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

runtime/lib/double.cc

 // Copyright (c) 2011, 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 "platform/math.h"
 
 #include "vm/dart_entry.h"
 #include "vm/double_conversion.h"
@@ skipping 61 matching lines @@
   return Double::New(left / right);
 }
 
 
 static RawInteger* DoubleToInteger(double val, const char* error_msg) {
   if (isinf(val) || isnan(val)) {
     const Array& args = Array::Handle(Array::New(1));
     args.SetAt(0, String::Handle(String::New(error_msg)));
     Exceptions::ThrowByType(Exceptions::kUnsupported, args);
   }
+  if (FLAG_truncate_ints_to_64_bits) {
+    // TODO(alexmarkov): decide on the double-to-integer conversion semantics
+    // in truncating mode.

[regis, 2017/07/07 20:10:29]

Something to discuss with the API people. You could pass a bool here indicating
whether to throw or to use min/max. I think it is safer to throw.

[alexmarkov, 2017/07/07 20:46:38]

Acknowledged.

+    int64_t ival = 0;
+    if (val <= static_cast<double>(kMinInt64)) {
+      ival = kMinInt64;
+    } else if (val >= static_cast<double>(kMaxInt64)) {
+      ival = kMaxInt64;
+    } else {  // representable in int64_t

[regis, 2017/07/07 20:10:29]

The convention is to use a capital letter and a period:
// Representable as int64_t.

[alexmarkov, 2017/07/07 20:46:38]

Done.

+      ival = static_cast<int64_t>(val);
+    }
+    return Integer::New(ival);
+  }
   if ((-1.0 < val) && (val < 1.0)) {
     return Smi::New(0);
   }
   DoubleInternals internals = DoubleInternals(val);
   ASSERT(!internals.IsSpecial());  // Only Infinity and NaN are special.
   uint64_t significand = internals.Significand();
   intptr_t exponent = internals.Exponent();
   if (exponent <= 0) {
     significand >>= -exponent;
     exponent = 0;
@@ skipping 217 matching lines @@
 DEFINE_NATIVE_ENTRY(Double_flipSignBit, 1) {
   const Double& arg = Double::CheckedHandle(arguments->NativeArgAt(0));
   const double in_val = arg.value();
   const int64_t bits = bit_cast<int64_t, double>(in_val) ^ kSignBitDouble;
   return Double::New(bit_cast<double, int64_t>(bits));
 }
 
 // Add here only functions using/referring to old-style casts.
 
 }  // namespace dart