<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On 31 August 2017 at 14:40, Hal Finkel 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 bgcolor="#FFFFFF" text="#000000"><span class="">
<p>On 08/31/2017 04:31 PM, Richard Smith
via cfe-dev wrote:<br></p>
<blockquote type="cite">
<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>
</blockquote>
<br></span>
Or we prevent inlining.</div></blockquote><div><br></div><div>Sure, I was considering that to be a form of interprocedural code motion :)</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><span class=""><blockquote type="cite">
<div dir="ltr">
<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>
</blockquote>
<br></span>
Good point. However, that's not a new problem, and we currently deal
with this by respecting the noimplicitfloat attribute (and I think
we'd definitely need to use that attribute if we allow fooling with
the FP environment).<br></div></blockquote><div><br></div><div>OK, so the idea would be that we'd lower a function containing FENV_ACCESS (or possibly an outlined block of such a function) with intrinsics for all FP operations, specially-annotated libm function calls, and noimplicitfloat and strictfp attributes to prevent generation of new FP operations and inlining into non-strictfp functions. Right? (And we could imagine a verifier check that ensures that you don't have pure FP operations inside a strictfp function.)</div><div><br></div><div>Given the function annotations, do we need special intrinsics at all, or could we instead require that passes check whether the enclosing function is marked strictfp before optimizing, in the same way that some optimizations must be gated by a check for noimplicitfloat?</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">
-Hal<div><div class="h5"><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<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 link="blue" vlink="purple" lang="EN-US">
<div class="m_-23198277102369654m_-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.</span></p>
<p class="MsoNormal"><a name="m_-23198277102369654_m_-2075974219545047674__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </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="m_-23198277102369654h5"><br>
<b>Subject:</b> Re: [cfe-dev] Why is #pragma
STDC FENV_ACCESS not supported?</div>
</div>
<div>
<div class="m_-23198277102369654h5">
<p class="MsoNormal"> </p>
<div>
<div>
<div>
<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:</p>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<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></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"> </p>
</div>
<div>
<p class="MsoNormal">fadd can be
hoisted past *anything*, can't it?</p>
</div>
<div>
<p class="MsoNormal"><a name="m_-23198277102369654_m_-2075974219545047674_m_2724022782041152477__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></a></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<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></p>
<div>
<div>
<p class="MsoNormal"><br>
<b>Subject:</b> Re:
[cfe-dev] Why is #pragma
STDC FENV_ACCESS not
supported?</p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> </p>
<div>
<div>
<div>
<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:</p>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
<div>
<div>
<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></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></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></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></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/D2702<wbr>8</a></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></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></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></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></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"> </p>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt">How do you deal with the hoisting-into-<span class="m_-23198277102369654m_-2075974219545047674m2724022782041152477gmail-il">fenv_access</span> prob<wbr>lem?
Eg:</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> </span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt">double f(double a, double b, double c) {</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> {</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt">#pragma STDC <span class="m_-23198277102369654m_-2075974219545047674m2724022782041152477gmail-il">FENV_ACCESS</span> ON</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> feenableexcept(FE_OVERFLOW);</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> double d = a * b;</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> fedisableexcept(FE_OVERFLOW);</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> }</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> return c * d;</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt">}</span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt"> </span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:9.5pt">What stops llvm from hoisting the second fmul up
to before the
fedisableexcept?</span></p>
</div>
</div>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
<div>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">-Andy</span></p>
<p class="MsoNormal"><a name="m_-23198277102369654_m_-2075974219545047674_m_2724022782041152477_m_1667421056975128"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> </span></a></p>
<div>
<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></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> </p>
<p> </p>
<div>
<p class="MsoNormal">On
08/31/2017
12:10 PM,
Richard Smith
via cfe-dev
wrote:</p>
</div>
<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.</p>
</div>
</blockquote>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
Yes. This is
what Andrew
Kaylor has
been working
on (cc'd).<br>
<br>
-Hal</p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal"> </p>
<div>
<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:</p>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
<div>
<div>
<p class="MsoNormal"><span style="font-size:18.0pt">^^^^^^</span></p>
</div>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
______________________________<wbr>_________________<br>
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mailing list<br>
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</blockquote>
</div>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt"> </p>
<pre>______________________________<wbr>_________________</pre>
<pre>cfe-dev mailing list</pre>
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</blockquote>
<p class="MsoNormal" style="margin-bottom:12.0pt"> </p>
<pre>-- </pre>
<pre>Hal Finkel</pre>
<pre>Lead, Compiler Technology and Programming Languages</pre>
<pre>Leadership Computing Facility</pre>
<pre>Argonne National Laboratory</pre>
</div>
</div>
</div>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">
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</div>
<p class="MsoNormal"> </p>
</div>
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</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">
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<fieldset class="m_-23198277102369654mimeAttachmentHeader"></fieldset>
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</pre>
</blockquote>
<pre class="m_-23198277102369654moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre></div></div></div><br>______________________________<wbr>_________________<br>
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