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

src/x64/code-stubs-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2244 matching lines @@
 
   // Calculate power with integer exponent.
   __ bind(&int_exponent);
   const XMMRegister double_scratch2 = double_exponent;
   // Back up exponent as we need to check if exponent is negative later.
   __ movq(scratch, exponent);  // Back up exponent.
   __ movsd(double_scratch, double_base);  // Back up base.
   __ 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);
   __ divsd(double_scratch2, double_result);
   __ movsd(double_result, double_scratch2);
   // Test whether result is zero.  Bail out to check for subnormal result.
   // Due to subnormals, x^-y == (1/x)^y does not hold in all cases.
   __ xorps(double_scratch2, double_scratch2);
   __ ucomisd(double_scratch2, double_result);
   // double_exponent aliased as double_scratch2 has already been overwritten
@@ skipping 4192 matching lines @@
 #endif
 
   __ Ret();
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64