<div dir="ltr"><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><br></div><div class="gmail_extra"><br><div class="gmail_quote">2017-11-04 4:29 GMT+08:00 Kaylor, Andrew via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span>:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal"><span style="color:rgb(31,73,125)">Copying the list on a discussion of potentially general interest….<u></u><u></u></span></p>
<p class="MsoNormal"><a name="m_4695229196957410940__MailEndCompose"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></a></p>
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<p class="MsoNormal"><b>From:</b> Kaylor, Andrew <br>
<b>Sent:</b> Friday, November 03, 2017 1:11 PM<br>
<b>To:</b> 'Ding, Wei' <<a href="mailto:Wei.Ding2@amd.com" target="_blank">Wei.Ding2@amd.com</a>>; Sumner, Brian <<a href="mailto:Brian.Sumner@amd.com" target="_blank">Brian.Sumner@amd.com</a>>; Arsenault, Matthew <<a href="mailto:Matthew.Arsenault@amd.com" target="_blank">Matthew.Arsenault@amd.com</a>><br>
<b>Subject:</b> RE: clarification needed for the constrained fp implementation.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">Hi Wei,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">I’ve been meaning to write something up for discussion on the LLVM Dev list about this. I hope you don’t mind if I copy the list now to accomplish that while also answering your questions. Eventually I create
a document describing this in more detail and less formally than the language definition.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">Basically, the “constraints” in the constrained FP intrinisics are constraints on the optimizer. They are a way of telling the optimizer what it can and cannot assume about rounding mode and FP exception behavior.
By default, the optimizer assumes that the rounding mode is round-to-nearest and that FP exceptions are being ignored. If the user code is going to do anything that invalidates these assumptions, then we need a way to make the optimizer stop assuming that.
That’s what the intrinisics do. Because most passes don’t recognize the intrinisics, they can’t do anything with the operations they represent and therefore can’t make any assumption about them.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">The intrinsics are not intended to do anything to change the rounding mode or FP exception handling state. I have an idea in mind for some additional intrinsics that would provide a way to control the FP environment.
There are already some target-specific mechanisms for doing that, but I’d like to have something that’s target independent. I’ll say more about this in a minute.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">I mentioned in my review comments that my work on this has been motivated by the STDC pragmas, and I think if I explain that it might make the semantics of the intrinsics seem a little more natural. The primary
pragma I have in mind here is the “STDC FENV_ACCESS” pragma. I believe this is part of the C99 standard, but compiler support for it is still mostly (if not entirely) missing. For instance, if you try to use this pragma with clang you will get a message
telling you that the pragma isn’t supported and it will have no other effect. We want to change that.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">Basically, what the “STDC FENV_ACCESS” pragma does is provide programmers with a way to tell the compiler that the program might change the FP environment. This pragma represents a setting that has only two
states -- on and off. The default setting of this state is documented as being implementation defined. In clang the default state will be off. The C99 standard states that accessing the FP environment (testing FP status flags, changing FP control modes,
etc.) when FENV_ACCESS is off is undefined behavior. The C99 standard provides library calls to access the environment (fesetround, fegetround, fetestexcept, etc.) but you can only safely use these if you have set FENV_ACCESS to the “on” state. A typical
usage might look like this:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">#include <fenv.h><u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">double someFunc(double A, double B, bool ForceRoundUp) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> #pragma STDC FENV_ACCESS ON<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> double Result;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> if (ForceRoundUp) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> int OldRM = fegetround();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> fesetround(FE_UPWARD);<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> Result = A/B;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> fesetround(OldRM);<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> } else {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> Result = A/B;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"> return Result;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> </span></p></div></div></blockquote><div><br></div><div><font color="#000000"> <div class="gmail_default" style="font-family:arial,helvetica,sans-serif;display:inline">... </div>abridge<div class="gmail_default" style="font-family:arial,helvetica,sans-serif;display:inline"> ...</div></font></div><div><span style="color:rgb(221,75,57)"><br></span></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_4695229196957410940WordSection1"><p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u></span></p>
<p class="MsoNormal"><br></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">What I’m thinking is that we need something like this:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">void llvm.set.roundingmode(i32 mode)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">i32 lllvm.get.roundingmode()<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">These would then get translated during instruction selection to target-specific instructions, which is equivalent to what fesetround() and fegetround() do. But I think it would also be useful to have something
like this:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">void llvm.begin.local.roundingmode(<wbr>i32 mode)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:rgb(31,73,125)">void llvm.end.local.roundingmode()</span></p></div></div></blockquote><div><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">A little bit of curiously. What if user doesn't restore the OldRM like this</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><br></div></div></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><span style="color:rgb(0,0,0)">double someFunc(double A, double B, bool ForceRoundUp) {<br></span><span style="color:rgb(0,0,0)"> #pragma STDC FENV_ACCESS ON<br></span><span style="color:rgb(0,0,0)"> double Result;<br></span><span style="color:rgb(0,0,0)"> if (ForceRoundUp) {<br></span><span style="color:rgb(0,0,0)"> int OldRM = fegetround();<br></span><span style="color:rgb(0,0,0)"> fesetround(FE_UPWARD);<br></span><span style="color:rgb(0,0,0)"> Result = A/B;<br></span><span style="color:rgb(0,0,0)"> <div class="gmail_default" style="font-family:arial,helvetica,sans-serif;display:inline">// </div>fesetround(OldRM);<br></span><span style="color:rgb(0,0,0)"> } else {<br></span><span style="color:rgb(0,0,0)"> Result = A/B;<br></span><span style="color:rgb(0,0,0)"> }<br></span><span style="color:rgb(0,0,0)"> return Result;</span><div class="gmail_extra"><p class="MsoNormal"><font color="#000000">}</font></p></div></blockquote><div class="gmail_extra"><br clear="all"><div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">Are we still going to generate <span style="font-family:arial,sans-serif"><font color="#000000">llvm.begin.local.roundingmode/</font></span><span style="font-family:arial,sans-serif"><font color="#000000">llvm.end.local.roundingmode?</font></span></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><span style="font-family:arial,sans-serif"><font color="#000000"><br></font></span></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><span style="font-family:arial,sans-serif"><font color="#000000">Regards,</font></span></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><span style="font-family:arial,sans-serif"><font color="#000000">chenwj</font></span></div><br></div>-- <br><div class="gmail_signature"><div dir="ltr"><div>Wei-Ren Chen (陳韋任)<br>Homepage: <a href="https://people.cs.nctu.edu.tw/~chenwj" target="_blank">https://people.cs.nctu.edu.tw/~chenwj</a></div></div></div>
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