WITH-ALIEN binds *alien-stack* even when not necessary

[Todd Williamson]

A foreign function call defined like this:

 (defun microseconds-now (now)
        (declare (type system-area-pointer now))
        (with-alien ((microseconds-now (function void (* int)) :extern "microseconds_now_p"))
          (alien-funcall microseconds-now now)))

generates code that does an unnecessary binding of *alien-stack*, which is a lot of overhead for a foreign function call, especially since nothing is actually allocated on the alien stack. Changing it to avoid the WITH-ALIEN (using extern-alien instead) solves the problem, so this is not really a big deal, though I think this would occur if all alien variables were allocated on the heap as well.

Perhaps the binding could be conditionalized on whether WITH-ALIEN actually allocates anything on the stack.

Tested version: 1.0.17.39
Platform: Linux x86-64

[Stas Boukarev (stassats)]

In 639b29cbdf3e9ff7d0897d299e7feb1889f09fd5