join points with arguments

compiler/gen/src/llvm/build.rs

@@ -41,7 +41,7 @@ pub enum OptLevel {
 #[derive(Default, Debug, Clone, PartialEq)]
 pub struct Scope<'a, 'ctx> {
     symbols: ImMap<Symbol, (Layout<'a>, PointerValue<'ctx>)>,
-    join_points: ImMap<JoinPointId, BasicBlock<'ctx>>,
+    join_points: ImMap<JoinPointId, (BasicBlock<'ctx>, &'a [PointerValue<'ctx>])>,
 }
 
 impl<'a, 'ctx> Scope<'a, 'ctx> {
@@ -604,11 +604,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
 
             result
         }
-        Ret(symbol) => {
+        Ret(symbol) => load_symbol(env, scope, symbol),
-            dbg!(symbol, &scope);
-
-            load_symbol(env, scope, symbol)
-        }
 
         Cond {
             branching_symbol,
@@ -714,11 +710,24 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             let builder = env.builder;
             let context = env.context;
 
+            let mut joinpoint_args = Vec::with_capacity_in(arguments.len(), env.arena);
+
+            for (_, layout) in arguments.iter() {
+                let btype = basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes);
+                joinpoint_args.push(create_entry_block_alloca(
+                    env,
+                    parent,
+                    btype,
+                    "joinpointarg",
+                ));
+            }
+
             // create new block
             let cont_block = context.append_basic_block(parent, "joinpointcont");
 
             // store this join point
-            scope.join_points.insert(*id, cont_block);
+            let joinpoint_args = joinpoint_args.into_bump_slice();
+            scope.join_points.insert(*id, (cont_block, joinpoint_args));
 
             // construct the blocks that may jump to this join point
             build_exp_stmt(env, layout_ids, scope, parent, remainder);
@@ -726,14 +735,11 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             // remove this join point again
             scope.join_points.remove(&id);
 
-            // Assumptions
+            for (ptr, (argument, layout)) in joinpoint_args.iter().zip(arguments.iter()) {
-            //
+                scope.insert(*argument, (layout.clone(), *ptr));
-            // - `remainder` is either a Cond or Switch where
+            }
-            // - all branches jump to this join point
+
-            //
-            // we should improve this in the future!
             let phi_block = builder.get_insert_block().unwrap();
-            //builder.build_unconditional_branch(cont_block);
 
             // put the cont block at the back
             builder.position_at_end(cont_block);
@@ -745,15 +751,20 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
 
             result
         }
-        Jump(join_point, _arguments) => {
+        Jump(join_point, arguments) => {
             let builder = env.builder;
             let context = env.context;
-            let cont_block = scope.join_points.get(join_point).unwrap();
+            let (cont_block, argument_pointers) = scope.join_points.get(join_point).unwrap();
-            let jmp = builder.build_unconditional_branch(*cont_block);
+
-            // builder.insert_instruction(&jmp, None);
+            for (pointer, argument) in argument_pointers.iter().zip(arguments.iter()) {
+                let value = load_symbol(env, scope, argument);
+                builder.build_store(*pointer, value);
+            }
+
+            builder.build_unconditional_branch(*cont_block);
 
             // This doesn't currently do anything
-            context.i64_type().const_int(0, false).into()
+            context.i64_type().const_zero().into()
         }
         _ => todo!("unsupported expr {:?}", stmt),
     }
@@ -1724,21 +1735,30 @@ fn build_switch_ir<'a, 'ctx, 'env>(
 
         let branch_val = build_exp_stmt(env, layout_ids, scope, parent, branch_expr);
 
+        if block.get_terminator().is_none() {
             builder.build_unconditional_branch(cont_block);
-
             incoming.push((branch_val, block));
         }
+    }
 
     // The block for the conditional's default branch.
     builder.position_at_end(default_block);
 
     let default_val = build_exp_stmt(env, layout_ids, scope, parent, default_branch);
 
+    if default_block.get_terminator().is_none() {
         builder.build_unconditional_branch(cont_block);
-
         incoming.push((default_val, default_block));
+    }
 
