[Fuzzer] Use liftoff when running d8, to avoid nondeterminism (#3402)

When running d8, run it in liftoff, to avoid tiering up causing nondeterminism
in the results.

When we do want to compare the tiers, we already do so in CompareVMs. This
fixes others places where we just wanted to run some JS in some VM.

1 files changed, 18 insertions, 5 deletions

diff --git a/scripts/fuzz_opt.py b/scripts/fuzz_opt.py
index 00031e3b7..509c25e3e 100755
--- a/scripts/fuzz_opt.py
+++ b/scripts/fuzz_opt.py
@@ -374,12 +374,25 @@ def run_bynterp(wasm, args):
         del os.environ['BINARYEN_MAX_INTERPRETER_DEPTH']
 
 
-def run_d8_js(js, args=[]):
-    return run_vm([shared.V8] + shared.V8_OPTS + [js] + (['--'] if args else []) + args)
+V8_LIFTOFF_ARGS = ['--liftoff', '--no-wasm-tier-up']
 
 
-def run_d8_wasm(wasm):
-    return run_d8_js(in_binaryen('scripts', 'fuzz_shell.js'), [wasm])
+# default to running with liftoff enabled, because we need to pick either
+# liftoff or turbofan for consistency (otherwise running the same command twice
+# may have different results due to NaN nondeterminism), and liftoff is faster
+# for small things
+def run_d8_js(js, args=[], liftoff=True):
+    cmd = [shared.V8] + shared.V8_OPTS
+    if liftoff:
+        cmd += V8_LIFTOFF_ARGS
+    cmd += [js]
+    if args:
+        cmd += ['--'] + args
+    return run_vm(cmd)
+
+
+def run_d8_wasm(wasm, liftoff=True):
+    return run_d8_js(in_binaryen('scripts', 'fuzz_shell.js'), [wasm], liftoff=liftoff)
 
 
 class TestCaseHandler:
@@ -463,7 +476,7 @@ class CompareVMs(TestCaseHandler):
             name = 'd8_liftoff'
 
             def run(self, wasm):
-                return super(D8Liftoff, self).run(wasm, extra_d8_flags=['--liftoff', '--no-wasm-tier-up'])
+                return super(D8Liftoff, self).run(wasm, extra_d8_flags=V8_LIFTOFF_ARGS)
 
         class D8TurboFan(D8):
             name = 'd8_turbofan'