[wasm] Disable Liftoff in single-threaded mode

For single-threaded, we can not tier up in the background. Hence we can
either tier up in foreground (which is pointless), or not tier up.
This CL disables Liftoff, so will compile TurboFan code right away.

R=ahaas@chromium.org

Bug: v8:10530
Change-Id: Icfdc26643ab219d617f844c2ea8e149870168dbc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208853Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67894}

src/flags/flag-definitions.h

@@ -724,6 +724,11 @@ DEFINE_BOOL(liftoff, false,
             "enable Liftoff, the baseline compiler for WebAssembly")
 DEFINE_IMPLICATION(future, liftoff)
 #endif
+// We can't tier up (from Liftoff to TurboFan) in single-threaded mode, hence
+// disable Liftoff in that configuration for now. The alternative is disabling
+// TurboFan, which would reduce peak performance considerably.
+// Note that for debugging, Liftoff will still be used.
+DEFINE_NEG_IMPLICATION(single_threaded, liftoff)
 DEFINE_DEBUG_BOOL(trace_liftoff, false,
                   "trace Liftoff, the baseline compiler for WebAssembly")
 DEFINE_BOOL(trace_wasm_memory, false,

src/wasm/module-compiler.cc

@@ -1481,8 +1481,9 @@ void RecompileNativeModule(NativeModule* native_module,
         }
       });
 
-  // We only wait for tier down. Tier up can happen in the background.
-  if (tiering_state == kTieredDown) {
+  // Always wait for tier down. Tier up usually happens in the background,
+  // except in single-threaded mode.
+  if (tiering_state == kTieredDown || FLAG_single_threaded) {
     // The main thread contributes to the compilation.
     constexpr Counters* kNoCounters = nullptr;
     while (ExecuteCompilationUnits(

test/mjsunit/mjsunit.status

@@ -962,8 +962,6 @@
   'wasm/atomics64-stress': [SKIP],
   'wasm/futex': [SKIP],
   'regress/wasm/regress-1067621': [SKIP],
-
-  'wasm/tier-down-to-liftoff': [SKIP],
 }],  # 'predictable == True'
 
 ##############################################################################