Ok, I think I understand. This means the test application I sent in my
previous mail shows the lack of alignment-awareness rather than an
actual bug, right?
// Christoffer
tis 2010-08-10 klockan 09:33 +0100 skrev Tom Hughes:
> On 10/08/10 09:00, Christoffer Haglund wrote:
>
> > Oh, sorry, that last reply wasn't very clear, let my try again. The
> > problem occurs inside some proprietary code, so unfortunately I can't
> > just copy the exact code that fails, but I'll try to describe it.
> >
> > There's a struct with the following definition
> >
> > typedef struct _session {
> > ...
> > char generic_buffer[255];
> > } session_t;
>
> Well the first question I'd be asking here, given that you are later
> accessing that buffer though an int pointer, is whether it is aligned on
> a 4 byte boundary or not..
>
> > The function has the following definition:
> >
> > status_t do_stuff(input_t * input, char * charPtr)
> > {
> > int n = calculate_n(input);
> > int value = calculate_value(input);
> > int * intPrt = (int *) charPtr;
> > for ( ; n < 0; n-- )
> > *intPtr++ = value;
> > }
> >
> > ...and when debugging I can see that calculate_n() returns 27 and
> > calculate_value() returns 0x000000 for the test case when the
> > application fails (which are correct values, compared to when the
> > application is built with another compiler or with "-fno-tree-vectorize").
>
> As I said above, my guess is that you have an alignment problem. On x86
> the alignment doesn't normally matter (though doing 4 byte accesses
> through an unaligned pointer is likely to be inefficient).
>
> Once you start vectorising and using SSE instructions that will change
> however as many SSE instructions do care about alignment and will fault
> if the data is not correctly aligned.
>
> Most probably the reason it works in valgrind is that in the process of
> annotating the code the actual instructions being used are changed and
> the ones valgrind has selected don't have the same alignment requirements.
>
> Tom
>
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