Improve range analysis for modulo operations.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index e362b95c60a81c0722e4a221e57ebd052271ada2..5cf4f8d62bc597ee77445545af7477d4930f3c04 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -1811,8 +1811,19 @@ 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.

[Michael Starzinger, 2013/05/29 11:59:54]

A comment why we need to go through negtive-absolutes here would be nice. After
discussing it offline I assume it's because of overflow at or near kMinInt.

+    int32_t positive_bound = -Min(NegAbs(b->lower()), NegAbs(b->upper())) - 1;

[Michael Starzinger, 2013/05/29 11:59:54]

Not completely sure but isn't there a parenthesis missing around the whole
expression after the "-" operator?

+
+    // 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);
+
+    // The result is bounded by its left operand, too.
+    result->Intersect(a);
+
+    if (left_can_be_negative) {
       result->set_can_be_minus_zero(true);
     }