Merge pull request #3699 from rtfeldman/wasm-fix-uninit-capacity

Wasm: fix uninitialised capacity in List literals

crates/compiler/gen_wasm/src/backend.rs

@@ -1479,7 +1479,14 @@ impl<'a> WasmBackend<'a> {
             // length of the list
             self.code_builder.get_local(stack_local_id);
             self.code_builder.i32_const(elems.len() as i32);
-            self.code_builder.i32_store(Align::Bytes4, stack_offset + 4);
+            self.code_builder
+                .i32_store(Align::Bytes4, stack_offset + 4 * Builtin::WRAPPER_LEN);
+
+            // capacity of the list
+            self.code_builder.get_local(stack_local_id);
+            self.code_builder.i32_const(elems.len() as i32);
+            self.code_builder
+                .i32_store(Align::Bytes4, stack_offset + 4 * Builtin::WRAPPER_CAPACITY);
 
             let mut elem_offset = 0;
 
@@ -1521,12 +1528,14 @@ impl<'a> WasmBackend<'a> {
         if let StoredValue::StackMemory { location, .. } = storage {
             let (local_id, offset) = location.local_and_offset(self.storage.stack_frame_pointer);
 
-            // This is a minor cheat.
+            // Store 12 bytes of zeros { elements: null, length: 0, capacity: 0 }
-            // What we want to write to stack memory is { elements: null, length: 0 }
+            debug_assert_eq!(Builtin::LIST_WORDS, 3);
-            // But instead of two 32-bit stores, we can do a single 64-bit store.
             self.code_builder.get_local(local_id);
             self.code_builder.i64_const(0);
             self.code_builder.i64_store(Align::Bytes4, offset);
+            self.code_builder.get_local(local_id);
+            self.code_builder.i32_const(0);
+            self.code_builder.i32_store(Align::Bytes4, offset + 8);
         } else {
             internal_error!("Unexpected storage for {:?}", sym)
         }

crates/compiler/test_gen/src/gen_list.rs

@@ -3354,3 +3354,18 @@ fn issue_3571_lowlevel_call_function_with_bool_lambda_set() {
         RocStr
     )
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn issue_3530_uninitialized_capacity_in_list_literal() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            [11,22,33]
+            "#
+        ),
+        3,
+        (usize, usize, usize),
+        |(_, _, cap)| cap
+    );
+}

crates/compiler/test_gen/src/helpers/from_wasm32_memory.rs

@@ -1,5 +1,6 @@
 use roc_error_macros::internal_error;
 use roc_gen_wasm::{round_up_to_alignment, wasm32_sized::Wasm32Sized};
+use roc_mono::layout::Builtin;
 use roc_std::{RocDec, RocList, RocOrder, RocResult, RocStr, I128, U128};
 use std::convert::TryInto;
 
@@ -57,8 +58,9 @@ impl FromWasm32Memory for RocStr {
 
         let str_words: &[u32; 3] = unsafe { std::mem::transmute(&str_bytes) };
 
-        let big_elem_ptr = str_words[0] as usize;
+        let big_elem_ptr = str_words[Builtin::WRAPPER_PTR as usize] as usize;
-        let big_length = str_words[1] as usize;
+        let big_length = str_words[Builtin::WRAPPER_LEN as usize] as usize;
+        let big_capacity = str_words[Builtin::WRAPPER_CAPACITY as usize] as usize;
 
         let last_byte = str_bytes[11];
         let is_small_str = last_byte >= 0x80;
@@ -70,16 +72,20 @@ impl FromWasm32Memory for RocStr {
             &memory_bytes[big_elem_ptr..][..big_length]
         };
 
-        unsafe { RocStr::from_slice_unchecked(slice) }
+        let mut roc_str = unsafe { RocStr::from_slice_unchecked(slice) };
+        if !is_small_str {
+            roc_str.reserve(big_capacity - big_length)
+        }
+        roc_str
     }
 }
 
 impl<T: FromWasm32Memory + Clone> FromWasm32Memory for RocList<T> {
     fn decode(memory: &[u8], offset: u32) -> Self {
-        let bytes = <u64 as FromWasm32Memory>::decode(memory, offset);
+        let elements = <u32 as FromWasm32Memory>::decode(memory, offset + 4 * Builtin::WRAPPER_PTR);
-
+        let length = <u32 as FromWasm32Memory>::decode(memory, offset + 4 * Builtin::WRAPPER_LEN);
-        let length = (bytes >> 32) as u32;
+        let capacity =
-        let elements = bytes as u32;
+            <u32 as FromWasm32Memory>::decode(memory, offset + 4 * Builtin::WRAPPER_CAPACITY);
 
         let mut items = Vec::with_capacity(length as usize);
 
@@ -91,7 +97,9 @@ impl<T: FromWasm32Memory + Clone> FromWasm32Memory for RocList<T> {
             items.push(item);
         }
 
-        RocList::from_slice(&items)
+        let mut list = RocList::with_capacity(capacity as usize);
+        list.extend_from_slice(&items);
+        list
     }
 }