Fuzzer: Standardize notation for exception prefixes (#6369)

We had exception: in one and exception thrown: in another. Making those
consistent allows fuzz_shell.js to print the exception after that prefix, which
makes debugging easier sometimes.

Also canonicalize tag names. Like funcref names, JS VMs print out the internal
name, which can change after opts, so canonicalize it.

scripts/fuzz_opt.py

@@ -478,6 +478,9 @@ def pick_initial_contents():
 # and also see the --dce workaround below that also links to those issues.
 V8_UNINITIALIZED_NONDEF_LOCAL = 'uninitialized non-defaultable local'
 
+# JS exceptions are logged as exception thrown: REASON
+EXCEPTION_PREFIX = 'exception thrown: '
+
 
 # given a call line that includes FUZZ_EXEC_CALL_PREFIX, return the export that
 # is called
@@ -585,7 +588,7 @@ def fix_double(x):
     out = re.sub(r'f64\.const (-?[nanN:abcdefxIity\d+-.]+)', fix_double, out)
 
     # mark traps from wasm-opt as exceptions, even though they didn't run in a vm
-    out = out.replace(TRAP_PREFIX, 'exception: ' + TRAP_PREFIX)
+    out = out.replace(TRAP_PREFIX, EXCEPTION_PREFIX + TRAP_PREFIX)
 
     # funcref(0) has the index of the function in it, and optimizations can
     # change that index, so ignore it
@@ -595,6 +598,9 @@ def fix_double(x):
     # to "N".
     out = re.sub(r'i31ref\((-?\d+)\)', r'\1', out)
 
+    # Tag names may change due to opts, so canonicalize them.
+    out = re.sub(r' tag\$\d+', ' tag', out)
+
     lines = out.splitlines()
     for i in range(len(lines)):
         line = lines[i]
@@ -604,7 +610,7 @@ def fix_double(x):
             # developer can see it.
             print(line)
             lines[i] = None
-        elif 'exception' in line:
+        elif EXCEPTION_PREFIX in line:
             # exceptions may differ when optimizing, but an exception should
             # occur, so ignore their types (also js engines print them out
             # slightly differently)

scripts/fuzz_shell.js

@@ -108,7 +108,7 @@ var instance;
 try {
   instance = new WebAssembly.Instance(module, imports);
 } catch (e) {
-  console.log('exception: failed to instantiate module');
+  console.log('exception thrown: failed to instantiate module');
   quit();
 }
 
@@ -143,7 +143,7 @@ for (var e in exports) {
       console.log('[fuzz-exec] note result: ' + e + ' => ' + printed(result));
     }
   } catch (e) {
-    console.log('exception!');// + [e, e.stack]);
+    console.log('exception thrown: ' + e);
   }
 }