Commit 46abf20454

@@ -8753,6 +8753,15 @@ pub const FuncGen = struct {
 
         const val_is_undef = if (try self.air.value(bin_op.rhs, mod)) |val| val.isUndefDeep(mod) else false;
         if (val_is_undef) {
+            const ptr_info = ptr_ty.ptrInfo(mod);
+            const needs_bitmask = (ptr_info.packed_offset.host_size != 0);
+            if (needs_bitmask) {
+                // TODO: only some bits are to be undef, we cannot write with a simple memset.
+                // meanwhile, ignore the write rather than stomping over valid bits.
+                // https://github.com/ziglang/zig/issues/15337
+                return .none;
+            }
+
             // Even if safety is disabled, we still emit a memset to undefined since it conveys
             // extra information to LLVM. However, safety makes the difference between using
             // 0xaa or actual undefined for the fill byte.

@@ -632,7 +632,6 @@ test "pointer to container level packed struct field" {
 test "store undefined to packed result location" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_llvm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;