Optimization question

Nikolaus Dunn nikdunn1979@gmail.com
Tue Sep 27 17:39:00 GMT 2016


Maybe I'm using the wrong terminology then or am possibly confused. I am 
under the impression that std::allocator points to std::new_allocator by 
default. In fact, in debugging with gdb, I've gotten into 
new_allocator.h. At any rate, in trying to contrive the smallest 
possible test program, I learned some new things that in my original 
pass, I mistook.

It turns out that std::vector seems to work fine with -O2 or without it.

std::ostringstream however does not call my new OR delete with no 
optimization. With -O2, it calls only my delete.

If I do not attempt to wrap malloc and free, I get the same result. 
std::vector calls my new and delete, ostringstream calls neither.

The command line I used to compile it is:
g++ -g -O2 --std=c++14 -Wl,-wrap,malloc -Wl,-wrap,free Test.c -o test.exe

Test.c:

#include <sstream>
#include <iostream>
#include <vector>

extern "C" void *__real_malloc(size_t);
extern "C" void __real_free(void *);

extern "C" void *__wrap_malloc(size_t nbytes) {
     printf("wrap_malloc: %ld bytes\n", nbytes);
     void * foo = __real_malloc(nbytes);
     printf("wrap_malloc address: %ld\n", foo);
     return foo;
}

extern "C" void __wrap_free(void *ptr) {
     printf("wrap_free: %ld\n", ptr);
     __real_free(ptr);
}

void* operator new (std::size_t size) throw (std::bad_alloc) {
     printf("new: %ld bytes\n", size);
     void *p = __wrap_malloc(size);
     printf("new address: %ld\n", p);
     return p;
}

void operator delete (void *p) {
     printf("delete: %ld\n", p);
     __wrap_free(p);
}

void operator delete (void *p, std::size_t) {
     printf("delete with size: %ld\n", p);
     __wrap_free(p);
}

int main(int argc, char ** argv) {
    printf("Creating new vector\n\n");
    std::vector<int> foo;

    printf("pushing an element\n\n");
    foo.push_back(5);

    printf("Creating new ostringstream\n\n");
    std::ostringstream msg2;

    printf("pushing once\n\n");
    msg2 << "Blah blah blah ";

    printf("pushing twice\n\n");
    msg2 << "boo dee doo" << std::endl;

    printf("printing it out\n\n");
    std::cout << msg2.str();
}


On 09/27/2016 01:10 PM, Jonathan Wakely wrote:
> * Nikolaus Dunn:
>> I've run into an issue where turning on compiler optimization using
>> -O1 or higher causes an issue in my program. Specifically, I am
>> replacing the new and delete operators globally to perform some real
>> time allocation tracking. When I compile with -O1 or -O2, my
>> implementation of new is not being called by STL classes, via the
>> std::allocator. My version of delete IS being called.
> That doesn't make much sense, because std::allocator<T> doesn't use
> new or delete for objects of type T, so neither should be called.
>
> Instead std::allocator<T> allocates untyped memory (e.g. via malloc)
> and then constructs objects into it with placement new-expressions.
>
> What do you replacement new and delete operators look like?




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