Improve the assembly code for power function with integer exponential on x64

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
===================================================================
--- src/x64/code-stubs-x64.cc	(revision 12493)
+++ src/x64/code-stubs-x64.cc	(working copy)
@@ -2262,21 +2262,28 @@
   __ movsd(double_scratch2, double_result);  // Load double_exponent with 1.
 
   // Get absolute value of exponent.
-  Label no_neg, while_true, no_multiply;
+  Label no_neg, while_true, while_false;
   __ testl(scratch, scratch);
   __ j(positive, &no_neg, Label::kNear);
   __ negl(scratch);
   __ bind(&no_neg);
 
+  __ j(zero, &while_false, Label::kNear);
+  __ shrl(scratch, Immediate(1));
+  // Above condition means CF=0 && ZF =0.
+  // So the last shifted bit is 0 and the rest is not 0.
+  __ j(above, &while_true, Label::kNear);
+  __ movsd(double_result, double_scratch);
+  __ j(zero, &while_false, Label::kNear);
+
   __ bind(&while_true);
   __ shrl(scratch, Immediate(1));
-  __ j(not_carry, &no_multiply, Label::kNear);
+  __ mulsd(double_scratch, double_scratch);
+  __ j(above, &while_true, Label::kNear);
   __ mulsd(double_result, double_scratch);
-  __ bind(&no_multiply);
-
-  __ mulsd(double_scratch, double_scratch);
   __ j(not_zero, &while_true);
 
+  __ bind(&while_false);
   // If the exponent is negative, return 1/result.
   __ testl(exponent, exponent);
   __ j(greater, &done);