Fix a DataFlowOpts bug (#1729)

We create some fake nodes for internal use, and were looking at one by mistake. This fixes that by

* Creating a non-ambiguous fake node, a call (which represents an unknown value properly, unlike a zero which we had before).
* Make DFO not rely on those values, if it knows a node is constant, apply those constant values.
Found by the fuzzer.

2 files changed, 10 insertions, 8 deletions

diff --git a/src/dataflow/graph.h b/src/dataflow/graph.h
index f81b4f989..57c03bb2e 100644
--- a/src/dataflow/graph.h
+++ b/src/dataflow/graph.h
@@ -739,13 +739,15 @@ struct Graph : public UnifiedExpressionVisitor<Graph, Node*> {
       // i1 zexts are a no-op for wasm
       return makeUse(node->values[0]);
     } else if (node->isVar()) {
-      // Nothing valid for us to read here.
-      // FIXME should we have a local index to get?
-      return Builder(*module).makeConst(LiteralUtils::makeLiteralZero(node->wasmType));
+      // Nothing valid for us to read here. Emit a call, representing an unknown
+      // variable value.
+      return Builder(*module).makeCall(FAKE_CALL, {}, node->wasmType);
     } else {
       WASM_UNREACHABLE(); // TODO
     }
   }
+
+  const Name FAKE_CALL = "fake$dfo$call";
 };
 
 } // namespace DataFlow
diff --git a/src/passes/DataFlowOpts.cpp b/src/passes/DataFlowOpts.cpp
index 05e975049..e32fcb700 100644
--- a/src/passes/DataFlowOpts.cpp
+++ b/src/passes/DataFlowOpts.cpp
@@ -117,11 +117,11 @@ struct DataFlowOpts : public WalkerPass<PostWalker<DataFlowOpts>> {
     for (Index i = 0; i < node->values.size(); i++) {
       if (node->values[i]->isConst()) {
         auto* currp = getIndexPointer(expr, i);
-        if (!(*currp)->is<Const>()) {
-          // Directly represent it as a constant.
-          auto* c = node->values[i]->expr->dynCast<Const>();
-          *currp = Builder(*getModule()).makeConst(c->value);
-        }
+        // Directly represent it as a constant. (Note that it may already be
+        // a constant, but for now to avoid corner cases just replace them
+        // all here.)
+        auto* c = node->values[i]->expr->dynCast<Const>();
+        *currp = Builder(*getModule()).makeConst(c->value);
       }
     }
     // Now we know that all our DataFlow inputs are constant, and all