On 20/01/2011, at 3:04 AM, Dave Goodell wrote:
> On Jan 19, 2011, at 9:52 AM CST, john skaller wrote:
>
>> On 20/01/2011, at 2:39 AM, Dave Goodell wrote:
>>
>>> On Jan 18, 2011, at 10:56 PM CST, john skaller wrote:
>
>> I could rewrite the GC so that the stack scan is in a separate subroutine,
>> and then just exclude that using Valgrinds nice suppression mechanism.
>
> Yes, although this doesn't get rid of the uninitialized values, which could
> potentially propagate elsewhere in your code. It just suppresses error
> _reporting_
Well there's no way to get rid of these "uninitialised" values. Most are in
fact initialised.
The problem is something like: when a subroutine is called the return address
is pushed
on the stack, along with callee-save registers.
I suspect Valgrind thinks these are uninitialised values.
In other cases: consider, I have a data structure:
struct { int x; long y; } a;
on the stack with say 4 bytes padding after 'x'. When I read the 8 byte word
at address "a" half is uninitialised. I'm not sure what Valgrind would say here:
most of my uninitialised values are Value8.
> Right. I was thinking this was an external thread or signal handler
> examining another stack, in which case you would need to scan the whole red
> zone. But if it all happens as a result of an explicit new/allocate call
> then scanning the red zone shouldn't be necessary.
Yeah. I'm not sure what happens on x86_64 Unix (OSX, Linux) with signals: I
have a feeling
they do not use the applications stack? or do they bump the RSP by the size of
the redzone
before calling the handler?
Anyhow, I've got some other test code exhibiting problems sometimes and not
others,
and I'm no closer to a solution. Sometime code works, sometimes it segfaults,
sometimes it just overwrites the wrong place and I trap the problem and report
it.
The behaviour is always the same for the same program, data, and GC tuning
parameters: the fault is unpredictable but thankfully not intermittent.
Unfortunately running Valgrind is one of the things the bug is sensitive to,
it runs and hides when running Valgrind :)
--
john skaller
skal...@users.sourceforge.net
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