VM::READ-CYCLE-COUNTER destroys live values in ebx and ecx registers

[Raymond Toy]

Consider the following code:

(eval-when (:compile-toplevel :execute)
(defmacro with-cycle-counter (&body body)
  (let ((hi0 (gensym))
	(hi1 (gensym))
	(lo0 (gensym))
	(lo1 (gensym)))
    `(multiple-value-bind (,lo0 ,hi0)
	 (vm::read-cycle-counter)
       (values (locally ,@body)
               (multiple-value-bind (,lo1 ,hi1)
		   (vm::read-cycle-counter)
                 (+ (ash (- ,hi1 ,hi0) 32)
                    (- ,lo1 ,lo0)))))))
)

(defun bar (x)
  (declare (type (and fixnum unsigned-byte) x) 
	   (optimize speed (safety 0)))
  (with-cycle-counter
    (let ((sum 0d0))
      (declare (double-float sum))
      (dotimes (k x)
	(declare (type (and fixnum unsigned-byte) k))
	(incf sum k))
      sum)))

When compiled, you get funny results like

* (bar 1000000)
0.0d0
408

This happens because READ-CYCLE-COUNTER uses the CPUID instruction that writes values to the eax, ebx, ecx, and edx registers, but the VOP for READ-CYCLE-COUNTER doesn't know that ebx and ecx are written. Thus the vop can cause any live values in the ebx and ecx registers to be destroyed, as happens in this example.

[Raymond Toy]

Clean up summary line.

[toy.raymond@…]

commit 3832e020b43c1270e0710097381fb718271e61bf Author: Raymond Toy <toy.raymond@…> Date: Tue Sep 25 21:09:46 2012 -0700

Fix ticket:63.