Optimization question

Jonathan Wakely jwakely.gcc@gmail.com
Tue Sep 27 17:49:00 GMT 2016


On 27 September 2016 at 18:39, Nikolaus Dunn wrote:
> 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.

Sorry, I completely misread your original mail, I thought you meant
new and delete operators for specific classes, not the global ones.

You're right that std::allocator uses the global new and delete operators.

> 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.

I can't reproduce this, I see stringstream calling both.


Creating new vector

pushing an element

new: 4 bytes
wrap_malloc: 4 bytes
wrap_malloc address: 0x1528c20
new address: 0x1528c20
Creating new ostringstream

pushing once

new: 513 bytes
wrap_malloc: 513 bytes
wrap_malloc address: 0x1528c40
new address: 0x1528c40
pushing twice

printing it out

new: 28 bytes
wrap_malloc: 28 bytes
wrap_malloc address: 0x1528e50
new address: 0x1528e50
Blah blah blah boo dee doo
delete: 0x1528e50
wrap_free: 0x1528e50
delete: 0x1528c40
wrap_free: 0x1528c40
delete: 0x1528c20
wrap_free: 0x1528c20



> 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);

N.B. compiling your code produces loads of warnings, the correct
format specifiers for printf are %zu and %p.



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