1 files changed, 11 insertions, 0 deletions
diff --git a/src/passes/ConstantFieldPropagation.cpp b/src/passes/ConstantFieldPropagation.cpp
index c5b95b15a..f97b440ea 100644
--- a/src/passes/ConstantFieldPropagation.cpp
+++ b/src/passes/ConstantFieldPropagation.cpp
@@ -27,6 +27,8 @@
 //        wasm GC programs we need to check for type escaping.
 //
 
+#include "ir/bits.h"
+#include "ir/gc-type-utils.h"
 #include "ir/module-utils.h"
 #include "ir/possible-constant.h"
 #include "ir/struct-utils.h"
@@ -105,6 +107,15 @@ struct FunctionOptimizer : public WalkerPass<PostWalker<FunctionOptimizer>> {
     // constant value. (Leave it to further optimizations to get rid of the
     // ref.)
     Expression* value = info.makeExpression(*getModule());
+    auto field = GCTypeUtils::getField(type, curr->index);
+    assert(field);
+    if (field->isPacked()) {
+      // We cannot just pass through a value that is packed, as the input gets
+      // truncated.
+      auto mask = Bits::lowBitMask(field->getByteSize() * 8);
+      value =
+        builder.makeBinary(AndInt32, value, builder.makeConst(int32_t(mask)));
+    }
     replaceCurrent(builder.makeSequence(
       builder.makeDrop(builder.makeRefAs(RefAsNonNull, curr->ref)), value));
     changed = true;