[Bug c++/45246] optimizer dereference

["pinskia at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from pinskia at gmail dot com  2010-08-10 12:36 -------
Subject: Re:   New: optimizer dereference



On Aug 10, 2010, at 1:00 AM, "attardi at di dot unipi dot it"
<gcc-bugzilla@gcc.gnu.org 
 > wrote:

> Code produced using -O2 handles dereferencing incorrectly.
> Here is a program that shows the bug:
>
> #include <iostream>
> #include <vector>
>
> class Derived : public std::vector<char *>
> {
> public:
>   Derived() {}
> };
>
> void* foo(void* arg) {
>     void* baseptr = 0;
>     *(std::vector<char*> **)&baseptr = (Derived *)arg;
>     return baseptr;
> }

Have it do
jlong a = 0;
memcpy(&a, &arg, sizeof(void*));

That is the most portable way of putting a pointer value in jlong.

>
> void* foo2(void* arg) {
>     void* baseptr = 0;
>     *(std::vector<char*> **)&baseptr = *(Derived **)&arg;
>     return baseptr;
> }
>
> int main()
> {
>   Derived* d = new Derived();
>   void* upcast = foo(d);
>   void* upcast2 = foo2(d);
>   std::cerr << d << ", " << upcast << ", " << upcast2 << std::endl;
> }
>
> If you compile without -O2 the two values upcast and upcast2 are the
> same, as they should be.
> Using -O2 the results are different.
>
> The bug occurrs with both:
>
> g++ (GCC) 4.1.2 20070925 (Red Hat 4.1.2-27)
> g++ (Ubuntu 4.4.1-4ubuntu9) 4.4.1
>
> This kind of code is generated by SWIG for Java, so I can't change  
> the source.
>
> Curiously the bug disappears if you put a print statement before the
> assignment to baseptr.
>
>
> -- 
>           Summary: optimizer dereference
>           Product: gcc
>           Version: 4.4.1
>            Status: UNCONFIRMED
>          Severity: critical
>          Priority: P3
>         Component: c++
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: attardi at di dot unipi dot it
> GCC build triplet: x86_64-unknown-linux-gnu
>  GCC host triplet: x86_64-unknown-linux-gnu
> GCC target triplet: x86_64-unknown-linux-gnu
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45246
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45246