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Re: [Xen-devel] [PATCH] x86-64: suppress some unneeded side effects of -

To: Jan Beulich <jbeulich@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86-64: suppress some unneeded side effects of -fPIC
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 13 Sep 2006 13:46:27 +0100
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Thread-topic: [Xen-devel] [PATCH] x86-64: suppress some unneeded side effects of -fPIC
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On 12/9/06 16:43, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> Full PIC mode isn't needed for Xen, namely is there no need to access data
> objects via the GOT or call functions through the PLT. Adding
> -fvisibility=hidden (if the compiler supports it) helps, although it doesn't
> do
> as good a job as would be desired (I'm in talks with our compiler guys to
> understand why).
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

What overheads does this remove? Does it actually shorten code sequences?

It seems that some gcc v3 compilers have -fvisibility=hidden too (e.g. My
gcc-3.4.4 does). I thus checked in a patch that checks for availability of
this feature in a 'test-gcc-flag' style of way (as defined in Config.mk),
rather than doing a gcc version test.

 -- Keir

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