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Re: [Qemu-devel] [PATCH 05/10] tcg: add jr opcode

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [PATCH 05/10] tcg: add jr opcode
Date: Sun, 16 Apr 2017 14:28:43 -0400
User-agent: Mutt/1.5.24 (2015-08-30)

On Sat, Apr 15, 2017 at 04:40:35 -0700, Richard Henderson wrote:
> On 04/11/2017 06:17 PM, Emilio G. Cota wrote:
> >This will be used by TCG targets to implement a fast path
> >for indirect branches.
> >
> >I only have implemented and tested this on an i386 host, so
> >make this opcode optional and mark it as not implemented by
> >other TCG backends.
> I don't think this is quite the right abstraction.  In particular, if we can
> always return a valid address from the helper, we can eliminate a
> conditional branch.
> I think this should work as follows:

Yes that's much better. In fact in the cover letter I forgot to
mention that the code by the paper authors does something similar
to avoid the branch.

I went with the design with a branch because (1) I wasn't sure that
exporting tb_ret_addr would get your approval and (2) my knowledge
of TCG backend code is shamefully poor.

Will work on a v2. Thanks for the feedback!


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