[turbofan] temporarily disable const-based load elimination

This is a safe to merge hot-fix to tackle https://crbug.com/983764.
To be reverted after merging to M77.

Bug: chromium:983764
Change-Id: I3cd27481f224b352ef6bcf9dde21a8f77616acff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1786285Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63570}

src/compiler/load-elimination.cc

@@ -629,6 +629,9 @@ LoadElimination::AbstractState::KillFields(Node* object, MaybeHandle<Name> name,
 
 LoadElimination::AbstractState const* LoadElimination::AbstractState::KillAll(
     Zone* zone) const {
+  // TODO(tebbi): This is not a good way to disable const load elimination.
+  //              It's just the safest to back-merge for crbug:983764.
+  if (FLAG_turbo_load_elimination_use_constness) {
     // Kill everything except for const fields
     for (size_t i = 0; i < const_fields_.size(); ++i) {
       if (const_fields_[i]) {
@@ -637,6 +640,7 @@ LoadElimination::AbstractState const* LoadElimination::AbstractState::KillAll(
         return that;
       }
     }
+  }
   return LoadElimination::empty_state();
 }
 

src/flags/flag-definitions.h

@@ -586,6 +586,8 @@ DEFINE_BOOL(analyze_environment_liveness, true,
 DEFINE_BOOL(trace_environment_liveness, false,
             "trace liveness of local variable slots")
 DEFINE_BOOL(turbo_load_elimination, true, "enable load elimination in TurboFan")
+DEFINE_BOOL(turbo_load_elimination_use_constness, false,
+            "use constness in TurboFan load elimination")
 DEFINE_BOOL(trace_turbo_load_elimination, false,
             "trace TurboFan load elimination")
 DEFINE_BOOL(turbo_profiling, false, "enable profiling in TurboFan")

test/mjsunit/compiler/load-elimination-const-field.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --allow-natives-syntax
+// Flags: --allow-natives-syntax --turbo-load-elimination-use-constness
 
 // Check that load elimination on const-marked fields works
 (function() {