On Fri, 2007-11-02 at 19:56 +0100, Olivier Galibert wrote: > On Sat, Nov 03, 2007 at 03:31:14AM +1100, skaller wrote: > > On Fri, 2007-11-02 at 07:39 -0700, Ian Lance Taylor wrote: > > > I think you need to look at the TLS access code before deciding that > > > it has bad performance. > > > > You already said it costs a register? That's a REALLY high cost > > to pay to support badly designed software. > > Not if you have a lot of registers (anything modern but i386) or if > the register can not really be used for anything else (%fs on i386 for > instance).
This is not true. If you use a register for any purpose like this, it can't be used for anything else and that has a cost. On x86_64 which I use, every register is valuable. Don't you dare take one away, it would have a serious performance impact AND it would stop ME using that register for something which my application might consider much more important, for example as a pointer to the minor heap in a copying collector (Ocaml does this, it is the reason it has very high performance.. I would do this in Felix too if I could figure out how to reload the variable in a callback invoked by foreign code). This applies to %fs on i386 too: if the compiler uses that register, other uses are denied, and the compiler can't tell which is more important. -- John Skaller <skaller at users dot sf dot net> Felix, successor to C++: http://felix.sf.net
