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Re: [Qemu-devel] [RFC PATCH 0/5] Add configure flag to disable TCG

From: Blue Swirl
Subject: Re: [Qemu-devel] [RFC PATCH 0/5] Add configure flag to disable TCG
Date: Sat, 3 Sep 2011 11:58:17 +0000

On Fri, Sep 2, 2011 at 9:47 PM, Anthony Liguori <address@hidden> wrote:
> Hi,
> There have been a few attempts in the past to allow TCG to be disabled
> at build time.  Recently, Alex made the suggestion that we could do it by 
> using
> the same trick that we used to introduce kvm support.  That involves 
> introducing
> a tcg_enabled() macro that will be (0) if TCG is disabled in the build.
> GCC is smart enough to do dead code elimination if it sees an if (0) and the
> result is that if you can do:

Is this also true for gcc optimization -O0? Didn't we have breakages
because of similar issues recently?

> if (tcg_enabled()) {
>  foo();
> }
> and it's more or less equivalent to:
> #ifdef CONFIG_TCG
>  foo();
> #endif
> Without the ugliness that comes from using the preprocessor.  I think this 
> ended
> up being pretty straight forward.  exec.c could use a fair bit of cleanup but
> other than that, this pretty much eliminates all of the TCG code from the 
> build.
> This absolutely is going to break non-x86 KVM builds if they use the
> --disable-tcg flag as I haven't tested those yet.  The normal TCG build
> shouldn't be affected at all though.
> In principle, the code assumes that you need KVM if you don't have TCG.  Of
> course, some extra logic could be added to allow for Xen if TCG isn't present.

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