Messages in this thread | | | Date | Mon, 19 Nov 2007 18:15:59 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [rfc 37/45] x86_64: Support for fast per cpu operations |
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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.
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