     // emit merge block
+    if incoming.is_empty() {
+        unsafe {
+            cont_block.delete().unwrap();
+        }
+        // produce unused garbage value
+        context.i64_type().const_zero().into()
+    } else {
         builder.position_at_end(cont_block);
 
         let phi = builder.build_phi(ret_type, "branch");
@@ -1749,6 +1769,7 @@ fn build_switch_ir<'a, 'ctx, 'env>(
 
         phi.as_basic_value()
     }
+}
 
 fn build_basic_phi2<'a, 'ctx, 'env, PassFn, FailFn>(
     env: &Env<'a, 'ctx, 'env>,

compiler/gen/tests/gen_tags.rs

@@ -663,17 +663,74 @@ mod gen_tags {
     }
 
     #[test]
-    fn join_points() {
+    fn join_point_if() {
         assert_evals_to_ir!(
             indoc!(
                 r#"
                 x =
                     if True then 1 else 2
 
-                5
+                x
                 "#
             ),
-            5,
+            1,
+            i64
+        );
+    }
+
+    #[test]
+    fn join_point_when() {
+        assert_evals_to_ir!(
+            indoc!(
+                r#"
+            x : [ Red, White, Blue ]
+            x = Blue
+
+            y = 
+                when x is
+                    Red -> 1
+                    White -> 2
+                    Blue -> 3.1
+
+            y
+            "#
+            ),
+            3.1,
+            f64
+        );
+    }
+
+    #[test]
+    fn join_point_with_cond_expr() {
+        assert_evals_to_ir!(
+            indoc!(
+                r#"
+            y = 
+                when 1 + 2 is
+                    3 -> 3
+                    1 -> 1
+                    _ -> 0
+
+            y
+            "#
+            ),
+            3,
+            i64
+        );
+
+        assert_evals_to_ir!(
+            indoc!(
+                r#"
+            y = 
+                if 1 + 2 > 0 then
+                    3
+                else
+                    0
+
+            y
+            "#
+            ),
+            3,
             i64
         );
     }

compiler/mono/src/decision_tree2.rs

@@ -873,7 +873,7 @@ enum Decider<'a, T> {
 
 #[derive(Clone, Debug, PartialEq)]
 enum Choice<'a> {
-    Inline(Stores<'a>, Stmt<'a>),
+    Inline(Stmt<'a>),
     Jump(Label),
 }
 
@@ -885,20 +885,17 @@ pub fn optimize_when<'a>(
     cond_symbol: Symbol,
     cond_layout: Layout<'a>,
     ret_layout: Layout<'a>,
-    opt_branches: Vec<(Pattern<'a>, Guard<'a>, Stores<'a>, Stmt<'a>)>,
+    opt_branches: bumpalo::collections::Vec<'a, (Pattern<'a>, Guard<'a>, Stmt<'a>)>,
 ) -> Stmt<'a> {
     let (patterns, _indexed_branches) = opt_branches
         .into_iter()
         .enumerate()
-        .map(|(index, (pattern, guard, stores, branch))| {
+        .map(|(index, (pattern, guard, branch))| {
-            (
+            ((guard, pattern, index as u64), (index as u64, branch))
-                (guard, pattern, index as u64),
-                (index as u64, stores, branch),
-            )
         })
         .unzip();
 
-    let indexed_branches: Vec<(u64, Stores<'a>, Stmt<'a>)> = _indexed_branches;
+    let indexed_branches: Vec<(u64, Stmt<'a>)> = _indexed_branches;
 
     let decision_tree = compile(patterns);
     let decider = tree_to_decider(decision_tree);
@@ -907,9 +904,8 @@ pub fn optimize_when<'a>(
     let mut choices = MutMap::default();
     let mut jumps = Vec::new();
 
-    for (index, stores, branch) in indexed_branches.into_iter() {
+    for (index, branch) in indexed_branches.into_iter() {
-        let ((branch_index, choice), opt_jump) =
+        let ((branch_index, choice), opt_jump) = create_choices(&target_counts, index, branch);
-            create_choices(&target_counts, index, stores, branch);
 
         if let Some(jump) = opt_jump {
             jumps.push(jump);
@@ -920,7 +916,7 @@ pub fn optimize_when<'a>(
 
     let choice_decider = insert_choices(&choices, decider);
 
-    let (stores, expr) = decide_to_branching(
+    let expr = decide_to_branching(
         env,
         cond_symbol,
         cond_layout,
@@ -932,7 +928,6 @@ pub fn optimize_when<'a>(
     // increase the jump counter by the number of jumps in this branching structure
     *env.jump_counter += jumps.len() as u64;
 
-    // Expr::Store(stores, env.arena.alloc(expr))
     expr
 }
 
@@ -1100,6 +1095,7 @@ fn test_to_equality<'a>(
             let lhs = Expr::Literal(Literal::Bool(test_bit));
             let lhs_symbol = env.unique_symbol();
             let (mut stores, rhs_symbol) = path_to_expr(env, cond_symbol, &path, &cond_layout);
+            stores.push((lhs_symbol, Layout::Builtin(Builtin::Int1), lhs));
 
