More precise range analysis for smi multiplication.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 37311)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -3087,7 +3087,7 @@
     case Token::kMUL: {
       const int64_t left_max = ConstantAbsMax(left_range);

[Vyacheslav Egorov (Google), 2014/06/13 13:59:29]

Can you test it for kMinInt64 * -1? Because I am bit concerned about what
ConstantAbsMax(kMinInt64) returns. 

Similar for kMinSmi * -1.

[Florian Schneider, 2014/06/16 10:45:10]

We're staying in smi range here: ConstantAbsMax returns a value between 0 and
-kMinSmi, which is always smaller than kMaxInt.

       const int64_t right_max = ConstantAbsMax(right_range);
-      if ((left_max < 0x7FFFFFFF) && (right_max < 0x7FFFFFFF)) {
+      if ((left_max == 0) || (right_max <= kMaxInt64 / left_max)) {
         // Product of left and right max values stays in 64 bit range.
         const int64_t result_max = left_max * right_max;
         if (Smi::IsValid64(result_max) && Smi::IsValid64(-result_max)) {