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<div class="moz-cite-prefix">On 9/12/19 5:06 AM, David Zarzycki via llvm-dev wrote:<br>
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<blockquote type="cite" cite="mid:FEAA3553-9828-4A7C-90D4-F5A3EABB1777@znu.io">I think adding a builtin to force CMOV or similar instructions on other architectures is long overdue. It’s generally useful, even if one isn’t mitigating speculative execution.</blockquote>
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<p>I believe that you can currently get this effect using __builtin_unpredictable in Clang. __builtin_unpredictable wasn't added for this purpose, and it's a hint not a forced behavior, but I believe that it causes the backend to prefer cmov to branches during
lowering.</p>
<p> -Hal<br>
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<blockquote type="cite" cite="mid:FEAA3553-9828-4A7C-90D4-F5A3EABB1777@znu.io">
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<div class="">On Sep 12, 2019, at 12:30 PM, Daan Sprenkels via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="" moz-do-not-send="true">llvm-dev@lists.llvm.org</a>> wrote:</div>
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<div class=""><span style="caret-color: rgb(0, 0, 0);
font-family: Menlo-Regular; font-size: 11px; font-style:
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0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
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!important;" class="">PS.
Perhaps, would there be interest to add such a feature to LLVM?</span><br style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
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<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
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!important;" class="">I
found this repository on GitHub[2], adding a `__builtin_ct_choose`</span><br style="caret-color: rgb(0, 0, 0); font-family:
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<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">intrinsic
to clang. But as far as I know, this has never been upstreamed.</span><br style="caret-color: rgb(0, 0, 0); font-family:
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<br style="caret-color: rgb(0, 0, 0); font-family:
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<span style="caret-color: rgb(0, 0, 0); font-family:
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0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">[1]:
Chandler Carruth described this trick at CppCon15:</span><br style="caret-color:
rgb(0, 0, 0); font-family: Menlo-Regular; font-size:
11px; font-style: normal; font-variant-caps: normal;
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<span style="caret-color: rgb(0, 0, 0); font-family:
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0px; text-transform: none; white-space: normal;
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!important;" class=""><</span><a href="https://youtu.be/nXaxk27zwlk?t=2472" style="font-family: Menlo-Regular; font-size: 11px;
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font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px;
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word-spacing: 0px; -webkit-text-size-adjust: auto;
-webkit-text-stroke-width: 0px;" class="" moz-do-not-send="true">https://youtu.be/nXaxk27zwlk?t=2472</a><span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
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0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">>.
See it in practice:</span><br style="caret-color: rgb(0, 0, 0); font-family:
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<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
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0px; text-transform: none; white-space: normal;
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text-decoration: none; float: none; display: inline
!important;" class=""><</span><a href="https://godbolt.org/z/UMPeku" style="font-family:
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start; text-indent: 0px; text-transform: none;
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-webkit-text-size-adjust: auto;
-webkit-text-stroke-width: 0px;" class="" moz-do-not-send="true">https://godbolt.org/z/UMPeku</a><span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
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0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">></span><br style="caret-color:
rgb(0, 0, 0); font-family: Menlo-Regular; font-size:
11px; font-style: normal; font-variant-caps: normal;
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start; text-indent: 0px; text-transform: none;
white-space: normal; word-spacing: 0px;
-webkit-text-stroke-width: 0px; text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">[2]:
<</span><a href="https://github.com/lmrs2/ct_choose" style="font-family: Menlo-Regular; font-size: 11px;
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font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px;
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word-spacing: 0px; -webkit-text-size-adjust: auto;
-webkit-text-stroke-width: 0px;" class="" moz-do-not-send="true">https://github.com/lmrs2/ct_choose</a><span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">>,</span><br style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
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letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
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text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class=""><</span><a href="https://github.com/lmrs2/llvm/commit/8f9a4d952100ae03d06f10aee237bf8b3331da89" style="font-family: Menlo-Regular; font-size: 11px;
font-style: normal; font-variant-caps: normal;
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auto; text-align: start; text-indent: 0px;
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word-spacing: 0px; -webkit-text-size-adjust: auto;
-webkit-text-stroke-width: 0px;" class="" moz-do-not-send="true">https://github.com/lmrs2/llvm/commit/8f9a4d952100ae03d06f10aee237bf8b3331da89</a><span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">>.</span><br style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
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text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
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!important;" class="">Later
published at S&P18.</span></div>
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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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