             (
                 stores,
@@ -1134,21 +1130,21 @@ fn decide_to_branching<'a>(
     cond_layout: Layout<'a>,
     ret_layout: Layout<'a>,
     decider: Decider<'a, Choice<'a>>,
-    jumps: &Vec<(u64, Stores<'a>, Stmt<'a>)>,
+    jumps: &Vec<(u64, Stmt<'a>)>,
-) -> (Stores<'a>, Stmt<'a>) {
+) -> Stmt<'a> {
     use Choice::*;
     use Decider::*;
 
     match decider {
         Leaf(Jump(label)) => {
             // we currently inline the jumps: does fewer jumps but produces a larger artifact
-            let (_, stores, expr) = jumps
+            let (_, expr) = jumps
                 .iter()
-                .find(|(l, _, _)| l == &label)
+                .find(|(l, _)| l == &label)
                 .expect("jump not in list of jumps");
-            (stores, expr.clone())
+            expr.clone()
         }
-        Leaf(Inline(stores, expr)) => (stores, expr),
+        Leaf(Inline(expr)) => expr,
         Chain {
             test_chain,
             success,
@@ -1156,7 +1152,7 @@ fn decide_to_branching<'a>(
         } => {
             // generate a switch based on the test chain
 
-            let (pass_stores, mut pass_expr) = decide_to_branching(
+            let pass_expr = decide_to_branching(
                 env,
                 cond_symbol,
                 cond_layout.clone(),
@@ -1165,15 +1161,7 @@ fn decide_to_branching<'a>(
                 jumps,
             );
 
-            dbg!(&pass_expr);
+            let fail_expr = decide_to_branching(
-
-            // TODO remove clone
-            for (symbol, layout, expr) in pass_stores.iter().cloned().rev() {
-                println!("{} {:?}", symbol, expr);
-                pass_expr = Stmt::Let(symbol, expr, layout, env.arena.alloc(pass_expr));
-            }
-
-            let (fail_stores, mut fail_expr) = decide_to_branching(
                 env,
                 cond_symbol,
                 cond_layout.clone(),
@@ -1182,11 +1170,6 @@ fn decide_to_branching<'a>(
                 jumps,
             );
 
-            // TODO remove clone
-            for (symbol, layout, expr) in fail_stores.iter().cloned().rev() {
-                fail_expr = Stmt::Let(symbol, expr, layout, env.arena.alloc(fail_expr));
-            }
-
             let fail = &*env.arena.alloc(fail_expr);
             let pass = &*env.arena.alloc(pass_expr);
 
@@ -1331,7 +1314,7 @@ fn decide_to_branching<'a>(
             );
 
             // (env.arena.alloc(stores), cond)
-            (&[], cond)
+            cond
         }
         FanOut {
             path,
@@ -1344,7 +1327,7 @@ fn decide_to_branching<'a>(
             let (cond, cond_stores_vec, cond_layout) =
                 path_to_expr_help2(env, cond_symbol, &path, cond_layout);
 
-            let (default_stores, mut default_branch) = decide_to_branching(
+            let default_branch = decide_to_branching(
                 env,
                 cond_symbol,
                 cond_layout.clone(),
@@ -1353,15 +1336,10 @@ fn decide_to_branching<'a>(
                 jumps,
             );
 
-            // TODO remove clone
-            for (symbol, layout, expr) in default_stores.iter().cloned() {
-                default_branch = Stmt::Let(symbol, expr, layout, env.arena.alloc(default_branch));
-            }
-
             let mut branches = bumpalo::collections::Vec::with_capacity_in(tests.len(), env.arena);
 
             for (test, decider) in tests {
-                let (stores, mut branch) = decide_to_branching(
+                let branch = decide_to_branching(
                     env,
                     cond_symbol,
                     cond_layout.clone(),
@@ -1370,11 +1348,6 @@ fn decide_to_branching<'a>(
                     jumps,
                 );
 
-                // TODO remove clone
-                for (symbol, layout, expr) in stores.iter().cloned() {
-                    branch = Stmt::Let(symbol, expr, layout, env.arena.alloc(branch));
-                }
-
                 let tag = match test {
                     Test::IsInt(v) => v as u64,
                     Test::IsFloat(v) => v as u64,
@@ -1386,7 +1359,6 @@ fn decide_to_branching<'a>(
 
                 branches.push((tag, branch));
             }
-            dbg!(&branches, &default_branch);
 
             let mut switch = Stmt::Switch {
                 cond_layout,
@@ -1401,7 +1373,7 @@ fn decide_to_branching<'a>(
             }
 
