Re: [rfc 37/45] x86_64: Support for fast per cpu operations

H. Peter Anvin
2007-11-20T02:18:53Z

Christoph Lameter wrote:
> On Mon, 19 Nov 2007, H. Peter Anvin wrote:
>
>> There was, at some point, discussion about using the gcc TLS mechanism, which
>> should permit even better code to be generated. Unfortunately, it would
>
> How would that be possible? Oh. You mean the discussion where I mentioned
> using the thread attribute?
>
>> require gcc to be able to reference %gs instead of %fs (and vice versa for
>> i386), which I don't think is available in anything except maybe the most
>> cutting-edge version of gcc.
>
> Right. That is why we do it in ASM here.
>
>> However, if we're doing a masssive revampt it would be good to get an idea of
>> how to migrate to that model eventually, or why it doesn't make sense at all.
>
> If you can tell me what the difference would be then we can discuss it.
> AFAICT there is no difference. Both use a segment register.
>

As far as I can tell from a *very* brief look at your code (which means
I might have misread it), these are the differences:

- gcc uses %fs:0 to contain a pointer to itself.
- gcc uses absolute offsets from the thread pointer, rather than adding
%rip. The %rip-based form is actually more efficient, but it does
affect the usable range off the base pointer.

-hpa