Re: [Qemu-devel] [PATCH] qemu-thread: add TLS wrappers

[Paolo Bonzini]

On 08/17/2011 10:37 AM, Blue Swirl wrote:
> >  I would agree (just did it for fun and to see how it looks like), but
> >  perhaps we can do something similar to what Windows does for OpenBSD too.
> >
> >  Though I'm quite disappointed and I must say OpenBSD should enter the 21st
> >  century.
>
> I can't justify their design decisions, but isn't __thread a
> non-standard extension by GCC anyway whereas POSIX threads are the
> standard? Are other compilers supporting it?

Visual C++ supports __declspec(thread).

pthread TLS is orders of magnitudes slower than __thread.  __thread in 
an executable is just as fast as a non-TLS variable access, 2 or 3 
machine language instructions at most.

Some pthread_getspecific implementation do a _linear walk_ of the keys. 
 In that case it may work to have something like Windows plus a single 
pthread_getspecific key.  On the other hand, Windows provides very good 
support for that thanks to its ordered subsections (.tls$something).  It 
only needs a bit of preprocessor magic.  Thinks such as linker scripts 
would be way out of scope.

Paolo