[wasm] Introduce --liftoff-only flag

This flag disables the implicit fallback to Turbofan when
Liftoff bails out due to an unsupported instruction/type.
Instead, Liftoff bailouts are treated as fatal errors.

This is meant for testing; it is not (yet?) a configuration
we support in production.

Change-Id: I04e2045f1976e202e65da0ba8e8d660c47859bf4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584949Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71721}

src/flags/flag-definitions.h

@@ -814,6 +814,11 @@ DEFINE_INT(trace_wasm_ast_start, 0,
 DEFINE_INT(trace_wasm_ast_end, 0, "end function for wasm AST trace (exclusive)")
 DEFINE_BOOL(liftoff, true,
             "enable Liftoff, the baseline compiler for WebAssembly")
+DEFINE_BOOL(liftoff_only, false,
+            "disallow Turbofan compilation for WebAssembly (for testing)")
+DEFINE_IMPLICATION(liftoff_only, liftoff)
+DEFINE_NEG_IMPLICATION(liftoff_only, wasm_tier_up)
+DEFINE_NEG_IMPLICATION(fuzzing, liftoff_only)
 DEFINE_BOOL(experimental_liftoff_extern_ref, false,
             "enable support for externref in Liftoff")
 // We can't tier up (from Liftoff to TurboFan) in single-threaded mode, hence

src/flags/flags.cc

@@ -269,10 +269,17 @@ struct Flag {
           break;
         case SetBy::kCommandLine:
           if (new_set_by == SetBy::kImplication && check_command_line_flags) {
+            if (is_bool_flag) {
+              FATAL(
+                  "Flag --%s: value implied by --%s conflicts with explicit "
+                  "specification\n%s",
+                  name(), implied_by, hint);
+            } else {
               FATAL(
                   "Flag --%s is implied by --%s but also specified "
                   "explicitly.\n%s",
                   name(), implied_by, hint);
+            }
           } else if (new_set_by == SetBy::kCommandLine &&
                      check_command_line_flags) {
             if (is_bool_flag) {

src/wasm/baseline/liftoff-compiler.cc

@@ -414,6 +414,10 @@ class LiftoffCompiler {
     decoder->errorf(decoder->pc_offset(), "unsupported liftoff operation: %s",
                     detail);
     UnuseLabels(decoder);
+    if (FLAG_liftoff_only) {
+      FATAL("--liftoff-only: treating bailout as fatal error. Cause: %s",
+            detail);
+    }
   }
 
   bool DidAssemblerBailout(FullDecoder* decoder) {

src/wasm/function-compiler.cc

@@ -196,6 +196,9 @@ WasmCompilationResult WasmCompilationUnit::ExecuteFunctionCompilation(
                                            func_body, func_index_,
                                            for_debugging_, counters, detected);
         if (result.succeeded()) break;
+        // In --liftoff-only mode, we should have aborted the process
+        // on bailout, i.e. before getting here.
+        DCHECK(!FLAG_liftoff_only);
       }
 
       // If Liftoff failed, fall back to turbofan.