[aliasing rules] Should operator new be excluded ?

[Yuri Victorovich]

In short my question is:
If I overload "operator new" for class A and return from it
instance of struct B (unrelated with A) as allocated memory
area for A should aliasing rules work and allow optimizer
to "merge" assemblies together ?

My opinion: NO, since aliasing rules talk about one lvalue
for access to two unrelated objects and one of these objects
is not constructed yet w/in operator new.

In fact GCC-3.3 applies aliasing rule in this case and code
breaks as the result.

If your answer is YES (aliasing rule applies) doesn't this
undermine meaning of overloading of "operator new" since
it's not much that you can do w/in it then ?

Sincerely,
Yuri

 

[tom_usenet]

In short my question is:
If I overload "operator new" for class A and return from it
instance of struct B (unrelated with A) as allocated memory
area for A should aliasing rules work and allow optimizer
to "merge" assemblies together ?

How can you return anything but raw memory from operator new? It
returns a void*. It isn't yet an instance of struct A, B or anything
else.

My opinion: NO, since aliasing rules talk about one lvalue
for access to two unrelated objects and one of these objects
is not constructed yet w/in operator new.

In fact GCC-3.3 applies aliasing rule in this case and code
breaks as the result.

If your answer is YES (aliasing rule applies) doesn't this
undermine meaning of overloading of "operator new" since
it's not much that you can do w/in it then ?

Post your code, it sounds like you might be misusing operator new.
operator new shouldn't have any effect on aliasing, since it is only a
memory allocation function, and nothing else.

Tom

 

[Attila Feher]

In short my question is:
If I overload "operator new" for class A and return from it
instance of struct B (unrelated with A) as allocated memory
area for A should aliasing rules work and allow optimizer
to "merge" assemblies together ?

My opinion: NO, since aliasing rules talk about one lvalue
for access to two unrelated objects and one of these objects
is not constructed yet w/in operator new.

In fact GCC-3.3 applies aliasing rule in this case and code
breaks as the result.

If your answer is YES (aliasing rule applies) doesn't this
undermine meaning of overloading of "operator new" since
it's not much that you can do w/in it then ?

Post the shortest, compilable code showing the problem you have. It is not
clear from your text what do you want to do.

 

[llewelly]

In short my question is:
If I overload "operator new" for class A and return from it
instance of struct B (unrelated with A) as allocated memory
area for A should aliasing rules work and allow optimizer
to "merge" assemblies together ?

My opinion: NO, since aliasing rules talk about one lvalue
for access to two unrelated objects and one of these objects
is not constructed yet w/in operator new.

In fact GCC-3.3 applies aliasing rule in this case and code
breaks as the result.

[snip]

Note gcc provides -fno-strict-aliasing if your non-standard code
breaks when aliasing optimizations are applied.

In some (all?) cases you can disable aliasing optimizations by
pointing a pointer to char at the struct.

 

[Yuri]

Here is an example of code for my previous post
It crashes if compiled with GCC-3.3 with -O3 only.

Yuri

----code start--------------------------------------

#include <stdio.h>

struct SList {
SList *next;
int i;
};
SList sl1, sl2;
SList *freed;

inline void *alc() {
void *r = freed;
// !!! HERE IS WHERE BUG IS IMPLANTED
freed = freed->next;
return (r);
}

class P {
public:
virtual int fn1() { return (1); }
virtual int fn2() { return (2); }
};
class C : public P {
public:
virtual int fn1() { return (3); }
virtual int fn2() { return (4); }
int i;
inline void * operator new(size_t sz) { return alc(); }
inline void operator delete(void *) { }
};
class D {
public:
virtual C* X_alc() { return (NULL); }
};
class E {
public:
virtual C* X_alc() { return (new C()); };
};
int
main(int argc[], const char *argv[]) {
C *c1, *c2;
E *a = new E;

// initialize
sl1.next = &sl2;
sl2.next = NULL;
freed = &sl1;


printf(" ** freed=%p **\n", freed);

printf(" ** alloc: %p **\n", (c1 = a->X_alc()));
printf(" ** freed=%p **\n", freed);
printf(" ** alloc: %p **\n", (c2 = a->X_alc()));
printf(" ** freed=%p **\n", freed);
delete c1;
delete c2;
return (0);
}