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From: "joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/54027] possible mis-optimization of signed left shift in c89 mode
Date: Thu, 19 Jul 2012 15:08:00 -0000	[thread overview]
Message-ID: <bug-54027-4-m3SwPs4dah@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-54027-4@http.gcc.gnu.org/bugzilla/>

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54027

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-07-19 15:07:54 UTC ---
On Thu, 19 Jul 2012, rguenth at gcc dot gnu.org wrote:

> Isn't it invoking undefined behavior by means of a signed integer overflow?
> (if shifts are not defined in terms of multiplies we may not internally
> fold x << 1 to x * 2).

Shifts in GCC are supposed to be defined as long as the shift amount is in 
range, independent of the LHS, so it should not be folding like that.  
(Although we document in implement-c.texi that this is subject to change 
for signed left shift, I don't think changing it would be a particularly 
good idea.)


  parent reply	other threads:[~2012-07-19 15:08 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-07-19  4:24 [Bug c/54027] New: " regehr at cs dot utah.edu
2012-07-19 10:07 ` [Bug c/54027] " rguenth at gcc dot gnu.org
2012-07-19 15:08 ` joseph at codesourcery dot com [this message]
2012-07-20  9:38 ` [Bug tree-optimization/54027] [4.8 Regression] " rguenth at gcc dot gnu.org
2012-08-09 13:31 ` markus at trippelsdorf dot de
2012-08-10  8:34 ` rguenth at gcc dot gnu.org
2012-08-10  8:39 ` rguenth at gcc dot gnu.org

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