             // make a jump table based on the tests
-            (&[], switch)
+            switch
         }
     }
 }
@@ -1601,19 +1573,15 @@ fn count_targets_help(decision_tree: &Decider<u64>, targets: &mut MutMap<u64, u6
 fn create_choices<'a>(
     target_counts: &MutMap<u64, u64>,
     target: u64,
-    stores: Stores<'a>,
     branch: Stmt<'a>,
-) -> ((u64, Choice<'a>), Option<(u64, Stores<'a>, Stmt<'a>)>) {
+) -> ((u64, Choice<'a>), Option<(u64, Stmt<'a>)>) {
     match target_counts.get(&target) {
         None => unreachable!(
             "this should never happen: {:?} not in {:?}",
             target, target_counts
         ),
-        Some(1) => ((target, Choice::Inline(stores, branch)), None),
+        Some(1) => ((target, Choice::Inline(branch)), None),
-        Some(_) => (
+        Some(_) => ((target, Choice::Jump(target)), Some((target, branch))),
-            (target, Choice::Jump(target)),
-            Some((target, stores, branch)),
-        ),
     }
 }
 

compiler/mono/src/experiment.rs

@@ -349,7 +349,7 @@ pub enum Stmt<'a> {
     Dec(Symbol, &'a Stmt<'a>),
     Join {
         id: JoinPointId,
-        arguments: &'a [Symbol],
+        arguments: &'a [(Symbol, Layout<'a>)],
         /// does not contain jumps to this id
         continuation: &'a Stmt<'a>,
         /// contains the jumps to this id
@@ -548,7 +548,7 @@ impl<'a> Stmt<'a> {
     ) -> Self {
         let mut layout_cache = LayoutCache::default();
 
-        from_can(env, can_expr, procs, &mut layout_cache)
+        dbg!(from_can(env, can_expr, procs, &mut layout_cache))
     }
     pub fn to_doc<'b, D, A>(&'b self, alloc: &'b D, parens: bool) -> DocBuilder<'b, D, A>
     where
@@ -625,17 +625,23 @@ impl<'a> Stmt<'a> {
                 arguments,
                 continuation,
                 remainder,
-            } => alloc.intersperse(
+            } => {
+                let it = arguments.iter().map(|(s, _)| symbol_to_doc(alloc, *s));
+
+                alloc.intersperse(
                     vec![
                         remainder.to_doc(alloc, false),
                         alloc
                             .text("joinpoint ")
                             .append(join_point_to_doc(alloc, *id))
+                            .append(" ".repeat(arguments.len().min(1)))
+                            .append(alloc.intersperse(it, alloc.space()))
                             .append(":"),
                         continuation.to_doc(alloc, false).indent(4),
                     ],
                     alloc.hardline(),
-            ),
+                )
+            }
             Jump(id, arguments) => {
                 let it = arguments.iter().map(|s| symbol_to_doc(alloc, *s));
 
@@ -708,7 +714,7 @@ fn patterns_to_when<'a>(
     // are only stores anyway, no branches.
     for (pattern_var, pattern) in patterns.into_iter() {
         let context = crate::pattern2::Context::BadArg;
-        let mono_pattern = from_can_pattern(env, procs, layout_cache, &pattern.value);
+        let mono_pattern = from_can_pattern(env, layout_cache, &pattern.value);
 
         match crate::pattern2::check(
             pattern.region,
@@ -1241,7 +1247,7 @@ pub fn with_hole<'a>(
                 .expect("invalid cond_layout");
 
             let id = JoinPointId(env.unique_symbol());
-            let jump = env.arena.alloc(Stmt::Jump(id, &[]));
+            let jump = env.arena.alloc(Stmt::Jump(id, env.arena.alloc([assigned])));
 
             let mut stmt = with_hole(env, final_else.value, procs, layout_cache, assigned, jump);
 
@@ -1270,18 +1276,66 @@ pub fn with_hole<'a>(
                 );
             }
 
-            let join = Stmt::Join {
+            let layout = layout_cache
+                .from_var(env.arena, branch_var, env.subs, env.pointer_size)
+                .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
+
+            Stmt::Join {
                 id,
-                arguments: &[],
+                arguments: env.arena.alloc([(assigned, layout)]),
                 remainder: env.arena.alloc(stmt),
                 continuation: hole,
-            };
+            }
-
-            // expr
-            join
         }
 
