Windows malloc replacement (e.g., tcmalloc) and dynamic crt linking

Q: A C++ program that is split accross several dlls should:

A) replace malloc?

B) ensure that allocation and de-allocation happens in the same dll module?

A: The correct answer is B. A c++ application design that incorporates multiple DLLs SHOULD ensure that a mechanism exists to ensure that things that are allocated on the heap in one dll, are free'd by the same dll module.

Why would you split a c++ program into several dlls anyway? By c++ program I mean that the objects and types you are dealing with are c++ templates, STL objects, classes etc. You CAN'T pass c++ objects accross dll boundries without either lot of very careful design and lots of compiler specific magic, or suffering from massive duplication of object code in the various dlls, and as a result an application that is extremely version sensitive. Any small change to a class definition will force a rebuild of all exe's and dll's, removing at least one of the major benefits of a dll approach to app development.

Either stick to a straight C interface between app and dll's, suffer hell, or just compile the entire c++ app as one exe.

It's a bold claim that a C++ program "should be equipped with a malloc replacement (like tcmalloc) for performance problems...."

"[In] 6 out of 8 popular benchmarks ... [real-sized applications] replacing back the custom allocator, in which people had invested significant amounts of time and money, ... with the system-provided dumb allocator [yielded] better performance. ... The simplest custom allocators, tuned for very special situations, are the only ones that can provide gains." --Andrei Alexandrescu

Most system allocators are about as good as a general purpose allocator can be. You can do better only if you have a very specific allocation pattern.

Typically, such special patterns apply only to a portion of the program, in which case, it's better to apply the custom allocator to the specific portion that can benefit than it is to globally replace the allocator.

C++ provides a few ways to selectively replace the allocator. For example, you can provide an allocator to an STL container or you can override new and delete on a class by class basis. Both of these give you much better control than any hack which globally replaces the allocator.

Note also that replacing malloc and free will not necessarily change the allocator used by operators new and delete. While the global new operator is typically implemented using malloc, there is no requirement that it do so. So replacing malloc may not even affect most of the allocations.

If you're using C, chances are you can wrap or replace key malloc and free calls with your custom allocator just where it matters and leave the rest of the program to use the default allocator. (If that's not the case, you might want to consider some refactoring.)

System allocators have decades of development behind them. They are stable and well-tested. They perform extremely well for general cases (in terms of raw speed, thread contention, and fragmentation). They have debugging versions for leak detection and support for tracking tools. Some even improve the security of your application by providing defenses against heap buffer overrun vulnerabilities. Chances are, the libraries you want to use have been tested only with the system allocator.

Most of the techniques to replace the system allocator forfeit these benefits. In some cases, they can even increase memory demand (because they can't be shared with the DLL runtime possibly used by other processes). They also tend to be extremely fragile in the face of changes in the compiler version, runtime version, and even OS version. Using a tweaked version of the runtime prevents your users from getting benefits of runtime updates from the OS vendor. Why give all that up when you can retain those benefits by applying a custom allocator just to the exceptional part of the program that can benefit from it?

Where does your premise "A C++ program that uses several DLLs and QT should be equipped with a malloc replacement" come from?

On Windows, if the all the dlls use the shared MSVCRT, then there is no need to replace malloc. By default, Qt builds against the shared MSVCRT dll.

One will run into problems if they:

1) mix dlls that use static linking vs using the shared VCRT

2) AND also free memory that was not allocated where it came from (ie, free memory in a statically linked dll that was allocated by the shared VCRT or vice versa).

Note that adding your own ref counted wrapper around a resource can help mitigate that problems associated with resources that need to be deallocated in particular ways (ie, a wrapper that disposes of one type of resource via a call back to the originating dll, a different wrapper for a resource that originates from another dll, etc).