close Warning: Can't synchronize with repository "(default)" ("(default)" is not readable or not a Git repository.). Look in the Trac log for more information.

Opened 16 years ago

Closed 15 years ago

#21 closed defect (fixed)

Modular arith bug 3

Reported by: Raymond Toy Owned by: somebody
Priority: minor Milestone:
Component: Core Version: 2008-08
Keywords: Cc:

Description

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.

Change History (3)

comment:1 Changed 16 years ago by Raymond Toy

Priority: majorminor

comment:2 Changed 15 years ago by Raymond Toy

This should be fixed in the 2008-12 snapshot.

We now delay logand-defopt-helperuntil after IR2 optimizations are done so that we give the compiler a chance to derive all of the variable types. Then we run this optimizer. This will cause some increase in compilation time for code that deals with modular arithmetic, but we should get better behaved modular arithmetic.

comment:3 Changed 15 years ago by Raymond Toy

Resolution: fixed
Status: newclosed
Note: See TracTickets for help on using tickets.