On Tue, Jan 15, 2013 at 03:54:03PM -0500, Trent Nelson wrote: > Howdy, > > I have an unusual requirement: I need to get the current thread ID > in as few instructions as possible. On Windows, I managed to come > up with this glorious hack: > > #ifdef WITH_INTRINSICS > # ifdef MS_WINDOWS > # include <intrin.h> > # if defined(MS_WIN64) > # pragma intrinsic(__readgsdword) > # define _Py_get_current_process_id() (__readgsdword(0x40)) > # define _Py_get_current_thread_id() (__readgsdword(0x48)) > # elif defined(MS_WIN32) > # pragma intrinsic(__readfsdword) > # define _Py_get_current_process_id() (__readfsdword(0x20)) > # define _Py_get_current_thread_id() (__readfsdword(0x24)) > > That exploits the fact that Windows uses the FS/GS registers to > store thread/process metadata. Could I use a similar approach on > FreeBSD to get the thread ID without the need for syscalls? The layout of the per-thread structure used by libthr is private and is not guaranteed to be stable even on the stable branches.
Yes, you could obtain the tid this way, but note explicitely that using it makes your application not binary compatible with any version of the FreeBSD except the one you compiled on. You could read the _thread_off_tid integer variable and use the value as offset from the %fs base to the long containing the unique thread id. But don't use this in anything except the private code. > > (I technically don't need the thread ID, I just need to get some > form of unique identifier for the current thread such that I can > compare it to a known global value that's been set to the "main > thread", in order to determine if I'm currently that thread or not. > As long as it's unique for each thread, and static for the lifetime > of the thread, that's fine.) > > The "am I the main thread?" comparison is made every ~50-100 opcodes, > which is why it needs to have the lowest overhead possible. On newer CPUs in amd64 mode, there is getfsbase instruction which reads the %fs register base. System guarantees that %fs base is unique among live threads.
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