 
     fn ptr_write(
         env: &mut Env<'a, '_>,
-        interner: &mut impl LayoutInterner<'a>,
+        _return_layout: InLayout<'a>,
-        return_layout: InLayout<'a>,
         ptr: Symbol,
         value: Symbol,
         next: &'a Stmt<'a>,
     ) -> Stmt<'a> {
         let box_write = Call {
             call_type: crate::ir::CallType::LowLevel {
-                op: roc_module::low_level::LowLevel::PtrWrite,
+                op: LowLevel::PtrStore,
-                update_mode: env.next_update_mode_id(),
+                // update_mode: env.next_update_mode_id(),
+                update_mode: UpdateModeId::BACKEND_DUMMY,
             },
             arguments: env.arena.alloc([ptr, value]),
         };
@@ -438,16 +570,13 @@ impl<'a> TrmcEnv<'a> {
         Stmt::Let(
             env.named_unique_symbol("_ptr_write_unit"),
             Expr::Call(box_write),
-            interner.insert_direct_no_semantic(LayoutRepr::Boxed(return_layout)),
+            // interner.insert_direct_no_semantic(LayoutRepr::Boxed(return_layout)),
+            Layout::UNIT,
             next,
         )
     }
 
-    pub fn init(
+    pub fn init<'i>(env: &mut Env<'a, 'i>, proc: &Proc<'a>) -> Proc<'a> {
-        env: &mut Env<'a, '_>,
-        interner: &mut impl LayoutInterner<'a>,
-        proc: &Proc<'a>,
-    ) -> Proc<'a> {
         let arena = env.arena;
         let return_layout = proc.ret_layout;
 
@@ -475,8 +604,9 @@ impl<'a> TrmcEnv<'a> {
         let null_symbol = env.named_unique_symbol("null");
         let let_null = |next| Stmt::Let(null_symbol, Expr::NullPointer, return_layout, next);
 
-        let box_return_layout =
+        let box_return_layout = env
-            interner.insert_direct_no_semantic(LayoutRepr::Boxed(return_layout));
+            .interner
+            .insert_direct_no_semantic(LayoutRepr::Boxed(return_layout));
         let box_null = Expr::ExprBox {
             symbol: null_symbol,
         };
@@ -508,7 +638,7 @@ impl<'a> TrmcEnv<'a> {
         let joinpoint = Stmt::Join {
             id: joinpoint_id,
             parameters: joinpoint_parameters.into_bump_slice(),
-            body: arena.alloc(this.walk_stmt(env, interner, &proc.body)),
+            body: arena.alloc(this.walk_stmt(env, &proc.body)),
             remainder: arena.alloc(jump_stmt),
         };
 
@@ -534,24 +664,19 @@ impl<'a> TrmcEnv<'a> {
         }
     }
 
-    fn walk_stmt(
+    fn walk_stmt(&mut self, env: &mut Env<'a, '_>, stmt: &Stmt<'a>) -> Stmt<'a> {
-        &mut self,
-        env: &mut Env<'a, '_>,
-        interner: &mut impl LayoutInterner<'a>,
-        stmt: &Stmt<'a>,
-    ) -> Stmt<'a> {
         let arena = env.arena;
 
         match stmt {
             Stmt::Let(symbol, expr, layout, next) => {
                 if self.recursive_call.is_none() {
-                    if let Some(call) = self.is_recursive_expr(expr) {
+                    if let Some(call) = Self::is_recursive_expr(expr, self.function_name) {
                         self.recursive_call = Some((*symbol, call));
-                        return self.walk_stmt(env, interner, next);
+                        return self.walk_stmt(env, next);
                     }
                 }
 
-                if let Some(cons_info) = self.is_terminal_constructor(stmt) {
+                if let Some(cons_info) = Self::is_terminal_constructor(stmt) {
                     match &self.recursive_call {
                         None => {
                             // this control flow path did not encounter a recursive call. Just
@@ -561,7 +686,7 @@ impl<'a> TrmcEnv<'a> {
 
                             let output = define_tag(arena.alloc(
                                 //
-                                self.non_trmc_return(env, interner, *symbol),
+                                self.non_trmc_return(env, *symbol),
                             ));
 
                             return output;
@@ -571,11 +696,21 @@ impl<'a> TrmcEnv<'a> {
                             // branch.
 
