Emcc fuzzing followups (#2812)

Avoid pass-debug when fuzzing emcc, as it can be slow and isn't
what we care about.

Clean up a loop.

Author: kripken

scripts/fuzz_opt.py

@@ -15,6 +15,7 @@
 script covers different options being passed)
 '''
 
+import contextlib
 import os
 import difflib
 import math
@@ -87,6 +88,17 @@ def randomize_pass_debug():
     print('randomized pass debug:', os.environ.get('BINARYEN_PASS_DEBUG', ''))
 
 
+@contextlib.contextmanager
+def no_pass_debug():
+    old_env = os.environ.copy()
+    if os.environ.get('BINARYEN_PASS_DEBUG'):
+        del os.environ['BINARYEN_PASS_DEBUG']
+    try:
+        yield
+    finally:
+        os.environ.update(old_env)
+
+
 def randomize_feature_opts():
     global FEATURE_OPTS
     FEATURE_OPTS = CONSTANT_FEATURE_OPTS[:]
@@ -380,7 +392,11 @@ def run(self, wasm):
                         compile_cmd += ['-Os']
                     else:
                         compile_cmd += ['-Oz']
-                run(compile_cmd)
+                # avoid pass-debug on the emcc invocation itself (which runs
+                # binaryen to optimize the wasm), as the wasm here can be very
+                # large and it isn't what we are focused on testing here
+                with no_pass_debug():
+                    run(compile_cmd)
                 return run_vm(['d8', 'a.out.js'])
 
             def can_run(self):

src/tools/wasm2c-wrapper.h

@@ -98,14 +98,13 @@ int main(int argc, char** argv) {
 
   // For each export in the wasm, emit code to call it and log its result,
   // similar to the other wrappers.
-  size_t exportIndex = 0;
-
-  for (auto& exp : wasm.exports) {
+  for (size_t i = 0; i < wasm.exports.size(); i++) {
+    auto& exp = wasm.exports[i];
     if (exp->kind != ExternalKind::Function) {
       continue;
     }
 
-    ret += "        case " + std::to_string(exportIndex++) + ":\n";
+    ret += "        case " + std::to_string(i) + ":\n";
 
     auto* func = wasm.getFunction(exp->value);