Fix large performance regression in Box2D, MD5, and SHA1 benchmarks

runtime/vm/intermediate_language_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 1552 matching lines @@
           __ popq(value_cid_reg);
         } else {
           ASSERT(field_cid == kIllegalCid);
           ASSERT(field_length == Field::kUnknownFixedLength);
           // At compile time we do not know the type of the field nor its
           // length. At execution time we may have set the class id and
           // list length so we compare the guarded length with the
           // list length here, without this check the list length could change
           // without triggering a deoptimization.
           Label check_array, length_compared, no_fixed_length;
+          // If length is negative the length guard is either disabled or
+          // has not been initialized, either way it is safe to skip the
+          // length check.
+          __ cmpq(field_length_operand, Immediate(Smi::RawValue(0)));
+          __ j(LESS, &skip_length_check);
           __ cmpq(value_cid_reg, Immediate(kNullCid));
           __ j(EQUAL, &no_fixed_length, Assembler::kNearJump);
           // Check for typed data array.
           __ cmpq(value_cid_reg, Immediate(kTypedDataFloat32x4ArrayCid));
           // Not a typed array or a regular array.
           __ j(GREATER, &no_fixed_length, Assembler::kNearJump);
           __ cmpq(value_cid_reg, Immediate(kTypedDataInt8ArrayCid));
           // Could still be a regular array.
           __ j(LESS, &check_array, Assembler::kNearJump);
           __ pushq(value_cid_reg);
@@ skipping 3260 matching lines @@
                          PcDescriptors::kOther,
                          locs());
   __ Drop(2);  // Discard type arguments and receiver.
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64