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Date: Wed, 24 Mar 2010 20:00:28 +0200
Message-ID: <f43fc5581003241100q5a3cab0dy7b9db163b1aef514@mail.gmail.com>
Subject: Re: [Qemu-devel] Re: Compile files only once: some planning
From: Blue Swirl <blauwirbel@gmail.com>
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To: Richard Henderson <rth@twiddle.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel <qemu-devel@nongnu.org>

On 3/24/10, Richard Henderson <rth@twiddle.net> wrote:
> On 03/24/2010 02:47 AM, Paolo Bonzini wrote:
>
> > 1) make CPUState define only common fields. Include CPUState at the
> > beginning of each per-target CPUXYZState.
> >
>
>  Irritatingly, the common fields contain quite big TLBs.  And the
>  offsets from the start of env affect the compactness of the code
>  generated from TCG.  We really really want the general registers
>  to come first to make sure that those offsets fit the host's
>  reg+offset addressing mode.

One trick is to define a fixed offset (about half CPUState size) and
make env point to CPUState + offset. Then the negative part of the
offset space can be used efficiently. But this just doubles the space
that can be accessed fast, so it's not a big win.