RE: GC / GCJ / and binutils (ld)

["Peter Blemel" <pblemel at hotmail dot com>]

> From: Andrew Haley <aph@redhat.com>
> To: "Peter Blemel" <pblemel@hotmail.com>
> CC: java-patches@gcc.gnu.org
> Subject: GC / GCJ / and binutils (ld)
> Date: Mon, 6 Sep 2004 13:34:00 +0100
>
> Peter Blemel writes:
>  > The macros in GCJ (specifically MS_TIME_DIFF) seem to ignore the fact 
> that a
>  > DWORD tick count even expressed in millis will wrap around every 49.7 
> days
>  > (E.g. MS Windows).   This is a problem if garbage collection is running
>  > right about that time, because attempts to limit the time used by GC 
> will
>  > not be successful if the clock 'rolls over' during GC.   It's not 
> terribly
>  > unusual for an embedded application to run for this long, and in the
>  > (unlikely) event that this occurs, the application will stop until gc
>  > completes.
>
> Why will there be a problem?  Even when the tick count wraps around,
> the time difference should still be correct.  Mind you, I don't know
> exactly what GetTickCount() does.
>
> Andrew.

The short version of this is that the MS_TIME_DIFF(a,b) macro basically 
subtracts b from a to compute 'elapsed time'.   'b' is held constant, taken 
at the beginning of some activity.   'a' is the current time.   This works 
fine while a > b, but when the clock rolls over after 'marking b' then a 
becomes less than b, and the difference is negative.  So the issue is "is 
this macro expected to always return a positive number?".   After looking 
again at where MS_TIME_DIFF is used, I see that the result is that GC will 
stop sooner than expected.   In the case of GC_timeout_stop_func() the 
result is cast to an unsigned long, and the negative difference becomes a 
large positive one, stopping GC.

At this point the issue is either picking nits or a theoretical discussion 
:-)  The quick answer is that I can just fix my macro to ensure that the 
result is positive and be done with it.     The answer to 'why is this a 
problem?' is longer and isn't likely to be interesting to most people 
(probably anyone but me).

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