Revert "[turbofan] Add fast path for single-character String#startsWith()"

This reverts commit e7e512da.

Reason for revert: chromium:963687, chromium:963731

Original change's description:
> [turbofan] Add fast path for single-character String#startsWith()
>
> This CL adds a fast path to String#startsWith(s) if s is a
> single character string.
>
> Bug: v8:8400
> Change-Id: Ibd6a9d1e46d98f41c198d2b579208e25003eedb0
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1525362
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61504}

TBR=sigurds@chromium.org,bmeurer@chromium.org,usharma1998@gmail.com

Change-Id: Ie09a96557102063ab15c3900563cf16ec57a47e3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8400
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1614605Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61562}

src/compiler/js-call-reducer.cc

@@ -3550,8 +3550,6 @@ Reduction JSCallReducer::ReduceJSCall(Node* node,
       return ReduceStringPrototypeSlice(node);
     case Builtins::kStringPrototypeSubstr:
       return ReduceStringPrototypeSubstr(node);
-    case Builtins::kStringPrototypeStartsWith:
-      return ReduceStringPrototypeStartsWith(node);
 #ifdef V8_INTL_SUPPORT
     case Builtins::kStringPrototypeToLowerCaseIntl:
       return ReduceStringPrototypeToLowerCaseIntl(node);
@@ -5004,84 +5002,6 @@ Reduction JSCallReducer::ReduceStringPrototypeStringAt(
   return Replace(value);
 }
 
-// ES section 21.1.3.20
-// String.prototype.startsWith ( searchString [ , position ] )
-Reduction JSCallReducer::ReduceStringPrototypeStartsWith(Node* node) {
-  DCHECK_EQ(IrOpcode::kJSCall, node->opcode());
-  CallParameters const& p = CallParametersOf(node->op());
-  if (p.speculation_mode() == SpeculationMode::kDisallowSpeculation) {
-    return NoChange();
-  }
-  if (node->op()->ValueInputCount() < 3) {
-    Node* value = jsgraph()->FalseConstant();
-    ReplaceWithValue(node, value);
-    return Replace(value);
-  }
-
-  Node* string = NodeProperties::GetValueInput(node, 1);
-  Node* search_string = NodeProperties::GetValueInput(node, 2);
-  Node* position = node->op()->ValueInputCount() >= 4
-                       ? NodeProperties::GetValueInput(node, 3)
-                       : jsgraph()->ZeroConstant();
-  Node* effect = NodeProperties::GetEffectInput(node);
-  Node* control = NodeProperties::GetControlInput(node);
-
-  HeapObjectMatcher m(search_string);
-  if (m.HasValue()) {
-    ObjectRef target_ref = m.Ref(broker());
-    if (target_ref.IsString()) {
-      StringRef str = target_ref.AsString();
-      if (str.length() == 1) {
-        string = effect = graph()->NewNode(
-            simplified()->CheckString(p.feedback()), string, effect, control);
-        position = effect = graph()->NewNode(
-            simplified()->CheckSmi(p.feedback()), position, effect, control);
-
-        Node* string_length =
-            graph()->NewNode(simplified()->StringLength(), string);
-        Node* unsigned_position = graph()->NewNode(
-            simplified()->NumberMax(), position, jsgraph()->ZeroConstant());
-
-        Node* check = graph()->NewNode(simplified()->NumberLessThan(),
-                                       unsigned_position, string_length);
-        Node* branch = graph()->NewNode(common()->Branch(BranchHint::kNone),
-                                        check, control);
-
-        Node* if_false = graph()->NewNode(common()->IfFalse(), branch);
-        Node* efalse = effect;
-        Node* vfalse = jsgraph()->FalseConstant();
-
-        Node* if_true = graph()->NewNode(common()->IfTrue(), branch);
-        Node* etrue = effect;
-        Node* vtrue;
-        {
-          Node* masked_position =
-              graph()->NewNode(simplified()->PoisonIndex(), unsigned_position);
-          Node* string_first = efalse =
-              graph()->NewNode(simplified()->StringCharCodeAt(), string,
-                               masked_position, efalse, control);
-
-          Node* search_first = jsgraph()->Constant(str.GetFirstChar());
-          vtrue = graph()->NewNode(simplified()->NumberEqual(), string_first,
-                                   search_first);
-        }
-
-        control = graph()->NewNode(common()->Merge(2), if_true, if_false);
-        Node* value =
-            graph()->NewNode(common()->Phi(MachineRepresentation::kTagged, 2),
-                             vtrue, vfalse, control);
-        effect =
-            graph()->NewNode(common()->EffectPhi(2), etrue, efalse, control);
-
-        ReplaceWithValue(node, value, effect, control);
-        return Replace(value);
-      }
-    }
-  }
-
-  return NoChange();
-}
-
 // ES section 21.1.3.1 String.prototype.charAt ( pos )
 Reduction JSCallReducer::ReduceStringPrototypeCharAt(Node* node) {
   DCHECK_EQ(IrOpcode::kJSCall, node->opcode());

