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Re: [Qemu-devel] env in TLS

From: Fabrice Bellard
Subject: Re: [Qemu-devel] env in TLS
Date: Mon, 11 Oct 2004 11:41:37 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.2) Gecko/20040803

Using TLS seems a good idea. I hesitated to implement it because I was not sure that segment overrides were as fast as an indexed memory access on all x86 processors. I think that imposing a gcc version >= 3.2 is no longer an issue. We must be sure that the other platforms support the __thread keyword though.


Piotr Krysik wrote:

Fabrice, how about moving env variable to TLS?

This could improve performance (based on http://lists.gnu.org/archive/html/qemu-devel/2004-09/msg00294.html

about 10% could be gained -- need to verify). Also additional register would be available for op.c (anyone to implement register renaming?).

The main problem with this approach is portability. AFAIK GCC <= 3.2 doesn't recognize __thread keyword. For x86 we could fall-back to the LDT method from qemu-fast. How about other platforms? In the worst case we could use global variable and forget about multi-processor qemu-softmmu and multi-thread in qemu-user for some platforms in the future.



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