2011/10/26 Michael Matz <m...@suse.de>:
> Hi,
>
> On Wed, 26 Oct 2011, Kai Tietz wrote:
>
>> well, if such a function is used as inline and we know for it that j has
>> value != 2, then we have here a big difference. For your first example,
>> we still have to do the memory access to *i, even if we are not
>> interested in result.
>
> Actually we don't have to preserve memory accesses. The interesting case
> is if the pointer has an invalid value. The behaviour of the access then
> is undefined, and it's okay to not do it at all. In case the pointer does
> point to an object the access (if it's value isn't needed) also isn't
> necessary. IOW: in "void f(int *p) { int i = *p; }" we can always remove
> the pointer read. So, I still maintain that the transformation on the
> original example was okay.
>
>
> Ciao,
> Michael.
So you would mean that memory dereferencing shouldn't be considered as
side-effect at all?
So we would happily cause by code 'if (i && *i != 0) an crash, as
memory-dereference has for you no side-effect?
In you special case it might be valid that, if first (and C-fold-const
doesn't know if the side-effect condition is really the first, as it
might be a sub-sequence of a condition) condition might trap or not,
to combine it. But branching has to cover the general cases. If you
find a way to determine that left-hand operand in fold_const's
branching code is really the left-most condition in chain, then we can
add such a special case, but I don't see here an easy way to determine
it.
Regards,
Kai
Hmm, not sure
