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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">With LLVM v5.0, I found failures in some of the ‘</span><span style="font-size:12.0pt;font-family:"Courier New"">gcc.c-torture/execute</span><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">’
tests due to a change in the optimisation where undefined behaviour is involved. The tests that fail are the ‘</span><span style="font-size:12.0pt;font-family:"Courier New"">20040409-[123].c</span><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">’
tests.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">The underlying failure is due to the optimisation of the following:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Courier New""> int test2(int x) { return x + INT_MIN; }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">from using an ADD instruction to using an OR instruction. The optimisation is entirely valid and will produce the correct result for all correct values of ‘</span><span style="font-size:12.0pt;font-family:"Courier New"">x</span><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">’,
but it introduces a change for architectures that have support for detecting integer overflow/underflow (signalling or quiet).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">For many systems the execution cost of ADD versus OR is the same, so there is no real reason to choose one versus the other, and UB is UB either way. However, it does impact
detection of out-of-range integer arithmetic for systems that have support for this.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">Is there a new trait in the TTI, or a call-back elsewhere that allows the target to choose whether it wants this optimisation to use an ADD or an OR (possibly in the cost-models)?
I would generally prefer to use ADD versus OR if the cost of execution is the same.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"> MartinO<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Book Antiqua",serif"><o:p> </o:p></span></p>
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