On 08/04/2011 10:52 AM, Richard Guenther wrote: > On Thu, Aug 4, 2011 at 11:50 AM, Andrew Haley <a...@redhat.com> wrote: >> On 08/04/2011 01:19 AM, Hans-Peter Nilsson wrote: >> >>> To make sure, it'd be nice if someone could perhaps grep an >>> entire GNU/Linux-or-other distribution including the kernel for >>> uses of asm-declared *local* registers that don't directly feed >>> into asms and not being the stack-pointer? Or can we get away >>> with just saying that local asm registers haven't had any other >>> documented meaning for the last seven years? >> >> It's the sort of thing that gets done in threaded interpreters, >> where you really need to keep a few pointers in registers and >> the interpreter itself is a very long function. gcc has always >> done a dreadful job of register allocation in such cases. > > Sure, but what I have seen people use global register variables > for this (which means they get taken away from the register allocator).
Not always though, and the x86 has so few registers that using a global register variable is very problematic. I suppose you could compile the threaded interpreter in a file of its own, but I'm not sure that has quite the same semantics as local register variables. The problem is that people who care about this stuff very much don't always read gcc@gcc.gnu.org so won't be heard. But in their own world (LISP, Forth) nice features like register variables and labels as values have led to gcc being the preferred compiler for this kind of work. Andrew.