-        When { .. } | If { .. } => todo!("when or if in expression requires join points"),
+        When {
+            cond_var,
+            expr_var,
+            region,
+            loc_cond,
+            branches,
+        } => {
+            let cond_symbol = if let roc_can::expr::Expr::Var(symbol) = loc_cond.value {
+                symbol
+            } else {
+                env.unique_symbol()
+            };
+
+            let id = JoinPointId(env.unique_symbol());
+
+            let mut stmt = from_can_when(
+                env,
+                cond_var,
+                expr_var,
+                region,
+                cond_symbol,
+                branches,
+                layout_cache,
+                procs,
+                Some((id, assigned)),
+            );
+
+            // TODO define condition
+
+            // define the `when` condition
+            if let roc_can::expr::Expr::Var(_) = loc_cond.value {
+                // do nothing
+            } else {
+                let hole = env.arena.alloc(stmt);
+                stmt = with_hole(env, loc_cond.value, procs, layout_cache, cond_symbol, hole);
+            };
+
+            let layout = layout_cache
+                .from_var(env.arena, expr_var, env.subs, env.pointer_size)
+                .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
+
+            Stmt::Join {
+                id,
+                arguments: bumpalo::vec![in env.arena; (assigned, layout)].into_bump_slice(),
+                remainder: env.arena.alloc(stmt),
+                continuation: hole,
+            }
+        }
 
         List { loc_elems, .. } if loc_elems.is_empty() => {
             // because an empty list has an unknown element type, it is handled differently
@@ -1687,6 +1741,7 @@ pub fn from_can<'a>(
                 branches,
                 layout_cache,
                 procs,
+                None,
             );
 
             if let roc_can::expr::Expr::Var(_) = loc_cond.value {
@@ -1704,7 +1759,7 @@ pub fn from_can<'a>(
     }
 }
 
-fn from_can_when<'a>(
+fn to_opt_branches<'a>(
     env: &mut Env<'a, '_>,
     cond_var: Variable,
     expr_var: Variable,
@@ -1712,75 +1767,13 @@ fn from_can_when<'a>(
     cond_symbol: Symbol,
     mut branches: std::vec::Vec<roc_can::expr::WhenBranch>,
     layout_cache: &mut LayoutCache<'a>,
-    procs: &mut Procs<'a>,
+) -> std::vec::Vec<(
-) -> Stmt<'a> {
+    Pattern<'a>,
-    if branches.is_empty() {
+    crate::decision_tree2::Guard<'a>,
-        // A when-expression with no branches is a runtime error.
+    roc_can::expr::Expr,
-        // We can't know what to return!
+)> {
-        Stmt::RuntimeError("Hit a 0-branch when expression")
+    debug_assert!(!branches.is_empty());
-    } else if branches.len() == 1 && branches[0].patterns.len() == 1 && branches[0].guard.is_none()
-    {
-        let first = branches.remove(0);
-        // A when-expression with exactly 1 branch is essentially a LetNonRec.
-        // As such, we can compile it direcly to a Store.
-        let arena = env.arena;
-        let mut stored = Vec::with_capacity_in(1, arena);
 
-        let bound_symbols = first
-            .patterns
-            .iter()
-            .map(|pat| roc_can::pattern::symbols_from_pattern(&pat.value))
-            .flatten()
-            .collect::<std::vec::Vec<_>>();
-
-        let loc_when_pattern = &first.patterns[0];
-
-        let mono_pattern = from_can_pattern(env, procs, layout_cache, &loc_when_pattern.value);
-
-        // record pattern matches can have 1 branch and typecheck, but may still not be exhaustive
-        let guard = if first.guard.is_some() {
-            Guard::HasGuard
-        } else {
-            Guard::NoGuard
-        };
-
-        let context = crate::pattern2::Context::BadCase;
-        match crate::pattern2::check(
-            region,
-            &[(
-                Located::at(loc_when_pattern.region, mono_pattern.clone()),
-                guard,
-            )],
-            context,
-        ) {
-            Ok(_) => {}
-            Err(errors) => {
-                for error in errors {
-                    env.problems.push(MonoProblem::PatternProblem(error))
-                }
-
-                // panic!("generate runtime error, should probably also optimize this");
-            }
-        }
-
-        let cond_layout = layout_cache
-            .from_var(env.arena, cond_var, env.subs, env.pointer_size)
-            .unwrap_or_else(|err| panic!("TODO turn this into a RuntimeError {:?}", err));
-
-        // NOTE this will still store shadowed names.
-        // that's fine: the branch throws a runtime error anyway
-        let mut ret = match store_pattern(env, &mono_pattern, cond_symbol, cond_layout, &mut stored)
-        {
-            Ok(_) => from_can(env, first.value.value, procs, layout_cache),
-            Err(message) => Stmt::RuntimeError(env.arena.alloc(message)),
-        };
-
-        for (symbol, layout, expr) in stored.iter().rev().cloned() {
-            ret = Stmt::Let(symbol, expr, layout, env.arena.alloc(ret));
-        }
-
-        ret
-    } else {
     let cond_layout = layout_cache
         .from_var(env.arena, cond_var, env.subs, env.pointer_size)
         .unwrap_or_else(|err| panic!("TODO turn this into a RuntimeError {:?}", err));
@@ -1789,8 +1782,6 @@ fn from_can_when<'a>(
     let mut opt_branches = std::vec::Vec::new();
 
