<div dir="ltr">I think that's also not enough; you'd get the same problem after inlining, and across modules with LTO. You would need to also prevent any interprocedural code motion across a FENV_ACCESS / non-FENV_ACCESS boundary.<div><br></div><div>And even that doesn't seem to be enough. Suppose that some scalar optimization pass finds a clever way to converts some integer operation into a floating-point operation, such that it can prove that the FP values never overflow (I believe Chandler has an example of this that comes up in some real crypto code). Now suppose there's a case where the integer operands are undef, but that the code in question is bypassed in that case. If the FP operations get hoisted, and you happen to have FP exceptions enabled, you have a potential miscompile.</div><div><div><br></div><div>Fundamentally, it seems to me that feenableexcept is unsound in the current LLVM IR model of floating point, if we assume that fadd, fmul, fsub etc do not have side-effects.<div class="gmail_extra"><br><div class="gmail_quote">On 31 August 2017 at 14:20, Kaylor, Andrew via cfe-dev <span dir="ltr"><<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div lang="EN-US" link="blue" vlink="purple">
<div class="m_-2075974219545047674WordSection1">
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">If that’s the case, we may need to use the constrained intrinsics for all FP operations when FENV_ACCESS is enabled anywhere in a function.<u></u><u></u></span></p>
<p class="MsoNormal"><a name="m_-2075974219545047674__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></a></p>
<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"> Richard Smith [mailto:<a href="mailto:richard@metafoo.co.uk" target="_blank">richard@metafoo.co.uk</a>]
<br>
<b>Sent:</b> Thursday, August 31, 2017 2:18 PM<br>
<b>To:</b> Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>><br>
<b>Cc:</b> Clang Dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>>; Marcus Johnson <<a href="mailto:bumblebritches57@gmail.com" target="_blank">bumblebritches57@gmail.com</a>>; <a href="mailto:wei.ding2@amd.com" target="_blank">wei.ding2@amd.com</a></span></p><div><div class="h5"><br>
<b>Subject:</b> Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?<u></u><u></u></div></div><p></p><div><div class="h5">
<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">On 31 August 2017 at 14:14, Kaylor, Andrew via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">I believe that we will rely on fedisableexcept() being marked as having unmodeled side-effects to prevent
a hoist like that.</span><u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">fadd can be hoisted past *anything*, can't it?<u></u><u></u></p>
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<p class="MsoNormal"><a name="m_-2075974219545047674_m_2724022782041152477__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></a><u></u><u></u></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"> Richard Smith [mailto:<a href="mailto:richard@metafoo.co.uk" target="_blank">richard@metafoo.co.uk</a>]
<br>
<b>Sent:</b> Thursday, August 31, 2017 2:09 PM<br>
<b>To:</b> Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>><br>
<b>Cc:</b> Marcus Johnson <<a href="mailto:bumblebritches57@gmail.com" target="_blank">bumblebritches57@gmail.com</a>>; Clang Dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>>;
<a href="mailto:wei.ding2@amd.com" target="_blank">wei.ding2@amd.com</a></span><u></u><u></u></p>
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<div>
<p class="MsoNormal"><br>
<b>Subject:</b> Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?<u></u><u></u></p>
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<div>
<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal">On 31 August 2017 at 11:09, Kaylor, Andrew via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">There are still a few things missing from the optimizer to get it completely robust, but I think there
is enough in place for front end work to begin. As I think I’ve demonstrated in my recent attempt to contribute a clang patch I’m not skilled enough with the front end to be the person to pull this off without an excessive amount of oversight, but as Erich
indicated we do have some good front end people here who have this on their TODO list. It’s just not at the top of the TODO list yet.</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">If anyone is interested in the details of the LLVM side of things, there are constrained FP intrinisics
(still marked as experimental at this point) documented in the language reference. The initial patch can be seen here:</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><a href="https://reviews.llvm.org/D27028" target="_blank">https://reviews.llvm.org/<wbr>D27028</a></span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">I’ve since added another group of intrinsics to handle the libm-equivalent intrinsics, and more recently
Wei Ding contributed an fma intrinsic.</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">The idea is that the front end will emit the constrained intrinsics in place of equivalent general