                             // TODO remove unwrap. also what if the symbol occurs more than once?
-                            let recursive_field_index = cons_info
+                            let opt_recursive_field_index =
-                                .arguments
+                                cons_info.arguments.iter().position(|s| *s == *call_symbol);
-                                .iter()
+
-                                .position(|s| *s == *call_symbol)
+                            let recursive_field_index = match opt_recursive_field_index {
-                                .unwrap();
+                                None => {
+                                    let next = self.walk_stmt(env, next);
+                                    return Stmt::Let(
+                                        *symbol,
+                                        expr.clone(),
+                                        *layout,
+                                        arena.alloc(next),
+                                    );
+                                }
+                                Some(v) => v,
+                            };
 
                             let mut arguments =
                                 Vec::from_iter_in(cons_info.arguments.iter().copied(), env.arena);
@@ -589,8 +724,11 @@ impl<'a> TrmcEnv<'a> {
 
                             let let_tag = |next| Stmt::Let(*symbol, tag_expr, *layout, next);
 
-                            let get_reference_expr = Expr::ExprBox {
+                            let get_reference_expr = Expr::UnionFieldPtrAtIndex {
-                                symbol: self.null_symbol,
+                                structure: *symbol,
+                                tag_id: cons_info.tag_id,
+                                union_layout: cons_info.tag_layout,
+                                index: recursive_field_index as _,
                             };
 
                             let new_hole_symbol = env.named_unique_symbol("newHole");
@@ -616,7 +754,6 @@ impl<'a> TrmcEnv<'a> {
                                     //
                                     Self::ptr_write(
                                         env,
-                                        interner,
                                         *layout,
                                         self.hole_symbol,
                                         *symbol,
@@ -630,7 +767,7 @@ impl<'a> TrmcEnv<'a> {
                     }
                 }
 
-                let next = self.walk_stmt(env, interner, next);
+                let next = self.walk_stmt(env, next);
                 Stmt::Let(*symbol, expr.clone(), *layout, arena.alloc(next))
             }
             Stmt::Switch {
@@ -646,14 +783,13 @@ impl<'a> TrmcEnv<'a> {
 
                 for (id, info, stmt) in branches.iter() {
                     self.recursive_call = opt_recursive_call.clone();
-                    let new_stmt = self.walk_stmt(env, interner, stmt);
+                    let new_stmt = self.walk_stmt(env, stmt);
 
                     new_branches.push((*id, info.clone(), new_stmt));
                 }
 
                 self.recursive_call = opt_recursive_call;
-                let new_default_branch =
+                let new_default_branch = &*arena.alloc(self.walk_stmt(env, default_branch.1));
-                    &*arena.alloc(self.walk_stmt(env, interner, default_branch.1));
 
