Improve range analysis for modulo operations.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/15952004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14884 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -1811,8 +1811,18 @@ Range* HMod::InferRange(Zone* zone) {
   if (representation().IsInteger32()) {
     Range* a = left()->range();
     Range* b = right()->range();
-    Range* result = new(zone) Range();
-    if (a->CanBeMinusZero() || a->CanBeNegative()) {
+
+    // The magnitude of the modulus is bounded by the right operand. Note that
+    // apart for the cases involving kMinInt, the calculation below is the same
+    // as Max(Abs(b->lower()), Abs(b->upper())) - 1.
+    int32_t positive_bound = -(Min(NegAbs(b->lower()), NegAbs(b->upper())) + 1);
+
+    // The result of the modulo operation has the sign of its left operand.
+    bool left_can_be_negative = a->CanBeMinusZero() || a->CanBeNegative();
+    Range* result = new(zone) Range(left_can_be_negative ? -positive_bound : 0,
+                                    a->CanBePositive() ? positive_bound : 0);
+
+    if (left_can_be_negative) {
       result->set_can_be_minus_zero(true);
     }
 

src/hydrogen-instructions.h

@@ -263,6 +263,7 @@ class Range: public ZoneObject {
   bool CanBeMinusZero() const { return CanBeZero() && can_be_minus_zero_; }
   bool CanBeZero() const { return upper_ >= 0 && lower_ <= 0; }
   bool CanBeNegative() const { return lower_ < 0; }
+  bool CanBePositive() const { return upper_ > 0; }
   bool Includes(int value) const { return lower_ <= value && upper_ >= value; }
   bool IsMostGeneric() const {
     return lower_ == kMinInt && upper_ == kMaxInt && CanBeMinusZero();

src/utils.h

@@ -232,6 +232,13 @@ T Min(T a, T b) {
 }
 
 
+// Returns the negative absolute value of its argument.
+template <typename T>
+T NegAbs(T a) {
+  return a < 0 ? a : -a;
+}
+
+
 inline int StrLength(const char* string) {
   size_t length = strlen(string);
   ASSERT(length == static_cast<size_t>(static_cast<int>(length)));