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<p class="MsoNormal"><span style="font-family:"Calisto MT",serif;color:#7030A0">See for example, the excellent discussion by Ian in his blog a while back:
<a href="http://www.airs.com/blog/archives/120">http://www.airs.com/blog/archives/120</a><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calisto MT",serif;color:#7030A0">-- Gaby<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ub-bounces@open-std.org [mailto:ub-bounces@open-std.org]
<b>On Behalf Of </b>Hubert Tong<br>
<b>Sent:</b> Tuesday, November 18, 2014 5:37 AM<br>
<b>To:</b> WG21 UB study group<br>
<b>Subject:</b> Re: [ub] Signed shifting<o:p></o:p></span></p>
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<p class="MsoNormal">UB for signed integer overflow is often relied on by optimizers to either increment 32-bit ints in 64-bit registers on RISC systems without enforcing exact wraparound or to perform loop optimizations:<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">for (int x = y; x < y + 32; ++x) f[x] += x;<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">to (possibly)<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">f[y] += y;<br>
f[y + 1] += y + 1;<br>
...<br>
f[y + 31] += y + 31;<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">where the original loop might not have performed a single iteration if wraparound is to reliably occur.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">-- HT<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">On Tue, Nov 18, 2014 at 4:04 AM, Jens Maurer <<a href="mailto:Jens.Maurer@gmx.net" target="_blank">Jens.Maurer@gmx.net</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On 11/18/2014 12:14 AM, Howard Hinnant wrote:<br>
> int<br>
> sign(int x)<br>
> {<br>
> constexpr int n = std::numeric_limits<int>::digits;<br>
> return (x >> n) | (static_cast<unsigned>(-x) >> n);<br>
> }<br>
<br>
That "-x" in there seems to cause undefined behavior on<br>
two's complement machines if "x" is std::numeric_limits<int>::min(),<br>
according to 5p4, it seems:<br>
<br>
"If during the evaluation of an expression, the result is [...] not<br>
in the range of representable values for its type, the behavior is<br>
undefined."<br>
<br>
<br>
Can we make the world a simpler and better place by prescribing two's<br>
complement for all signed integer operations in C++, thereby enshrining<br>
what everybody expects, anyway?<br>
<span style="color:#888888"><br>
<span class="hoenzb">Jens</span></span><o:p></o:p></p>
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<p class="MsoNormal"><br>
_______________________________________________<br>
ub mailing list<br>
<a href="mailto:ub@isocpp.open-std.org">ub@isocpp.open-std.org</a><br>
<a href="http://www.open-std.org/mailman/listinfo/ub" target="_blank">http://www.open-std.org/mailman/listinfo/ub</a><o:p></o:p></p>
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