Odd GCC Behavior
Odd GCC Behavior
- Subject: Odd GCC Behavior
- From: "J. Aaron Pendergrass" <email@hidden>
- Date: Mon, 30 Jun 2008 09:21:02 -0400
Hello list,
I just ran into what seems to be fairly peculiar behavior of Apple's
GCC (v4.0.1 PPC) that seemed worth
pointing out. It is hard to say if it is really a bug since the C
spec is fairly non-deterministic,
but it is peculiar.
Here's some sample code that demonstrates the behavior:
{
long stack[4] = {1,2,3,4};
long *sp = test + 4; /* top of stack is at (sp - 1) */
/* add the top of stack to the next element and pop */
*(sp - 2) += *(--sp);
}
One might expect this to result in a stack like:
{1, 2, 7, 4}
This is the behavior of GNU GCC (v 4.2.3 on Ubuntu x86-32) and Apple
GCC (v4.0.1)
on Intel. Another reasonable guess might be that the prefix-decrement
would
be evaluated before the lhs is resolved to a value which should result
in:
{1, 6, 3, 4}
But in fact it results in:
{1, 7, 3, 4}
This just seems bizarre to me.
The most bizarre part in my mind is that one might think the program
above should be identical to:
{
long stack[4] = {1,2,3,4}
int sp = 4;
stack[sp-2] += stack[--sp];
}
but this snippet actually results in {1,2,7,4}.
Again, I'm not sure that this is really a bug since the C spec is
annoyingly vague about exactly
when prefix increment/decrement is supposed to happen. But it seems
like it should at least be
consistent between the two cases.
Does anyone have insight into this?
Thanks,
-Aaron
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