Fuzzer: CompareVMs: Do not compare when hitting a host limitation (#5562)

For example, we might hit an allocation limit in the wasm, but the
optimized wasm might optimize that allocation out. So we need to
ignore comparisons in such cases, as we cannot expect the output
to be identical. We already do similar things for FuzzExec and
#5560 adds it for TrapsNeverHappen; this adds it to CompareVMs.

1 files changed, 18 insertions, 2 deletions

diff --git a/scripts/fuzz_opt.py b/scripts/fuzz_opt.py
index 982a735ec..3608e339e 100755
--- a/scripts/fuzz_opt.py
+++ b/scripts/fuzz_opt.py
@@ -581,6 +581,7 @@ ignored_vm_runs = 0
 
 def note_ignored_vm_run():
     global ignored_vm_runs
+    print('(ignore VM run)')
     ignored_vm_runs += 1
 
 
@@ -838,7 +839,10 @@ class CompareVMs(TestCaseHandler):
                 # NaNs can differ from wasm VMs
                 return not NANS
 
-        self.vms = [BinaryenInterpreter(),
+        # the binaryen interpreter is specifically useful for various things
+        self.bynterpreter = BinaryenInterpreter()
+
+        self.vms = [self.bynterpreter,
                     D8(),
                     D8Liftoff(),
                     D8TurboFan(),
@@ -848,7 +852,20 @@ class CompareVMs(TestCaseHandler):
                     ]
 
     def handle_pair(self, input, before_wasm, after_wasm, opts):
+        global ignored_vm_runs
+        ignored_before = ignored_vm_runs
+
         before = self.run_vms(before_wasm)
+
+        # if the binaryen interpreter hit a host limitation on the original
+        # testcase, or for some other reason we need to ignore this, then stop
+        # (otherwise, a host limitation on say allocations may be hit in the
+        # 'before' but not in the 'after' as optimizations may remove it).
+        if before[self.bynterpreter] == IGNORE:
+            # the ignoring should have been noted during run_vms()
+            assert(ignored_vm_runs > ignored_before)
+            return
+
         after = self.run_vms(after_wasm)
         self.compare_before_and_after(before, after)
 
@@ -861,7 +878,6 @@ class CompareVMs(TestCaseHandler):
                 vm_results[vm] = fix_output(vm.run(wasm))
 
         # compare between the vms on this specific input
-
         first_vm = None
         for vm in vm_results.keys():
             if vm.can_compare_to_others():