PERFORCE change 50188 for review

[Daniel Eischen]

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 at ntpoff, %eax
> > >
> > > it should generate:
> > >
> > > 	movl	%gs:0, %eax
> > > 	movl	x at 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.

-- 
Dan Eischen