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Re: [Xen-devel] [PATCH] x86/time: fix scale_delta() inline assembly

  • To: Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
  • From: Keir Fraser <keir.xen@xxxxxxxxx>
  • Date: Mon, 26 Nov 2012 19:22:46 +0000
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxx>
  • Delivery-date: Mon, 26 Nov 2012 19:23:25 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>
  • Thread-index: Ac3MC2oT+Xrwgn2fxUaVAJzi0dVEEw==
  • Thread-topic: [Xen-devel] [PATCH] x86/time: fix scale_delta() inline assembly

On 26/11/2012 17:12, "Ian Campbell" <Ian.Campbell@xxxxxxxxxx> wrote:

>> That's how things are with multi-insn asm-s (and even with single-
>> insn ones when an output different from an input get tied together
>> through using the same register). I continue to think that using the
>> numbered forms here is better.
> I think the important difference here is that the mul generates its
> result specifically in rdx:rax rather than user specified registers.
> Since the output is not specified via the constraints IMHO using the
> constraints when subsequently consuming those outputs is confusing.
> I'd agree with you if mul was something like "mul %2,%0,%1" (where %2*%
> rax =>%0:%1) then "shrd $32,%0,%1" would be perfectly fine.
> Anyway I guess we'll have to agree to disagree. In any case your opinion
> trumps mine on this code ;-)

I didn't find Jan's asm particularly confusing, any more than it kind of
fundamentally is. It's fine by me for it to go in as it is.

 -- Keir

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