Steel Bank Common Lisp

On 26 January 2010 19:20, Tamas K Papp <tkpapp@...> wrote:
> In my code, I was relying on (log 0) signalling an error.  It worked
> fine.  After a bit of development (basically a few hours), I noticed
> my code misbehaving, and found that the cause is:
>
> CL-USER> (sb-int:get-floating-point-modes)
> (:TRAPS NIL :ROUNDING-MODE :NEAREST :CURRENT-EXCEPTIONS (:INEXACT)
>  :ACCRUED-EXCEPTIONS (:INEXACT) :FAST-MODE NIL :PRECISION :53-BIT)
>
> I never explicitly unset the traps.  Is there anything else in SBCL
> that could unset them without my knowledge?

One of the reason setting traps isn't supported is just this. :/ SBCL
should not be unsetting them behind your back, but calls to libc may
do so -- and SBCL doesn't notice if that happens.

Cheers,

 -- Nikodemus

On Fri, 26 Feb 2010 17:18:04 +0200, Nikodemus Siivola wrote:

> On 26 January 2010 19:20, Tamas K Papp <tkpapp@...> wrote:
>> In my code, I was relying on (log 0) signalling an error.  It worked
>> fine.  After a bit of development (basically a few hours), I noticed my
>> code misbehaving, and found that the cause is:
>>
>> CL-USER> (sb-int:get-floating-point-modes) (:TRAPS NIL :ROUNDING-MODE
>> :NEAREST :CURRENT-EXCEPTIONS (:INEXACT)
>>  :ACCRUED-EXCEPTIONS (:INEXACT) :FAST-MODE NIL :PRECISION :53-BIT)
>>
>> I never explicitly unset the traps.  Is there anything else in SBCL
>> that could unset them without my knowledge?
> 
> One of the reason setting traps isn't supported is just this. :/ SBCL
> should not be unsetting them behind your back, but calls to libc may do
> so -- and SBCL doesn't notice if that happens.

What do you mean by calls to libc?  I am not calling any libc functions
directly, does SBCL call anything indirectly?  I am calling LAPACK
routines.

Would wrapping my code in the macro below be an acceptable way to find
when they are changed?  Or could it be something outside my function 
calls, called by SBCL itself?

(defun get-traps ()
  (getf (sb-int:get-floating-point-modes) :traps))

(defmacro with-traps-checked (&body body)
  "Check if floating point traps were changed during body."
  (let ((traps-before (gensym "traps"))
        (traps-after (gensym "traps")))
    `(let ((,traps-before (get-traps)))
       (multiple-value-prog1
           (progn 
             ,@body)
         (let ((,traps-after (get-traps)))
           (unless (equal ,traps-before ,traps-after)
             (error "Floating point traps were changed from ~A to ~A."
                    ,traps-before ,traps-after)))))))

Thanks,

Tamas

On 26 February 2010 19:11, Tamas K Papp <tkpapp@...> wrote:

> What do you mean by calls to libc?  I am not calling any libc functions
> directly, does SBCL call anything indirectly?  I am calling LAPACK
> routines.

Sorry about the vagueness: I meant that most likely cause is a foreign
call frobbing the traps -- be it libc, lapack, or something else.

> Would wrapping my code in the macro below be an acceptable way to find
> when they are changed?  Or could it be something outside my function
> calls, called by SBCL itself?

Well, at least it can help narrow it down. As mentioned on eg. Darwin
calling almost any foreign function will reset the traps, though, so
there aren't very many guarantees.

To get traps to behave I believe we would need to restore them after
every foreign call (possibly depending on compiler policy.)

Cheers,

 -- Nikodemus