Option to truncate integers to 64 bits, part 1 (core VM changes)

runtime/lib/integers.cc

 // Copyright (c) 2012, 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/bootstrap_natives.h"
 
 #include "include/dart_api.h"
 #include "vm/dart_entry.h"
 #include "vm/dart_api_impl.h"
 #include "vm/exceptions.h"
 #include "vm/isolate.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/symbols.h"
 
 namespace dart {
 
 DEFINE_FLAG(bool,
             trace_intrinsified_natives,
             false,
             "Report if any of the intrinsified natives are called");
 
 // Smi natives.
 
 // Returns false if integer is in wrong representation, e.g., as is a Bigint
 // when it could have been a Smi.
 static bool CheckInteger(const Integer& i) {
   if (i.IsBigint()) {
+    ASSERT(!FLAG_truncate_ints_to_64_bits);
     const Bigint& bigint = Bigint::Cast(i);
     return !bigint.FitsIntoSmi() && !bigint.FitsIntoInt64();
   }
   if (i.IsMint()) {
     const Mint& mint = Mint::Cast(i);
     return !Smi::IsValid(mint.value());
   }
   return true;
 }
 
@@ skipping 213 matching lines @@
                                         const Smi& amount) {
   if (amount.Value() < 0) {
     Exceptions::ThrowArgumentError(amount);
   }
   if (value.IsSmi()) {
     const Smi& smi_value = Smi::Cast(value);
     return smi_value.ShiftOp(kind, amount, Heap::kNew);
   }
   if (value.IsMint()) {
     const int64_t mint_value = value.AsInt64Value();
-    const int count = Utils::HighestBit(mint_value);
     intptr_t shift_count = amount.Value();
-    if (kind == Token::kSHR) {
-      shift_count = -shift_count;
-    }
-    if ((count + shift_count) < Mint::kBits) {
-      switch (kind) {
-        case Token::kSHL:
-          return Integer::New(mint_value << shift_count, Heap::kNew);
-        case Token::kSHR:
-          shift_count =
-              (-shift_count > Mint::kBits) ? Mint::kBits : -shift_count;
-          return Integer::New(mint_value >> shift_count, Heap::kNew);
-        default:
-          UNIMPLEMENTED();
-      }
-    } else {
-      // Overflow in shift, use Bigints
-      return Integer::null();
+    switch (kind) {
+      case Token::kSHL:
+        if (FLAG_truncate_ints_to_64_bits) {
+          return Integer::New(
+              Utils::ShiftLeftWithTruncation(mint_value, shift_count),
+              Heap::kNew);
+        } else {
+          const int count = Utils::HighestBit(mint_value);
+          if (shift_count < (Mint::kBits - count)) {
+            return Integer::New(mint_value << shift_count, Heap::kNew);
+          } else {
+            // Overflow in shift, use Bigints
+            return Integer::null();
+          }
+        }
+      case Token::kSHR:
+        shift_count = Utils::Minimum(shift_count, Mint::kBits);
+        return Integer::New(mint_value >> shift_count, Heap::kNew);
+      default:
+        UNIMPLEMENTED();
     }
   } else {
     ASSERT(value.IsBigint());
   }
+  ASSERT(!FLAG_truncate_ints_to_64_bits);
   return Integer::null();
 }
 
 
 DEFINE_NATIVE_ENTRY(Smi_bitAndFromSmi, 2) {
   const Smi& left = Smi::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Smi, right, arguments->NativeArgAt(1));
   if (FLAG_trace_intrinsified_natives) {
     OS::Print("Smi_bitAndFromSmi %s & %s\n", left.ToCString(),
               right.ToCString());
@@ skipping 106 matching lines @@
 }
 
 
 DEFINE_NATIVE_ENTRY(Bigint_getDigits, 1) {
   const Bigint& bigint = Bigint::CheckedHandle(arguments->NativeArgAt(0));
   return bigint.digits();
 }
 
 
 DEFINE_NATIVE_ENTRY(Bigint_allocate, 4) {
+  // TODO(alexmarkov): Consider throwing error if --truncate-ints-to-64-bits.
   if (FLAG_limit_ints_to_64_bits) {

[siva, 2017/07/07 23:11:20]

Not sure I understand why we should have two flags
--limit_ints_to_64_bits and --truncate_ints_to_64_bits

[alexmarkov, 2017/07/10 16:58:12]

Done.

     // The allocated Bigint value is not necessarily out of range, but it may
     // be used as an operand in an operation resulting in a Bigint.
     Exceptions::ThrowRangeErrorMsg(
         "Integer operand requires conversion to Bigint");
   }
   // First arg is null type arguments, since class Bigint is not parameterized.
   const Bool& neg = Bool::CheckedHandle(arguments->NativeArgAt(1));
   const Smi& used = Smi::CheckedHandle(arguments->NativeArgAt(2));
   const TypedData& digits = TypedData::CheckedHandle(arguments->NativeArgAt(3));
   ASSERT(!digits.IsNull());
   return Bigint::New(neg.value(), used.Value(), digits);
 }
 
 }  // namespace dart