Re: [eigen] Help on solving a race condition

[Rhys Ulerich <rhys.ulerich@xxxxxxxxx>]

> Then I guess the only safe and clean solution is to request users to
> call a Eigen::init_parallel() or something.

That's safe but I'd not call it clean.  It's error prone.  And verbose.

> The problem with this approach, is that the cache-sizes are recomputed
>for every thread.

That doesn't seem terrible to me if I don't have to explicitly call
Eigen::init_parallel() as a tradeoff.  Is this a slow process?

>> This solution is openmp specific; what if someone wants to use pthreads or
>> some other threading system ?

Why not #ifdef on compiler versions to dig out the vendor-specific
thread-local storage extension when OpenMP isn't active:

#if defined(_OPENMP)
# define TLS
#elseif defined(__GNUC__) && __GNUC__ > 3
# define TLS __thread
#elseif....
# define TLS ...
#else
# define TLS
#endif

static TLS std::ptrdiff_t m_l1CacheSize = 0;
static TLS std::ptrdiff_t m_l2CacheSize = 0;
#pragma omp threadprivate(m_l1CacheSize, m_l2CacheSize)

#ifdef TLS
#undef TLS
#endif

Notice that no vendor-specific threadlocal keyword is used when OMP is
active because OMP is the better way to go.

It's a bit ugly from the implementer's side, but it the number of
vendor-specific thread-local keywords should be small.  From the end
user's perspective, however, it is seamless.

- Rhys