> On Sep 14, 2018, at 7:07 PM, Sergei Trofimovich <sly...@gentoo.org> wrote:
>
> On Fri, 14 Sep 2018 11:54:57 -0400
> Richard Yao <r...@gentoo.org> wrote:
>
>>>> My read of this is that the warning occurs regardless of optimization
>>>> level, but it could somehow be improved by optimization.
>>>>
>>>> As for the last, it is for uninitialized variable reads. However, I think
>>>> you are misinterpreting the claim. The way that optimization level could
>>>> affect warning generation would be if the warning generation came after
>>>> optimization passes that could hide reads. That means that -O3 would
>>>> prevent the warning.
> ...
>> Either provide code examples that generate warnings in a way that
>> demonstrates that I am incorrect
>
> To make code behave differently it needs substantial amount of code
> to provide you an example. You need to him O2<->O3 behaviour delta
> after all. But I will try (for a different warning, it should not matter
> much).
Thanks. I had been incorrect about -O3 giving not us some additional
information for warnings. My apologies for the confusion.
>
> Below is a reduced example of a larger C++ program.
> Many thanks to Ulya for providing recent example!
>
> In this example -O3 manages to inline/const-propagate deep enough
> and find out potential null-deref. -O2 was not able to do it.
>
> $ bash -x ./mk_.sh
> + LANG=C
> + g++-8.2.0 -O2 -c 1.cc -Wnull-dereference
> + g++-8.2.0 -O3 -c 1.cc -Wnull-dereference
> 1.cc: In function 'bool foo(std::vector<int>)':
> 1.cc:3:22: warning: null pointer dereference [-Wnull-dereference]
> typename h = e - d >> *g;
> ~~~~~~^~~~~
> 1.cc:3:22: warning: null pointer dereference [-Wnull-dereference]
> typename h = e - d >> *g;
> ~~~~~~^~~~~
>
> $ cat 1.cc
> #include <vector>
> template <typename a, typename b> a c(a d, a e, b f, int *g) {
> typename h = e - d >> *g;
> for (; h;) if (f(*d)) if (f(*d)) return d;
> return d;
> }
> template <typename i, typename b> i o(i d, i e, b f, int *g) {
> return c(d, e, f, g);
> }
> template <typename i, typename b> i oo(i d, i e, b f, int *g) {
> return c(d, e, f, g);
> }
> template <typename j, typename b> j k(j d, j e, b f, int *g) {
> return o(d, e, f, g);
> }
> template <typename j, typename b> j kk(j d, j e, b f, int *g) {
> return oo(d, e, f, g);
> }
> bool cmpp(int);
> bool foo(std::vector<int> l) {
> std::vector<int>::const_iterator ib,
> ie = l.end(), m, n = k(ib, ie, cmpp, 0) = kk(m, ie, cmpp, 0);
> return m == n;
> }
>
> --
>
> Sergei
>
