<div dir="ltr"><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Sep 14, 2020 at 3:00 PM Sanjay Patel <<a href="mailto:spatel@rotateright.com">spatel@rotateright.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Sorry - I misread your example and the problem. I see now where LibCallSimplifier creates the select...but we are immediately erasing that select with the code from the godbolt example.<br></div><div>Does the real motivating case have no uses of the pow() result value?</div></div></blockquote><div>Using the result value (assign to global volatile) doesn't cause the sqrt call to be avoided either.<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Sep 14, 2020 at 1:03 PM Sanjay Patel <<a href="mailto:spatel@rotateright.com" target="_blank">spatel@rotateright.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Yes, I mean just bail out on the transform in LibCallSimplifier::replacePowWithSqrt() -> getSqrtCall(). If we can't prove the call behaves the same with errno, then give up.<br></div><div>I'm not sure where the select / branching happens, but I don't see that happening in the initial transform (called from -instcombine)<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Sep 14, 2020 at 12:58 PM Hubert Tong <<a href="mailto:hubert.reinterpretcast@gmail.com" target="_blank">hubert.reinterpretcast@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Sep 14, 2020 at 12:45 PM Sanjay Patel <<a href="mailto:spatel@rotateright.com" target="_blank">spatel@rotateright.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Yes, that looks like a bug. The transform is ok in general for negative numbers, but -Inf is a special-case for pow(), right?<br></div><div>If so, we probably need an extra check of the input with "isKnownNeverInfinity()".</div></div></blockquote><div>There is an extra check there already, but it uses "select" instead of branching. One question is if branching is okay to use instead. Or perhaps you mean the transform should not be done unless "isKnownNeverInfinity()" returns true.<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>If there are other errno divergences for edge case values, we may need to check other conditions.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Sep 12, 2020 at 9:37 PM Hubert Tong via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>The transformation in <span style="font-family:monospace">LibCallSimplifier::replacePowWithSqrt</span> with respect to -Inf uses a select instruction, which based on the observed behaviour, incorporates the side effects of the unchosen branch. This means that (for <span style="font-family:monospace">pow</span>) a call to <span style="font-family:monospace">sqrt(</span>-Inf<span style="font-family:monospace">)</span> is materialized. Such a call is specified as having a domain error (C17 subclause 7.12.7.5) since the operand is less than zero. Contrast this with <span style="font-family:monospace">pow(</span>-Inf<span style="font-family:monospace">, 0.5)</span>, which is specified by C17 subclause F.10.4.4 as having a result of +Inf (indicating an exact result for the operation and, since IEEE Std 754-2008 subclause 9.1.1 states that domain errors are to be indicated by a NaN result, a lack of a domain error).</div><div><br></div><div>It is noted that the above statements were made notwithstanding the ERRORS section of pow() in POSIX.1-2017 XSH Chapter 3, which specifies a domain error except perhaps by deference to the C standard due to a conflict between the POSIX and the C wording.</div><div><br></div><div>The transformation in question causes EDOM for
<span style="font-family:monospace">pow(</span>-Inf<span style="font-family:monospace">, 0.5)</span>
even on platforms where the system library implementation of <span style="font-family:monospace">pow</span> does not cause this situation to arise.</div><div><br></div><div>A sample program that (on some platforms, such as Linux on x86-64) completes successfully with optimizations off, and aborts with LLVM's optimization follows; <span style="font-family:monospace">-fmath-errno</span> does not help, and it is not expected to, because it is designed to retain setting errno to non-zero (not to prevent spuriously setting errno).</div><div><br></div><div><span style="font-family:monospace">#include <errno.h><br>volatile double inf = -__builtin_inf();<br><br>double pow(double, double);<br>void abort(void);<br>int main(void) {<br> errno = 0;<br> pow(inf, 0.5);<br> if (errno != 0) abort();<br>}</span></div><div><br></div><div>Compiler Explorer link: <a href="https://godbolt.org/z/5Wr66M" target="_blank">https://godbolt.org/z/5Wr66M</a></div><div><br></div><div>Is the transformation actually valid in some way? If not, I see no instances where the LibCallSimplier generates conditional branches. Retaining the transformation
in some way
without generating conditional branches would probably involve bailing out if infinities are still in play.<br></div><div><br></div></div>
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