[Fuzzing] Add an env var to avoid modifying the given wasm file in fuzz_opt.py (#7703)

Normally we want to fix it up (remove NaNs etc., as we do normally), but
the changes in each fuzzing iteration can make things hard to debug. Add an
env var to simply trust the given wasm file and use it without changes.

Author: kripken

scripts/fuzz_opt.py

@@ -1874,15 +1874,21 @@ def test_one(random_input, given_wasm):
     print()
 
     if given_wasm:
-        # if given a wasm file we want to use it as is, but we also want to
-        # apply properties like not having any NaNs, which the original fuzz
-        # wasm had applied. that is, we need to preserve properties like not
-        # having nans through reduction.
-        try:
-            run([in_bin('wasm-opt'), given_wasm, '-o', abspath('a.wasm')] + GEN_ARGS + FEATURE_OPTS)
-        except Exception as e:
-            print("Internal error in fuzzer! Could not run given wasm")
-            raise e
+        # We are given a wasm file to operate on. By default we modify it in the
+        # usual ways, like running DeNAN on it, which is important in many cases
+        # (imagine the reducer generates a NAN, then we need to restore the
+        # property of not having any). However, in some cases we do need to
+        # trust the wasm is correct, or it is simpler to debug things without
+        # constant changes in each reduction cycle, so we have an env var to
+        # control that, BINARYEN_TRUST_GIVEN_WASM.
+        if os.environ.get('BINARYEN_TRUST_GIVEN_WASM'):
+            shutil.copyfile(given_wasm, abspath('a.wasm'))
+        else:
+            try:
+                run([in_bin('wasm-opt'), given_wasm, '-o', abspath('a.wasm')] + GEN_ARGS + FEATURE_OPTS)
+            except Exception as e:
+                print("Internal error in fuzzer! Could not run given wasm")
+                raise e
     else:
         # emit the target features section so that reduction can work later,
         # without needing to specify the features
@@ -2351,7 +2357,9 @@ def get_random_opts():
 (If it does not, then one possible issue is that the fuzzer fails to write a
 valid binary. If so, you can print the output of the fuzzer's first command
 (using -ttf / --translate-to-fuzz) in text form and run the reduction from that,
-passing --text to the reducer.)
+passing --text to the reducer. Another possible fix is to avoid re-processing
+the wasm for fuzzing in each iteration, by adding
+BINARYEN_TRUST_GIVEN_WASM=1 in the env.)
 
 You can also read "%(reduce_sh)s" which has been filled out for you and includes
 docs and suggestions.