Fix Range::Mul for fully non-positive ranges.

runtime/vm/flow_graph_range_analysis.cc

 // Copyright (c) 2014, 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/flow_graph_range_analysis.h"
 
 #include "vm/bit_vector.h"
 #include "vm/il_printer.h"
 
 namespace dart {
@@ skipping 2523 matching lines @@
   ASSERT(right_range != NULL);
   ASSERT(result_min != NULL);
   ASSERT(result_max != NULL);
 
   const int64_t left_max = ConstantAbsMax(left_range);
   const int64_t right_max = ConstantAbsMax(right_range);
   if ((left_max <= -kSmiMin) && (right_max <= -kSmiMin) &&
       ((left_max == 0) || (right_max <= kMaxInt64 / left_max))) {
     // Product of left and right max values stays in 64 bit range.
     const int64_t mul_max = left_max * right_max;
-    const int64_t r_min =
-        OnlyPositiveOrZero(*left_range, *right_range) ? 0 : -mul_max;
-    *result_min = RangeBoundary::FromConstant(r_min);
-    const int64_t r_max =
-        OnlyNegativeOrZero(*left_range, *right_range) ? 0 : mul_max;
-    *result_max = RangeBoundary::FromConstant(r_max);
+    if (OnlyPositiveOrZero(*left_range, *right_range) ||
+        OnlyNegativeOrZero(*left_range, *right_range)) {
+      // If both ranges are of the same sign then the range of the result
+      // is positive and is between multiplications of absolute minimums
+      // and absolute maximums.
+      const int64_t mul_min =
+          ConstantAbsMin(left_range) * ConstantAbsMin(right_range);
+      *result_min = RangeBoundary::FromConstant(mul_min);
+      *result_max = RangeBoundary::FromConstant(mul_max);
+    } else {
+      // If ranges have mixed signs then use conservative approximation:
+      // absolute value of the result is less or equal to multiplication
+      // of absolute maximums.
+      *result_min = RangeBoundary::FromConstant(-mul_max);
+      *result_max = RangeBoundary::FromConstant(mul_max);
+    }
     return;
   }
 
   // TODO(vegorov): handle mixed sign case that leads to (-Infinity, 0] range.
   if (OnlyPositiveOrZero(*left_range, *right_range) ||
       OnlyNegativeOrZero(*left_range, *right_range)) {
     *result_min = RangeBoundary::FromConstant(0);
     *result_max = RangeBoundary::PositiveInfinity();
     return;
   }
@@ skipping 19 matching lines @@
 int64_t Range::ConstantAbsMax(const Range* range) {
   if (range == NULL) {
     return RangeBoundary::kMax;
   }
   const int64_t abs_min = Utils::Abs(Range::ConstantMin(range).ConstantValue());
   const int64_t abs_max = Utils::Abs(Range::ConstantMax(range).ConstantValue());
   return Utils::Maximum(abs_min, abs_max);
 }
 
 
+// Return the minimum absolute value included in range.
+int64_t Range::ConstantAbsMin(const Range* range) {
+  if (range == NULL) {
+    return 0;
+  }
+  const int64_t abs_min = Utils::Abs(Range::ConstantMin(range).ConstantValue());
+  const int64_t abs_max = Utils::Abs(Range::ConstantMax(range).ConstantValue());
+  return Utils::Minimum(abs_min, abs_max);
+}
+
+
 void Range::BinaryOp(const Token::Kind op,
                      const Range* left_range,
                      const Range* right_range,
                      Definition* left_defn,
                      Range* result) {
   ASSERT(left_range != NULL);
   ASSERT(right_range != NULL);
 
   // Both left and right ranges are finite.
   ASSERT(left_range->IsFinite());
@@ skipping 544 matching lines @@
     }
   } while (CanonicalizeMaxBoundary(&max) ||
            CanonicalizeMinBoundary(&canonical_length));
 
   // Failed to prove that maximum is bounded with array length.
   return false;
 }
 
 
 }  // namespace dart