     for when_branch in branches {
-            let mono_expr = from_can(env, when_branch.value.value, procs, layout_cache);
-
         let exhaustive_guard = if when_branch.guard.is_some() {
             Guard::HasGuard
         } else {
@@ -1798,92 +1789,23 @@ fn from_can_when<'a>(
         };
 
         for loc_pattern in when_branch.patterns {
-                let mono_pattern = from_can_pattern(env, procs, layout_cache, &loc_pattern.value);
+            let mono_pattern = from_can_pattern(env, layout_cache, &loc_pattern.value);
 
             loc_branches.push((
                 Located::at(loc_pattern.region, mono_pattern.clone()),
                 exhaustive_guard.clone(),
             ));
 
-                let mut stores = Vec::with_capacity_in(1, env.arena);
+            // TODO implement guard again
+            let mono_guard = crate::decision_tree2::Guard::NoGuard;
 
-                let (mono_guard, stores, expr) = match store_pattern(
+            opt_branches.push((mono_pattern, mono_guard, when_branch.value.value.clone()));
-                    env,
-                    &mono_pattern,
-                    cond_symbol,
-                    cond_layout.clone(),
-                    &mut stores,
-                ) {
-                    Ok(_) => {
-                        // if the branch is guarded, the guard can use variables bound in the
-                        // pattern. They must be available, so we give the stores to the
-                        // decision_tree. A branch with guard can only be entered with the guard
-                        // evaluated, so variables will also be loaded in the branch's body expr.
-                        //
-                        // otherwise, we modify the branch's expression to include the stores
-                        if let Some(loc_guard) = when_branch.guard.clone() {
-                            let guard_symbol = env.unique_symbol();
-                            let id = JoinPointId(env.unique_symbol());
-
-                            let hole = env.arena.alloc(Stmt::Jump(id, &[]));
-                            let mut stmt = with_hole(
-                                env,
-                                loc_guard.value,
-                                procs,
-                                layout_cache,
-                                guard_symbol,
-                                hole,
-                            );
-
-                            for (symbol, expr, layout) in stores.into_iter().rev() {
-                                stmt = Stmt::Let(symbol, layout, expr, env.arena.alloc(stmt));
-                            }
-                            (
-                                crate::decision_tree2::Guard::Guard {
-                                    stmt,
-                                    id,
-                                    symbol: guard_symbol,
-                                },
-                                &[] as &[_],
-                                mono_expr.clone(),
-                            )
-                        } else {
-                            (
-                                crate::decision_tree2::Guard::NoGuard,
-                                stores.into_bump_slice(),
-                                mono_expr.clone(),
-                            )
-                        }
-                    }
-                    Err(message) => {
-                        // when the pattern is invalid, a guard must give a runtime error too
-                        if when_branch.guard.is_some() {
-                            /*
-                            (
-                                crate::decision_tree2::Guard::Guard {
-                                    stores: &[],
-                                    expr: Stmt::RuntimeError(env.arena.alloc(message)),
-                                },
-                                &[] as &[_],
-                                // we can never hit this
-                                Stmt::RuntimeError(&"invalid pattern with guard: unreachable"),
-                            )
-                                */
-                            todo!()
-                        } else {
-                            (
-                                crate::decision_tree2::Guard::NoGuard,
-                                &[] as &[_],
-                                Stmt::RuntimeError(env.arena.alloc(message)),
-                            )
-                        }
-                    }
-                };
-
-                opt_branches.push((mono_pattern, mono_guard, stores, expr));
         }
     }
 
