Re: Optimization of conditional access to globals: thread-unsafe?

[Florian Weimer <fw at deneb dot enyo dot de>]

* Bart Van Assche:

> As known the compiler may not reorder any access to any static
> variable, global variable or dynamically allocated data with a call to
> a function that is not declared inline.

I assume you mean "defined in another translation unit" instead of "not
declared inline".  Still, I don't think this is true as far as static
variables are concerned (both file-scoped and function-scoped ones).

There may only be a few cases where you can prove that a call to an
extern function does not access a file-scoped static variable, so doing
that optimization may not be worthwhile.  But the optimization is not
forbidden per se.

> Variables that are allocated on the stack of a thread can only be
> shared with another thread only by passing a pointer to that variable
> to another thread first. Passing such a pointer inhibits reordering of
> accesses to shared local variables with a call to a function that is
> not declared inline.

And this isn't really specific to threads.

> Maybe it's a good idea to add a chapter to the gcc manual about
> multithreaded programming, such that gcc users who did not follow this
> discussion can look up this kind of information easily ?

If there's a real chance that something like a C or C++ memory model
will be standardized in the forseeable future (three years, perhaps), it
might be unwise to set in stone a potentially conflicting set of rules
for GCC.