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Re: [Commits] Rev 3353: Performed re-factoring and re-structuring of the code for mwl#248: in file:///home/igor/maria/maria-5.5-mwl248-changes/


Kristian Nielsen <knielsen@xxxxxxxxxxxxxxx> writes:

> Or compilers
> could optimise based on knowledge that no external code can modify a private
> member (so it does not need to be spilled over a call to pthread_mutex_lock()
> or similar). But none of this would give a problem with bzero-initialisation
> after alloc.

Hm, thinking more - I am not sure I was correct with this statement.

Consider the following code:

    class GenId {
        int current;
        int next() { return current++; }

    GenId *p= malloc(sizeof(GenId));
    memset(p, 0, sizeof(GenId));

What is to prevent the compiler from moving the code around, so that the
access to GenId::current inside p->next() happens before the memset()?

After all, GenId::current is private, there is no code outside of class GenId
that is allowed to change it. So this would seem a "correct" optimisation, and
the code will run with an uninitialised value for current.

This is precisely the kind of problem we want desparately to avoid: A new
compiler on a new platform comes out, and it does this optimisation, and some
poor seniour developer has to spend days to find a way to reproduce and figure
out why some strange case crashes.

So I can only agree more with Davi here: let's avoid this. Make all members
public. Or use placement new instead of bzero() to initialise.

 - Kristian.