Revise assertions and fix bug in the implementation of shifts

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/compiler.h"
@@ skipping 6054 matching lines @@
 
   Label* deopt = NULL;
   if (CanDeoptimize()) {
     deopt = compiler->AddDeoptStub(deopt_id(), ICData::kDeoptBinaryMintOp);
   }
   if (locs()->in(1).IsConstant()) {
     // Code for a constant shift amount.
     ASSERT(locs()->in(1).constant().IsSmi());
     const int64_t shift =
         reinterpret_cast<int64_t>(locs()->in(1).constant().raw()) >> 1;
-    // TODO(alexmarkov): revise and uncomment the following assertions
-    // ASSERT(!has_shift_count_check());
-    // ASSERT((0 <= shift) && (shift < 64));
+    ASSERT(shift >= 0);
     switch (op_kind()) {
       case Token::kSHR:
-        __ sarq(left, Immediate(shift));
+        __ sarq(left,
+                Immediate(Utils::Minimum<int64_t>(shift, kBitsPerWord - 1)));
         break;
       case Token::kSHL: {
+        ASSERT(shift < 64);
         if (can_overflow()) {
           // Check for overflow.
           Register temp = locs()->temp(0).reg();
           __ movq(temp, left);
           __ shlq(left, Immediate(shift));
           __ sarq(left, Immediate(shift));
           __ cmpq(left, temp);
           __ j(NOT_EQUAL, deopt);  // Overflow.
         }
         // Shift for result now we know there is no overflow.
         __ shlq(left, Immediate(shift));
         break;
       }
       default:
         UNREACHABLE();
     }
   } else {
     // Code for a variable shift amount.
-    // Deoptimize if shift count is > 63.
-    // sarl operation masks the count to 5 bits and
-    // shrd is undefined with count > operand size (32)
+    // Deoptimize if shift count is > 63 or negative.
+    // Sarq and shlq instructions mask the count to 6 bits.
     __ SmiUntag(RCX);
-    if (has_shift_count_check()) {
+    if (!IsShiftCountInRange()) {
       __ cmpq(RCX, Immediate(kMintShiftCountLimit));
       __ j(ABOVE, deopt);
     }
     Label done, large_shift;
     switch (op_kind()) {
       case Token::kSHR: {
         __ sarq(left, RCX);
         break;
       }
       case Token::kSHL: {
@@ skipping 533 matching lines @@
   compiler->AddCurrentDescriptor(stub_kind_, deopt_id_, token_pos());
   compiler->RecordSafepoint(locs());
 }
 
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64