keep wvals if only dep needs changing

if both CUs have the SC in question in their
dependency lists, but at different indices, we
can find the right entry and only change the
index.

src/spesh/inline.c

@@ -230,9 +230,19 @@ static void fix_str(MVMThreadContext *tc, MVMSpeshGraph *inliner,
 }
 static void fix_wval(MVMThreadContext *tc, MVMSpeshGraph *inliner,
                      MVMSpeshGraph *inlinee, MVMSpeshIns *to_fix) {
-    /* Resolve object, then just put it into a spesh slot. (Could do some
-     * smarter things like trying to see if the SC is referenced by both
-     * compilation units, too.) */
+    /* We have three different ways to make a wval refer to the right
+     * object after an inline:
+     * - If a simple change of the dep parameter lets us refer to the
+     *   same SC in the inliner's dependency list, just change that.
+     * - If the object had already been deserialized, we can just
+     *   stash it away in a spesh slot.
+     * - If the object hadn't been deserialized yet, an indirect lookup
+     *   via a spesh slot holding the actual SC is possible.
+     *
+     * The last option is needed because deserializing objects causes
+     * allocations, which lead to GC, which is not supported inside
+     * the spesh process.
+     */
     MVMCompUnit *targetcu = inliner->sf->body.cu;
     MVMCompUnit *sourcecu = inlinee->sf->body.cu;
     MVMint16     dep      = to_fix->operands[1].lit_i16;
@@ -242,6 +252,14 @@ static void fix_wval(MVMThreadContext *tc, MVMSpeshGraph *inliner,
     if (dep >= 0 && dep < sourcecu->body.num_scs) {
         MVMSerializationContext *sc = MVM_sc_get_sc(tc, sourcecu, dep);
         if (sc) {
+            MVMuint16 otherdep;
+            for (otherdep = 0; otherdep < targetcu->body.num_scs; otherdep++) {
+                MVMSerializationContext *othersc = MVM_sc_get_sc(tc, targetcu, otherdep);
+                if (sc == othersc) {
+                    to_fix->operands[1].lit_i16 = otherdep;
+                    return;
+                }
+            }
             if (MVM_sc_is_object_immediately_available(tc, sc, idx)) {
                 MVMint16 ss  = MVM_spesh_add_spesh_slot_try_reuse(tc, inliner, (MVMCollectable *)MVM_sc_get_object(tc, sc, idx));
                 to_fix->info = MVM_op_get_op(MVM_OP_sp_getspeshslot);