Re: libgcc: strange optimization

[Richard Guenther <richard.guenther@gmail.com>]

On Thu, Aug 4, 2011 at 1:10 PM, Andrew Haley <aph@redhat.com> wrote:
> On 08/04/2011 10:52 AM, Richard Guenther wrote:
>> On Thu, Aug 4, 2011 at 11:50 AM, Andrew Haley <aph@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.

Richard.

> Andrew.
>