From 015452a9dc39a5065ac87cbdf27b7df8bb6622be Mon Sep 17 00:00:00 2001
From: Thomas Lively <tlively@google.com>
Date: Thu, 17 Nov 2022 15:01:22 -0800
Subject: [PATCH] Do not compare reference values across executions

Since we optimize assuming a closed world, optimizations can change the types
and structure of GC data even in externally-visible ways. Because differences
are expected, the fuzzer already did not compare reference-typed values from
before and after optimizations when running with nominal typing. Update it to
not compare these values under any type system.
---
 src/tools/execution-results.h      | 28 ++++++++++------------------
 test/lit/exec/no-compare-refs.wast | 22 ++++++++++++++++++++++
 2 files changed, 32 insertions(+), 18 deletions(-)
 create mode 100644 test/lit/exec/no-compare-refs.wast

diff --git a/src/tools/execution-results.h b/src/tools/execution-results.h
index 723ddffda46..925c9b6d816 100644
--- a/src/tools/execution-results.h
+++ b/src/tools/execution-results.h
@@ -143,25 +143,17 @@ struct ExecutionResults {
   }
 
   bool areEqual(Literal a, Literal b) {
-    // We allow nulls to have different types (as they compare equal regardless)
-    // but anything else must have an identical type.
-    // We cannot do this in nominal typing, however, as different modules will
-    // have different types in general. We could perhaps compare the entire
-    // graph structurally TODO
-    if (getTypeSystem() != TypeSystem::Nominal) {
-      if (a.type != b.type && !(a.isNull() && b.isNull())) {
-        std::cout << "types not identical! " << a << " != " << b << '\n';
-        return false;
-      }
-    }
     if (a.type.isRef()) {
-      // Don't compare references - only their types. There are several issues
-      // here that we can't fully handle, see
-      // https://github.com/WebAssembly/binaryen/issues/3378, but the core issue
-      // is that we are comparing results between two separate wasm modules (and
-      // a separate instance of each) - we can't really identify an identical
-      // reference between such things. We can only compare things structurally,
-      // for which we compare the types.
+      // Don't compare references. There are several issues here that we can't
+      // fully handle, see https://github.com/WebAssembly/binaryen/issues/3378,
+      // but the core issue is that since we optimize assuming a closed world,
+      // the types and structure of GC data can arbitrarily change after
+      // optimizations, even in ways that are externally visible from outside
+      // the module.
+      //
+      // TODO: Once we support optimizing under some form of open-world
+      // assumption, we should be able to check that the types and/or structure
+      // of GC data passed out of the module does not change.
       return true;
     }
     if (a != b) {
diff --git a/test/lit/exec/no-compare-refs.wast b/test/lit/exec/no-compare-refs.wast
new file mode 100644
index 00000000000..bb9766a3122
--- /dev/null
+++ b/test/lit/exec/no-compare-refs.wast
@@ -0,0 +1,22 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --output=fuzz-exec and should not be edited.
+
+;; RUN: wasm-opt %s --hybrid -all --signature-pruning --fuzz-exec -q -o /dev/null 2>&1 | filecheck %s
+
+(module
+  (type $f (func (param i32)))
+
+  (func $no-use-param (type $f) (param i32)
+    (unreachable)
+  )
+
+  ;; The type of the reference this function returns will change as a result of
+  ;; signature pruning. The fuzzer should not complain about this.
+  ;; CHECK:      [fuzz-exec] calling return-ref
+  ;; CHECK-NEXT: [fuzz-exec] note result: return-ref => funcref
+  (func "return-ref" (result funcref)
+    (ref.func $no-use-param)
+  )
+)
+;; CHECK:      [fuzz-exec] calling return-ref
+;; CHECK-NEXT: [fuzz-exec] note result: return-ref => funcref
+;; CHECK-NEXT: [fuzz-exec] comparing return-ref