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From: Richard Henderson <rth@twiddle.net>
To: malc <av1474@comtv.ru>
Cc: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] TCG sar UB (fwd)
Date: Wed, 07 Sep 2011 10:49:38 +0530	[thread overview]
Message-ID: <4E66FEEA.2060703@twiddle.net> (raw)
In-Reply-To: <alpine.LNX.2.00.1109061917370.2314@linmac>

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
result.

> 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.


r~

  reply	other threads:[~2011-09-07  5:19 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <alpine.LNX.2.00.1109031417430.1511@linmac>
2011-09-04  2:27 ` [Qemu-devel] TCG sar UB (fwd) Richard Henderson
2011-09-04  2:33   ` malc
2011-09-06 14:48     ` Richard Henderson
2011-09-06 15:20       ` malc
2011-09-07  5:19         ` Richard Henderson [this message]
2011-09-07 16:04     ` malc
2011-09-08  8:23       ` [Qemu-devel] [PATCH] target-i386: Compute all flag data inside %cl != 0 test Richard Henderson
2011-09-08 14:12         ` malc

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