for referenced values, use allocas instead of phi nodes

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

@@ -3292,14 +3292,18 @@ pub(crate) fn build_exp_stmt<'a, 'ctx>(
                         layout_interner.get_repr(param.layout),
                     );
 
-                    let phi_type = if layout_interner.is_passed_by_reference(param.layout) {
-                        basic_type.ptr_type(AddressSpace::default()).into()
+                    use crate::llvm::scope::JoinPointArg::*;
+                    let joinpoint_arg = if layout_interner.is_passed_by_reference(param.layout) {
+                        Alloca(create_entry_block_alloca(
+                            env,
+                            basic_type,
+                            "joinpoint_arg_alloca",
+                        ))
                     } else {
-                        basic_type
+                        Phi(env.builder.new_build_phi(basic_type, "joinpointarg"))
                     };
 
-                    let phi_node = env.builder.new_build_phi(phi_type, "joinpointarg");
-                    joinpoint_args.push(phi_node);
+                    joinpoint_args.push(joinpoint_arg);
                 }
 
                 builder.position_at_end(current);
@@ -3351,14 +3355,30 @@ pub(crate) fn build_exp_stmt<'a, 'ctx>(
         Jump(join_point, arguments) => {
             let builder = env.builder;
             let context = env.context;
-            let (cont_block, argument_phi_values) = scope.get_join_point(*join_point).unwrap();
+            let (cont_block, joinpoint_args) = scope.get_join_point(*join_point).unwrap();
 
             let current_block = builder.get_insert_block().unwrap();
 
-            for (phi_value, argument) in argument_phi_values.iter().zip(arguments.iter()) {
-                let (value, _) = scope.load_symbol_and_layout(argument);
+            for (joinpoint_arg, argument) in joinpoint_args.iter().zip(arguments.iter()) {
+                let (value, layout) = scope.load_symbol_and_layout(argument);
 
-                phi_value.add_incoming(&[(&value, current_block)]);
+                match joinpoint_arg {
+                    crate::llvm::scope::JoinPointArg::Alloca(alloca) => {
+                        let (size, alignment) = layout_interner.stack_size_and_alignment(layout);
+                        builder
+                            .build_memcpy(
+                                *alloca,
+                                alignment,
+                                value.into_pointer_value(),
+                                alignment,
+                                env.ptr_int().const_int(size as _, false),
+                            )
+                            .unwrap();
+                    }
+                    crate::llvm::scope::JoinPointArg::Phi(phi) => {
+                        phi.add_incoming(&[(&value, current_block)]);
+                    }
+                }
             }
 
             builder.new_build_unconditional_branch(*cont_block);

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

@@ -1,6 +1,6 @@
 use inkwell::{
     basic_block::BasicBlock,
-    values::{BasicValueEnum, FunctionValue, PhiValue},
+    values::{BasicValue, BasicValueEnum, FunctionValue, PhiValue, PointerValue},
 };
 use roc_collections::ImMap;
 use roc_module::symbol::{ModuleId, Symbol};
@@ -13,12 +13,18 @@ use roc_mono::{
 pub(crate) struct Scope<'a, 'ctx> {
     symbols: ImMap<Symbol, (InLayout<'a>, BasicValueEnum<'ctx>)>,
     top_level_thunks: ImMap<Symbol, (ProcLayout<'a>, FunctionValue<'ctx>)>,
-    join_points: ImMap<JoinPointId, (BasicBlock<'ctx>, Vec<PhiValue<'ctx>>)>,
+    join_points: ImMap<JoinPointId, (BasicBlock<'ctx>, Vec<JoinPointArg<'ctx>>)>,
 }
 
 #[derive(Debug)]
 pub(crate) struct JoinPointNotFound;
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) enum JoinPointArg<'ctx> {
+    Alloca(PointerValue<'ctx>),
+    Phi(PhiValue<'ctx>),
+}
+
 impl<'a, 'ctx> Scope<'a, 'ctx> {
     pub fn insert(&mut self, symbol: Symbol, layout: InLayout<'a>, value: BasicValueEnum<'ctx>) {
         self.symbols.insert(symbol, (layout, value));
@@ -62,7 +68,7 @@ impl<'a, 'ctx> Scope<'a, 'ctx> {
         &mut self,
         join_point_id: JoinPointId,
         bb: BasicBlock<'ctx>,
-        phis: Vec<PhiValue<'ctx>>,
+        phis: Vec<JoinPointArg<'ctx>>,
     ) {
         self.join_points.insert(join_point_id, (bb, phis));
     }
@@ -74,7 +80,7 @@ impl<'a, 'ctx> Scope<'a, 'ctx> {
     pub fn get_join_point(
         &self,
         join_point_id: JoinPointId,
-    ) -> Option<&(BasicBlock<'ctx>, Vec<PhiValue<'ctx>>)> {
+    ) -> Option<&(BasicBlock<'ctx>, Vec<JoinPointArg<'ctx>>)> {
         self.join_points.get(&join_point_id)
     }
 
@@ -89,9 +95,17 @@ impl<'a, 'ctx> Scope<'a, 'ctx> {
             .ok_or(JoinPointNotFound)?
             .1;
 
-        for (phi_value, param) in ref_join_points.iter().zip(parameters.into_iter()) {
-            let value = phi_value.as_basic_value();
-            self.symbols.insert(param.symbol, (param.layout, value));
+        for (joinpoint_arg, param) in ref_join_points.iter().zip(parameters.into_iter()) {
+            match joinpoint_arg {
+                crate::llvm::scope::JoinPointArg::Alloca(alloca) => {
+                    self.symbols
+                        .insert(param.symbol, (param.layout, alloca.as_basic_value_enum()));
+                }
+                crate::llvm::scope::JoinPointArg::Phi(phi) => {
+                    self.symbols
+                        .insert(param.symbol, (param.layout, phi.as_basic_value()));
+                }
+            }
         }
 
         Ok(())

crates/compiler/test_gen/src/gen_primitives.rs

@@ -4664,3 +4664,40 @@ fn multiple_uses_of_bool_true_tag_union() {
         bool
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-dev", feature = "gen-wasm"))]
+fn issue_6139() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            buggy = \node, seen ->
+                if List.contains seen node then
+                    seen
+                else
+                    # node = "B"
+                    nextNode = stepNode node
+
+                    # node = "C"
+                    buggy nextNode (List.append seen node)
+
+            stepNode = \node ->
+                when node is
+                    "A" -> "B"
+                    "B" -> "C"
+                    "C" -> "D"
+                    "D" -> "A"
+                    _ -> crash ""
+
+            buggy "A" []
+            "#
+        ),
+        RocList::from_slice(&[
+            RocStr::from("A"),
+            RocStr::from("B"),
+            RocStr::from("C"),
+            RocStr::from("D"),
+        ]),
+        RocList<RocStr>
+    );
+}