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<div class="moz-cite-prefix">On 10/4/19 5:46 AM, Joan Lluch via llvm-dev wrote:<br>
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<blockquote type="cite" cite="mid:01C51DE9-24BB-43EF-BADC-C2384AA4DBBD@icloud.com">
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Hi all,
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<div class="">As a continuation of this thread, I was about to fill a bug report requesting the modification of <font class="" size="3"><span style="white-space: pre-wrap;" class="">DAGTypeLegalizer::ExpandIntRes_SIGN_EXTEND, in order to avoid the creation
of Shifts for targets with no native support. For example by generating a ‘select' equivalent to a<0 ? -1 : 0
</span></font>instead of an arithmetic shift right. For targets with no multiple shifts or word extension instructions, the select is much cheaper.</div>
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<div class="">However, I found that the early InstCombine pass spoils such optimisation by creating shifts on their own as a transform of (supposedly) equivalent code. </div>
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...
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<div class=""><span style="background-color: rgb(255, 255,
255);" class="">I strongly suggest the above gets fixed. HOWEVER, even after the DAG combiner is fixed, the issues will remain due to InstCombine doing essentially the same thing </span><span style="background-color: rgb(255, 255,
255);" class="">much
earlier. Consider code like this:</span></div>
<div class=""><span style="background-color: rgb(255, 255,
255);" class=""><br class="">
</span></div>
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<div style="margin: 0px; font-size: 11px; line-height:
normal; font-family: Monaco; background-color: rgb(255,
255, 255);" class="">
<span style="color: rgb(186, 45,
162);" class="">int</span> test0( <span style="color:
rgb(186, 45, 162);" class="">int</span> a )</div>
<div style="margin: 0px; font-size: 11px; line-height:
normal; font-family: Monaco; background-color: rgb(255,
255, 255);" class="">
{</div>
<div style="margin: 0px; font-size: 11px; line-height:
normal; font-family: Monaco; background-color: rgb(255,
255, 255);" class="">
<span style="color: rgb(186, 45,
162);" class="">return</span> a<<span style="color:
rgb(39, 42, 216);" class="">0</span> ? -<span style="color: rgb(39, 42, 216);" class="">1</span> : <span style="color: rgb(39, 42, 216);" class="">0</span>;</div>
<div style="margin: 0px; font-size: 11px; line-height:
normal; font-family: Monaco; background-color: rgb(255,
255, 255);" class="">
}</div>
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font-family: Monaco; background-color: rgb(255, 255, 255);" class="">
<br class="">
</div>
<div style="margin: 0px; font-size: 11px; line-height: normal;
font-family: Monaco; background-color: rgb(255, 255, 255);" class="">
<div style="margin: 0px; line-height: normal;" class=""><span style="color: rgb(186, 45, 162);" class="">int</span> test1( <span style="color: rgb(186, 45, 162);" class="">int</span> a )</div>
<div style="margin: 0px; line-height: normal;" class="">{</div>
<div style="margin: 0px; line-height: normal;" class=""> <span style="color: rgb(186, 45, 162);" class="">return</span> a<<span style="color: rgb(39, 42, 216);" class="">0</span> ?
<span style="color: rgb(39, 42, 216);" class="">1</span> : <span style="color: rgb(39, 42, 216);" class="">0</span>;</div>
<div style="margin: 0px; line-height: normal;" class="">}</div>
<div style="margin: 0px; line-height: normal;" class=""><br class="">
</div>
<div style="margin: 0px; line-height: normal;" class="">
<div style="margin: 0px; line-height: normal;" class=""><span style="color: rgb(186, 45, 162);" class="">int</span> test2( <span style="color: rgb(186, 45, 162);" class="">int</span> a )</div>
<div style="margin: 0px; line-height: normal;" class="">{</div>
<div style="margin: 0px; line-height: normal;" class=""> <span style="color: rgb(186, 45, 162);" class="">return</span> a<<span style="color: rgb(39, 42, 216);" class="">0</span> ? <font class="" color="#272ad8">2</font> : <span style="color: rgb(39, 42, 216);" class="">0</span>;</div>
<div style="margin: 0px; line-height: normal;" class="">}</div>
<div style="margin: 0px; line-height: normal;" class=""><br class="">
</div>
<div style="margin: 0px; line-height: normal;" class="">
<div style="margin: 0px; line-height: normal;" class=""><span style="color: rgb(186, 45, 162);" class="">int</span> test3( <span style="color: rgb(186, 45, 162);" class="">int</span> a )</div>
<div style="margin: 0px; line-height: normal;" class="">{</div>
<div style="margin: 0px; line-height: normal;" class=""> <span style="color: rgb(186, 45, 162);" class="">return</span> a<<span style="color: rgb(39, 42, 216);" class="">0</span> ?
<font class="" color="#272ad8">3</font> : <span style="color: rgb(39, 42, 216);" class="">0</span>;</div>
<div style="margin: 0px; line-height: normal;" class="">}</div>
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<div class=""><span style="background-color: rgb(255, 255,
255);" class=""><br class="">
</span></div>
<div class=""><span style="background-color: rgb(255, 255,
255);" class="">In all cases, InstCombine converts the above into Shifts, right into the IR, which the backend can’t do anything about.</span></div>
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<p>Why not? It seems like we could pattern match the shifts back into selects/branches?</p>
<p> -Hal<br>
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<pre class="moz-quote-pre" wrap="">_______________________________________________
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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