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Re: [Qemu-devel] TCG sar UB (fwd)

From: Richard Henderson
Subject: Re: [Qemu-devel] TCG sar UB (fwd)
Date: Wed, 07 Sep 2011 10:49:38 +0530
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110817 Fedora/3.1.12-1.fc14 Thunderbird/3.1.12

On 09/06/2011 08:50 PM, malc wrote:
> Correct me if i'm wrong, previously the code worked like this:
> mov tmp, 0
> sub tmp, 1
> sar r, r, tmp
> Still UB as far as TCG is concerned but since no immediates are involved
> things worked, now, with constant folding, we are asked to sar by -1 
> directly.

You are exactly correct.

That's why I thought my patch to re-arrange the order of operations
and only perform the subtraction inside the %cl != 0 test was a good
idea.  No point in performing the shift if we're not going to use the

> I did that when first hit this problem, but decided not to push it.

I think pushing it is a good idea.  Just because the result is not
defined, according to tcg/README, is no reason to SIGILL.


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