On 08/04/2011 12:19 PM, Richard Guenther wrote: > On Thu, Aug 4, 2011 at 1:10 PM, Andrew Haley <a...@redhat.com> wrote: >> 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. > > Well, the uses won't break with the idea - they would simply work > like if they were not using local register variables.
I don't understand this remark. Surely if they work like they were not using local register variables, you'll get dreadful register allocation. But this is a big reason to use gcc. Efficient code really does matter to people writing this kind of thing. Andrew.
