#21: Modular arith bug 3 --------------------+------------------------------------------------------- Reporter: rtoy | Owner: somebody Type: defect | Status: new Priority: major | Milestone: Component: Core | Version: 2008-08 Keywords: | --------------------+------------------------------------------------------- The following code doesn't use modular arithmetic for ash:
{{{ (defun bug (v) (declare (type (unsigned-byte 32) v) (optimize (speed 3) (safety 0))) (logand #xffffffff (logxor v (ash v (- -16))))) }}}
You get compiler warnings about {{{ASH}}}. But the equivalent code is ok:
{{{ (defun bug-a (v) (declare (type (unsigned-byte 32) v) (optimize (speed 3) (safety 0))) (ldb (byte 32 0) (logxor v (ash v (- -16))))) }}}
Also, if in {{{bug}}}, you change {{{(- -16)}}} to the obvious {{{16}}}, the compiler notes are gone, and the generated code is as expected.
What appears to be happening is that when {{{logand-defopt-helper}}} is run, the compiler doesn't know the type of the shift for {{{ASH}}}. Hence, it can't do modular arithmetic stuff. However, when the code is finally generated, you can see that the shift is now a known constant value of 16. I don't know why.