FP operations or intrinsics in scopes where FENV_ACCESS is enabled. This will prevent the optimizer from making optimizations that assume default fenv settings (which is what we want the optimizer to do in all other cases). Eventually, we’ll want to go back
and teach specific optimizations to understand the intrinsics so that where possible optimizations can be performed in a manner consistent with dynamic rounding modes and strict exception handling.</span><u></u><u></u></p>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt">How do you deal with the hoisting-into-<span class="m_-2075974219545047674m2724022782041152477gmail-il">fenv_access</span> <wbr>problem? Eg:</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt">double f(double a, double b, double c) {</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> {</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt">#pragma STDC <span class="m_-2075974219545047674m2724022782041152477gmail-il">FENV_ACCESS</span> ON</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> feenableexcept(FE_OVERFLOW);</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> double d = a * b;</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> fedisableexcept(FE_OVERFLOW);</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> }</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> return c * d;</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt">}</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt"> </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:9.5pt">What stops llvm from hoisting the second fmul up to before the fedisableexcept?</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span><u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">-Andy</span><u></u><u></u></p>
<p class="MsoNormal"><a name="m_-2075974219545047674_m_2724022782041152477_m_1667421056975128"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></a><u></u><u></u></p>
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<div style="border:none;border-top:solid #e1e1e1 1.0pt;padding:3.0pt 0in 0in 0in">
<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"> Hal Finkel [mailto:<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>]
<br>
<b>Sent:</b> Thursday, August 31, 2017 10:45 AM<br>
<b>To:</b> Richard Smith <<a href="mailto:richard@metafoo.co.uk" target="_blank">richard@metafoo.co.uk</a>>; Marcus Johnson <<a href="mailto:bumblebritches57@gmail.com" target="_blank">bumblebritches57@gmail.com</a>><br>
<b>Cc:</b> Clang Dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>>; Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>><br>
<b>Subject:</b> Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?</span><u></u><u></u></p>
</div>
</div>
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<div>
<p class="MsoNormal"> <u></u><u></u></p>
<p> <u></u><u></u></p>
<div>
<p class="MsoNormal">On 08/31/2017 12:10 PM, Richard Smith via cfe-dev wrote:<u></u><u></u></p>
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<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">Because no-one has implemented it. Patches would be welcome, but will need to start with a design and implementation of the requisite llvm extensions.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
Yes. This is what Andrew Kaylor has been working on (cc'd).<br>
<br>
-Hal<u></u><u></u></p>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal">On 31 Aug 2017 10:06, "Marcus Johnson via cfe-dev" <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:18.0pt">^^^^^^</span><u></u><u></u></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
______________________________<wbr>_________________<br>
cfe-dev mailing list<br>
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<a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" target="_blank">http://lists.llvm.org/cgi-bin/<wbr>mailman/listinfo/cfe-dev</a><u></u><u></u></p>
</blockquote>
</div>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><u></u> <u></u></p>
<pre>______________________________<wbr>_________________<u></u><u></u></pre>
<pre>cfe-dev mailing list<u></u><u></u></pre>
<pre><a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><u></u><u></u></pre>
<pre><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" target="_blank">http://lists.llvm.org/cgi-bin/<wbr>mailman/listinfo/cfe-dev</a><u></u><u></u></pre>
</blockquote>
<p class="MsoNormal" style="margin-bottom:12.0pt"> <u></u><u></u></p>
<pre>-- <u></u><u></u></pre>
<pre>Hal Finkel<u></u><u></u></pre>
<pre>Lead, Compiler Technology and Programming Languages<u></u><u></u></pre>
<pre>Leadership Computing Facility<u></u><u></u></pre>
<pre>Argonne National Laboratory<u></u><u></u></pre>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
______________________________<wbr>_________________<br>
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</blockquote>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
______________________________<wbr>_________________<br>
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</blockquote>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
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