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

runtime/vm/intermediate_language_mips.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_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 1588 matching lines @@
           __ subu(CMPRES1, TMP, CMPRES1);
         } 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.
+          __ lw(CMPRES1, field_length_operand);
+          __ BranchSignedLess(CMPRES1, 0, &skip_length_check);
           __ BranchEqual(value_cid_reg, kNullCid, &no_fixed_length);
           // Check for typed data array.
           __ BranchSignedGreater(value_cid_reg, kTypedDataFloat32x4ArrayCid,
                                  &no_fixed_length);
           __ BranchSignedLess(value_cid_reg, kTypedDataInt8ArrayCid,
                               &check_array);
           __ lw(TMP, FieldAddress(value_reg, TypedData::length_offset()));
           __ lw(CMPRES1, field_length_operand);
           __ subu(CMPRES1, TMP, CMPRES1);
           __ b(&length_compared);
@@ skipping 2483 matching lines @@
   compiler->GenerateCall(token_pos(),
                          &label,
                          PcDescriptors::kOther,
                          locs());
   __ Drop(2);  // Discard type arguments and receiver.
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS