[Wasm GC] Properly handle packed field truncation in StructNew (#5570)

2 files changed, 21 insertions, 2 deletions

diff --git a/src/wasm-interpreter.h b/src/wasm-interpreter.h
index 8e82bdf80..afc193f71 100644
--- a/src/wasm-interpreter.h
+++ b/src/wasm-interpreter.h
@@ -1541,14 +1541,15 @@ public:
     const auto& fields = heapType.getStruct().fields;
     Literals data(fields.size());
     for (Index i = 0; i < fields.size(); i++) {
+      auto& field = fields[i];
       if (curr->isWithDefault()) {
-        data[i] = Literal::makeZero(fields[i].type);
+        data[i] = Literal::makeZero(field.type);
       } else {
         auto value = self()->visit(curr->operands[i]);
         if (value.breaking()) {
           return value;
         }
-        data[i] = value.getSingleValue();
+        data[i] = truncateForPacking(value.getSingleValue(), field);
       }
     }
     return Literal(std::make_shared<GCData>(curr->type.getHeapType(), data),
diff --git a/test/lit/passes/precompute-gc.wast b/test/lit/passes/precompute-gc.wast
index 0c5b9f8b8..782cf9b06 100644
--- a/test/lit/passes/precompute-gc.wast
+++ b/test/lit/passes/precompute-gc.wast
@@ -23,6 +23,8 @@
  ;; NOMNL:      (type $B (struct (field (mut f64))))
  (type $B (struct (field (mut f64))))
 
+ (type $struct_i8 (struct (field i8)))
+
  (type $array16 (array (mut i16)))
 
  (type $func-return-i32 (func (result i32)))
@@ -1469,4 +1471,20 @@
    (local.get $s)
   )
  )
+
+ ;; CHECK:      (func $struct.new.packed (type $func-return-i32) (result i32)
+ ;; CHECK-NEXT:  (i32.const 120)
+ ;; CHECK-NEXT: )
+ ;; NOMNL:      (func $struct.new.packed (type $func-return-i32) (result i32)
+ ;; NOMNL-NEXT:  (i32.const 120)
+ ;; NOMNL-NEXT: )
+ (func $struct.new.packed (result i32)
+  ;; Truncation happens when we write to this packed i8 field, so the result we
+  ;; read back is 0x12345678 & 0xff which is 0x78 == 120.
+  (struct.get_s $struct_i8 0
+   (struct.new $struct_i8
+    (i32.const 0x12345678)
+   )
+  )
+ )
 )