Fuzzer: Adjust feature fuzzing frequency (WebAssembly#6305)

We used to fuzz MVP 1/3, all 1/3, and a mixture 1/3, but that gives far too much
priority to the MVP which is increasingly less important. It is also a good idea to
give "all" more priority as that enables more initial content to run (the fuzzer will
discard initial content if it doesn't validate with the features chosen in the current
iteration).

Also (NFC) rename POSSIBLE_FEATURE_OPTS to make the code easier to follow.

Author: kripken

scripts/fuzz_opt.py

@@ -126,16 +126,27 @@ def no_pass_debug():
 def randomize_feature_opts():
     global FEATURE_OPTS
     FEATURE_OPTS = CONSTANT_FEATURE_OPTS[:]
-    # 1/3 the time apply all the possible opts, 1/3 none of them, to maximize
-    # coverage both ways, and 1/3 pick each one randomly
-    if random.random() < 0.33333:
-        FEATURE_OPTS += POSSIBLE_FEATURE_OPTS
-    elif random.random() < 0.5:
-        for possible in POSSIBLE_FEATURE_OPTS:
+
+    if random.random() < 0.1:
+        # 10% of the time disable all features, i.e., fuzz the MVP featureset.
+        # Fuzzing that is less and less important as more features get enabled
+        # by default, but we don't want to lose all coverage for it entirely
+        # (and the odds of randomly not selecting any feature, below, is too
+        # small - at 17 features it is far less than 1%).
+        FEATURE_OPTS += FEATURE_DISABLE_FLAGS
+    elif random.random() < 0.333:
+        # 1/3 of the remaining 90% pick each feature randomly.
+        for possible in FEATURE_DISABLE_FLAGS:
             if random.random() < 0.5:
                 FEATURE_OPTS.append(possible)
                 if possible in IMPLIED_FEATURE_OPTS:
                     FEATURE_OPTS.extend(IMPLIED_FEATURE_OPTS[possible])
+    else:
+        # 2/3 of the remaining 90% use them all. This is useful to maximize
+        # coverage, as enabling more features enables more optimizations and
+        # code paths, and also allows all initial contents to run.
+        pass
+
     print('randomized feature opts:', '\n  ' + '\n  '.join(FEATURE_OPTS))
 
     # Pick closed or open with equal probability as both matter.
@@ -928,9 +939,6 @@ def compare_before_and_after(self, before, after):
             if vm in after and vm.can_compare_to_self():
                 compare(before[vm], after[vm], 'CompareVMs between before and after: ' + vm.name)
 
-    def can_run_on_feature_opts(self, feature_opts):
-        return True
-
 
 # Check for determinism - the same command must have the same output.
 class CheckDeterminism(TestCaseHandler):
@@ -1591,11 +1599,10 @@ def get_random_opts():
 
 # main
 
-# possible feature options that are sometimes passed to the tools. this
-# contains the list of all possible feature flags we can disable (after
-# we enable all before that in the constant options)
-POSSIBLE_FEATURE_OPTS = run([in_bin('wasm-opt'), '--print-features', in_binaryen('test', 'hello_world.wat')] + CONSTANT_FEATURE_OPTS).replace('--enable', '--disable').strip().split('\n')
-print('POSSIBLE_FEATURE_OPTS:', POSSIBLE_FEATURE_OPTS)
+# list of all the flags to disable all the features. if all of these are added
+# then we target the MVP.
+FEATURE_DISABLE_FLAGS = run([in_bin('wasm-opt'), '--print-features', in_binaryen('test', 'hello_world.wat')] + CONSTANT_FEATURE_OPTS).replace('--enable', '--disable').strip().split('\n')
+print('FEATURE_DISABLE_FLAGS:', FEATURE_DISABLE_FLAGS)
 
 # some features depend on other features, so if a required feature is
 # disabled, its dependent features need to be disabled as well.