+    // NOTE exhaustiveness is checked after the construction of all the branches
+    // In contrast to elm (currently), we still do codegen even if a pattern is non-exhaustive.
+    // So we not only report exhaustiveness errors, but also correct them
     let context = crate::pattern2::Context::BadCase;
     match crate::pattern2::check(region, &loc_branches, context) {
         Ok(_) => {}
@@ -1914,41 +1836,105 @@ fn from_can_when<'a>(
                 opt_branches.push((
                     Pattern::Underscore,
                     crate::decision_tree2::Guard::NoGuard,
-                        &[],
+                    roc_can::expr::Expr::RuntimeError(
-                        Stmt::RuntimeError("non-exhaustive pattern match"),
+                        roc_problem::can::RuntimeError::NonExhaustivePattern,
+                    ),
                 ));
             }
         }
     }
 
+    opt_branches
+}
+
+fn from_can_when<'a>(
+    env: &mut Env<'a, '_>,
+    cond_var: Variable,
+    expr_var: Variable,
+    region: Region,
+    cond_symbol: Symbol,
+    branches: std::vec::Vec<roc_can::expr::WhenBranch>,
+    layout_cache: &mut LayoutCache<'a>,
+    procs: &mut Procs<'a>,
+    join_point: Option<(JoinPointId, Symbol)>,
+) -> Stmt<'a> {
+    if branches.is_empty() {
+        // A when-expression with no branches is a runtime error.
+        // We can't know what to return!
+        return Stmt::RuntimeError("Hit a 0-branch when expression");
+    }
+    let opt_branches = to_opt_branches(
+        env,
+        cond_var,
+        expr_var,
+        region,
+        cond_symbol,
+        branches,
+        layout_cache,
+    );
+
+    let cond_layout = layout_cache
+        .from_var(env.arena, cond_var, env.subs, env.pointer_size)
+        .unwrap_or_else(|err| panic!("TODO turn this into a RuntimeError {:?}", err));
+
     let ret_layout = layout_cache
         .from_var(env.arena, expr_var, env.subs, env.pointer_size)
         .unwrap_or_else(|err| panic!("TODO turn this into a RuntimeError {:?}", err));
 
+    let arena = env.arena;
+    let it = opt_branches.into_iter().map(|(pattern, guard, can_expr)| {
+        let mut stores = Vec::with_capacity_in(1, env.arena);
+        let res_stores =
+            store_pattern(env, &pattern, cond_symbol, cond_layout.clone(), &mut stores);
+        let mut stmt = match join_point {
+            None => from_can(env, can_expr, procs, layout_cache),
+            Some((id, _symbol)) => {
+                let symbol = env.unique_symbol();
+                let arguments = bumpalo::vec![in env.arena; symbol].into_bump_slice();
+                let jump = env.arena.alloc(Stmt::Jump(id, arguments));
+
+                with_hole(env, can_expr, procs, layout_cache, symbol, jump)
+            }
+        };
+
+        match res_stores {
+            Ok(_) => {
+                for (symbol, layout, expr) in stores.into_iter().rev() {
+                    stmt = Stmt::Let(symbol, expr, layout, env.arena.alloc(stmt));
+                }
+
+                (pattern, guard, stmt)
+            }
+            Err(msg) => (
+                Pattern::Underscore,
+                guard,
+                Stmt::RuntimeError(env.arena.alloc(msg)),
+            ),
+        }
+    });
+    let mono_branches = Vec::from_iter_in(it, arena);
+
     crate::decision_tree2::optimize_when(
         env,
         cond_symbol,
         cond_layout.clone(),
         ret_layout,
-            opt_branches,
+        mono_branches,
     )
 }
-}
 
 fn store_pattern<'a>(
     env: &mut Env<'a, '_>,
     can_pat: &Pattern<'a>,
     outer_symbol: Symbol,
-    layout: Layout<'a>,
+    _layout: Layout<'a>,
     stored: &mut Vec<'a, (Symbol, Layout<'a>, Expr<'a>)>,
 ) -> Result<(), String> {
     use Pattern::*;
 
     match can_pat {
         Identifier(symbol) => {
-            // let load = Expr::Load(outer_symbol);
+            // TODO surely something should happen here?
-            // stored.push((*symbol, layout, load))
-            // todo!()
         }
         Underscore => {
             // Since _ is never read, it's safe to reassign it.
@@ -2343,7 +2329,7 @@ pub struct RecordDestruct<'a> {
 #[derive(Clone, Debug, PartialEq)]
 pub enum DestructType<'a> {
     Required,
-    Optional(Stmt<'a>),
+    Optional(roc_can::expr::Expr),
     Guard(Pattern<'a>),
 }
 
