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

The change removes one unused multiply and reschedules
the shift, multiply and jump instructions to reduce
stall. Experiment shows it improve about 20% performance
on x64 for exponetials from about 100 to 2000.

Review URL: https://chromiumcodereview.appspot.com/10939013
Patch from Xi Qian <xi.qian@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12535 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

diff --git a/src/ia32/code-stubs-ia32.cc b/src/ia32/code-stubs-ia32.cc
index 1fdbaf1..96d45d3 100644 (file)
--- a/src/ia32/code-stubs-ia32.cc
+++ b/src/ia32/code-stubs-ia32.cc
@@ -3213,7 +3213,7 @@ void MathPowStub::Generate(MacroAssembler* masm) {
   __ movsd(double_scratch2, double_result);  // Load double_exponent with 1.
 
   // Get absolute value of exponent.
-  Label no_neg, while_true, no_multiply, while_false;
+  Label no_neg, while_true, while_false;
   __ test(scratch, scratch);
   __ j(positive, &no_neg, Label::kNear);
   __ neg(scratch);

diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 62a89b8..c5025cf 100644 (file)
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -2262,21 +2262,28 @@ void MathPowStub::Generate(MacroAssembler* masm) {
   __ 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);
 
-  __ bind(&while_true);
+  __ j(zero, &while_false, Label::kNear);
   __ shrl(scratch, Immediate(1));
-  __ j(not_carry, &no_multiply, Label::kNear);
-  __ mulsd(double_result, double_scratch);
-  __ bind(&no_multiply);
+  // Above condition means CF==0 && ZF==0.  This means that the
+  // bit that has been shifted out is 0 and the result 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));
   __ mulsd(double_scratch, double_scratch);
+  __ j(above, &while_true, Label::kNear);
+  __ mulsd(double_result, double_scratch);
   __ j(not_zero, &while_true);
 
+  __ bind(&while_false);
   // If the exponent is negative, return 1/result.
   __ testl(exponent, exponent);
   __ j(greater, &done);