Jeff King <peff@xxxxxxxx> writes:
> On Tue, Mar 19, 2013 at 11:29:36AM +0100, Thomas Rast wrote:
>
>> > Ah, indeed. Putting:
>> >
>> > fprintf(stderr, "%lu\n", base->obj->delta_depth);
>> >
>> > before the conditional reveals that base->obj->delta_depth is
>> > uninitialized, which is the real problem. I'm sure there is some
>> > perfectly logical explanation for why valgrind can't detect its use
>> > during the assignment, but I'm not sure what it is.
>>
>> That's simply because you would get far too much noise. It only reports
>> an uninitialized value when it actually gets used in a conditional or
>> for output (syscalls), which is when they matter.
>
> Would it? I would think any computation you start with an undefined
> value would be suspect (and you would want to know about it as soon as
> possible, before the tainted value gets output). I was assuming it was a
> performance issue or something.
Now consider
// somewhere on the stack
struct foo {
char c;
int i;
} a, b;
a.c = a.i = 0;
memcpy(&b, &a, sizeof(struct foo));
The compiler could legitimately leave the padding between c and i
uninitialized, and with your proposed "early" reporting the memcpy would
complain.
--
Thomas Rast
trast@{inf,student}.ethz.ch
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