@@ -2356,7 +2342,6 @@ pub struct WhenBranch<'a> {
 
 pub fn from_can_pattern<'a>(
     env: &mut Env<'a, '_>,
-    procs: &mut Procs<'a>,
     layout_cache: &mut LayoutCache<'a>,
     can_pattern: &roc_can::pattern::Pattern,
 ) -> Pattern<'a> {
@@ -2466,7 +2451,7 @@ pub fn from_can_pattern<'a>(
                     let mut mono_args = Vec::with_capacity_in(arguments.len(), env.arena);
                     for ((_, loc_pat), layout) in arguments.iter().zip(field_layouts.iter()) {
                         mono_args.push((
-                            from_can_pattern(env, procs, layout_cache, &loc_pat.value),
+                            from_can_pattern(env, layout_cache, &loc_pat.value),
                             layout.clone(),
                         ));
                     }
@@ -2508,7 +2493,7 @@ pub fn from_can_pattern<'a>(
                     let it = argument_layouts[1..].iter();
                     for ((_, loc_pat), layout) in arguments.iter().zip(it) {
                         mono_args.push((
-                            from_can_pattern(env, procs, layout_cache, &loc_pat.value),
+                            from_can_pattern(env, layout_cache, &loc_pat.value),
                             layout.clone(),
                         ));
                     }
@@ -2561,7 +2546,6 @@ pub fn from_can_pattern<'a>(
 
                         mono_destructs.push(from_can_record_destruct(
                             env,
-                            procs,
                             layout_cache,
                             &destruct.value,
                             field_layout.clone(),
@@ -2597,7 +2581,6 @@ pub fn from_can_pattern<'a>(
 
 fn from_can_record_destruct<'a>(
     env: &mut Env<'a, '_>,
-    procs: &mut Procs<'a>,
     layout_cache: &mut LayoutCache<'a>,
     can_rd: &roc_can::pattern::RecordDestruct,
     field_layout: Layout<'a>,
@@ -2609,11 +2592,11 @@ fn from_can_record_destruct<'a>(
         typ: match &can_rd.typ {
             roc_can::pattern::DestructType::Required => DestructType::Required,
             roc_can::pattern::DestructType::Optional(_, loc_expr) => {
-                DestructType::Optional(from_can(env, loc_expr.value.clone(), procs, layout_cache))
+                DestructType::Optional(loc_expr.value.clone())
+            }
+            roc_can::pattern::DestructType::Guard(_, loc_pattern) => {
+                DestructType::Guard(from_can_pattern(env, layout_cache, &loc_pattern.value))
             }
-            roc_can::pattern::DestructType::Guard(_, loc_pattern) => DestructType::Guard(
-                from_can_pattern(env, procs, layout_cache, &loc_pattern.value),
-            ),
         },
     }
 }

compiler/mono/tests/test_mono.rs

@@ -1555,4 +1555,35 @@ mod test_mono {
             ),
         )
     }
+
+    #[test]
+    fn when_joinpoint() {
+        compiles_to_ir(
+            r#"
+            x : [ Red, White, Blue ]
+            x = Blue
+
+            y = 
+                when x is
+                    Red -> 1
+                    White -> 2
+                    Blue -> 3
+
+            y
+            "#,
+            indoc!(
+                r#"
+                procedure List.5 (#Attr.2, #Attr.3):
+                    let Test.3 = lowlevel ListPush #Attr.2 #Attr.3;
+                    ret Test.3;
+
+                let Test.4 = 1i64;
+                let Test.1 = Array [Test.4];
+                let Test.2 = 2i64;
+                let Test.0 = CallByName List.5 Test.1 Test.2;
+                ret Test.0;
+                "#
+            ),
+        )
+    }
 }

compiler/problem/src/can.rs

@@ -123,6 +123,8 @@ pub enum RuntimeError {
     InvalidInt(IntErrorKind, Base, Region, Box<str>),
     CircularDef(Vec<Symbol>, Vec<(Region /* pattern */, Region /* expr */)>),
 
+    NonExhaustivePattern,
+
     /// When the author specifies a type annotation but no implementation
     NoImplementation,
 }