Improve the assembly code for power function with integer exponential.

src/ia32/code-stubs-ia32.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 3195 matching lines @@
   }
 
   // Calculate power with integer exponent.
   __ bind(&int_exponent);
   const XMMRegister double_scratch2 = double_exponent;
   __ mov(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, no_multiply, while_false;

[Vyacheslav Egorov (Google), 2012/09/17 22:12:02]

I have an impression that no_multiply is not used anymore.

   __ test(scratch, scratch);
   __ j(positive, &no_neg, Label::kNear);
   __ neg(scratch);
   __ bind(&no_neg);
 
+  __ j(zero, &while_false, Label::kNear);
+  __ shr(scratch, 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);
   __ shr(scratch, 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);
   // scratch has the original value of the exponent - if the exponent is
   // negative, return 1/result.
   __ test(exponent, exponent);
   __ j(positive, &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);  // Result cannot be NaN.
@@ skipping 4263 matching lines @@
   // Restore ecx.
   __ pop(ecx);
   __ ret(0);
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32