src/compiler/js-call-reducer.h

@@ -122,7 +122,6 @@ class V8_EXPORT_PRIVATE JSCallReducer final : public AdvancedReducer {
   Reduction ReduceStringPrototypeStringAt(
       const Operator* string_access_operator, Node* node);
   Reduction ReduceStringPrototypeCharAt(Node* node);
-  Reduction ReduceStringPrototypeStartsWith(Node* node);
 
 #ifdef V8_INTL_SUPPORT
   Reduction ReduceStringPrototypeToLowerCaseIntl(Node* node);

test/mjsunit/compiler/string-startswith.js

-// Copyright 2019 the V8 project authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// Flags: --allow-natives-syntax --opt
-
-(function() {
-  function foo(string) { return string.startsWith('a'); }
-
-  %PrepareFunctionForOptimization(foo);
-  assertEquals(false, foo(''));
-  assertEquals(true, foo('a'));
-  assertEquals(false, foo('ba'));
-  assertEquals(true, foo('abc'));
-  %OptimizeFunctionOnNextCall(foo);
-  assertEquals(false, foo(''));
-  assertEquals(true, foo('a'));
-  assertEquals(false, foo('ba'));
-  assertEquals(true, foo('abc'));
-  assertOptimized(foo);
-})();
-
-(function() {
-  function f() { return "abc".startsWith(); }
-
-  %PrepareFunctionForOptimization(f);
-  assertEquals(false, f());
-  assertEquals(false, f());
-  %OptimizeFunctionOnNextCall(f);
-  assertEquals(false, f());
-  assertOptimized(f);
-})();
-
-(function() {
-  function g(n) { return "abc".startsWith("a", n); }
-
-  %PrepareFunctionForOptimization(g);
-  assertEquals(true, g(-1));
-  assertEquals(true, g(0));
-  assertEquals(false, g(1));
-  assertEquals(false, g(2));
-  assertEquals(false, g(3));
-  assertEquals(false, g(4));
-  %OptimizeFunctionOnNextCall(g);
-  assertEquals(true, g(-1));
-  assertEquals(true, g(0));
-  assertEquals(false, g(1));
-  assertEquals(false, g(2));
-  assertEquals(false, g(3));
-  assertEquals(false, g(4));
-  assertOptimized(g);
-})();
-
-(function() {
-  function g(n) { return "cba".startsWith("a", n); }
-
-  %PrepareFunctionForOptimization(g);
-  assertEquals(false, g(-1));
-  assertEquals(false, g(0));
-  assertEquals(false, g(1));
-  assertEquals(true, g(2));
-  assertEquals(false, g(3));
-  assertEquals(false, g(4));
-  %OptimizeFunctionOnNextCall(g);
-  assertEquals(false, g(-1));
-  assertEquals(false, g(0));
-  assertEquals(false, g(1));
-  assertEquals(true, g(2));
-  assertEquals(false, g(3));
-  assertEquals(false, g(4));
-  assertOptimized(g);
-})();