Date: Sat, 03 Apr 2004 14:17:23 -0800 From: Julian Elischer <julian@elischer.org> To: Daniel Eischen <eischen@vigrid.com> Cc: threads@freebsd.org Subject: Re: PERFORCE change 50188 for review Message-ID: <406F37F3.3050501@elischer.org> In-Reply-To: <Pine.GSO.4.10.10404031701480.18223-100000@pcnet5.pcnet.com> References: <Pine.GSO.4.10.10404031701480.18223-100000@pcnet5.pcnet.com>
next in thread | previous in thread | raw e-mail | index | archive | help
Daniel Eischen wrote: > On Sat, 3 Apr 2004, Doug Rabson wrote: > > >>On Saturday 03 April 2004 19:22, Daniel Eischen wrote: >> >>>On Sat, 3 Apr 2004, Doug Rabson wrote: >>> >>>>I was just wandering around the internet looking at the scenery and >>>>I ended up here: >>>>http://gcc.gnu.org/onlinedocs/gcc/i386-and-x86-64-Options.html. >>> >>>Neat. >>> >>> >>>>This document describes a new options (which is not supported by >>>>the compiler in current right now), -mno-tls-direct-seg-refs. This >>>>looks like it will do everything we need for both i386 and amd64, >>>>i.e. instead of code like: >>>> >>>> movl %gs:x@ntpoff, %eax >>>> >>>>it should generate: >>>> >>>> movl %gs:0, %eax >>>> movl x@ntpoff(%eax), %eax >>> >>>That's what I thought the SUN ABI was supposed to do, no? >>>Perhaps I should go back and read the TLS spec... >> >>The main difference, (for me anyway) is that the calling convention for >>tls_get_addr in the sun abi is a standard stack-based convention. This >>leads to bulky code sequences which are hard for the linker to >>transform when it realises that it can change a reference from e.g. >>global dynamic to local exec. > > > Oh, I was really only thinking that the tls_get_addr function > and everything else would be pretty much the same as the > GNU convention, except that there would be one extra > instruction for __thread references (like you show > above). I think this is what we were going on from the > start. > > >>>>Although I'm still not quite convinced that we can't do the first >>>>version with essentially zero cost for i386 at least. >>> >>>I think it might get messy trying to manage LDTs. Extra >>>locking will be needed when you need to borrow them from >>>other threads, and you need to make sure those other threads >>>aren't running and aren't scope system. You might as well >>>make a system call to continue the thread and let the >>>kernel do all the work. >> >>Probably. If we can arrange to reduce the syscall cost somewhat (e.g. >>with sysenter/sysexit instead of int $80), perhaps this still isn't too >>much of a problem. I think that most programs should do far fewer >>context switches than most other work. > > > But everything else being equal, it's so much easier > for the one extra instruction in the TLS reference. > Talking with Peter, it may be feasible to use the kernel to set %fs:0 to point to per-thread data as there is a very fast way to make syscalls (12 clocks vs 300 clocks, or so he says) so that leaves us only with problems on the x86. The option above is what I thought we were going to do all along for x86 -- +------------------------------------+ ______ _ __ | __--_|\ Julian Elischer | \ U \/ / hard at work in | / \ julian@elischer.org +------>x USA \ a very strange | ( OZ ) \___ ___ | country ! +- X_.---._/ presently in San Francisco \_/ \\ v
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?406F37F3.3050501>