                 Stmt::Switch {
                     cond_symbol: *cond_symbol,
@@ -666,42 +802,92 @@ impl<'a> TrmcEnv<'a> {
             Stmt::Ret(symbol) => {
                 // write the symbol we're supposed to return into the hole
                 // then read initial_symbol and return its contents
-                self.non_trmc_return(env, interner, *symbol)
+                self.non_trmc_return(env, *symbol)
             }
-            Stmt::Refcounting(_, _) => todo!(),
+            Stmt::Refcounting(op, next) => {
-            Stmt::Expect { .. } => todo!(),
+                let new_next = self.walk_stmt(env, next);
-            Stmt::ExpectFx { .. } => todo!(),
+                Stmt::Refcounting(*op, arena.alloc(new_next))
-            Stmt::Dbg { .. } => todo!(),
+            }
-            Stmt::Join { .. } => todo!(),
+            Stmt::Expect {
-            Stmt::Jump(_, _) => todo!(),
+                condition,
-            Stmt::Crash(_, _) => todo!(),
+                region,
+                lookups,
+                variables,
+                remainder,
+            } => Stmt::Expect {
+                condition: *condition,
+                region: *region,
+                lookups,
+                variables,
+                remainder: arena.alloc(self.walk_stmt(env, remainder)),
+            },
+            Stmt::ExpectFx {
+                condition,
+                region,
+                lookups,
+                variables,
+                remainder,
+            } => Stmt::Expect {
+                condition: *condition,
+                region: *region,
+                lookups,
+                variables,
+                remainder: arena.alloc(self.walk_stmt(env, remainder)),
+            },
+            Stmt::Dbg {
+                symbol,
+                variable,
+                remainder,
+            } => Stmt::Dbg {
+                symbol: *symbol,
+                variable: *variable,
+                remainder: arena.alloc(self.walk_stmt(env, remainder)),
+            },
+            Stmt::Join {
+                id,
+                parameters,
+                body,
+                remainder,
+            } => {
+                let new_body = self.walk_stmt(env, body);
+                let new_remainder = self.walk_stmt(env, remainder);
+
+                Stmt::Join {
+                    id: *id,
+                    parameters,
+                    body: arena.alloc(new_body),
+                    remainder: arena.alloc(new_remainder),
+                }
+            }
+            Stmt::Jump(id, arguments) => Stmt::Jump(*id, arguments),
+            Stmt::Crash(symbol, crash_tag) => Stmt::Crash(*symbol, *crash_tag),
         }
     }
 
-    fn non_trmc_return(
+    fn non_trmc_return(&mut self, env: &mut Env<'a, '_>, value_symbol: Symbol) -> Stmt<'a> {
-        &mut self,
-        env: &mut Env<'a, '_>,
-        interner: &mut impl LayoutInterner<'a>,
-        value_symbol: Symbol,
-    ) -> Stmt<'a> {
         let arena = env.arena;
         let layout = self.return_layout;
 
-        let unbox_expr = Expr::ExprUnbox {
-            symbol: self.initial_box_symbol,
-        };
         let final_symbol = env.named_unique_symbol("final");
-        let unbox = |next| Stmt::Let(final_symbol, unbox_expr, layout, next);
+
+        let call = Call {
+            call_type: CallType::LowLevel {
+                op: LowLevel::PtrLoad,
+                update_mode: UpdateModeId::BACKEND_DUMMY,
+            },
+            arguments: &*arena.alloc([self.initial_box_symbol]),
+        };
+
+        let ptr_load = |next| Stmt::Let(final_symbol, Expr::Call(call), layout, next);
 
         Self::ptr_write(
             env,
-            interner,
             layout,
             self.hole_symbol,
             value_symbol,
             arena.alloc(
                 //
-                unbox(arena.alloc(Stmt::Ret(final_symbol))),
+                ptr_load(arena.alloc(Stmt::Ret(final_symbol))),
             ),
         )
     }

examples/platform-switching/rocLovesZig.roc

@@ -3,4 +3,29 @@ app "rocLovesZig"
     imports []
     provides [main] to pf
 
-main = "Roc <3 Zig!\n"
+LinkedList a : [Nil, Cons a (LinkedList a)]
+
+map : LinkedList a, (a -> b) -> LinkedList b
+map = \list, f ->
+    when list is
+        Nil -> Nil
+        Cons x xs -> Cons (f x) (map xs f)
+
+unfold : a, Nat -> LinkedList a
+unfold = \value, n ->
+    when n is
+        0 -> Nil
+        _ -> Cons value (unfold value (n - 1))
+
+length : LinkedList a -> I64
+length = \list ->
+    when list is
+        Nil -> 0
+        Cons _ rest -> 1 + length rest
+
+main : Str 
+main = 
+    unfold 42 5
+        |> map (\x -> x + 1i64) 
+        |> length
+        |> Num.toStr