<div dir="ltr"><div>Unfortunately, it's not as easy as "any undef in --> undef out". That's a big reason for moving away from undef in IR.</div><div><br></div><div>If you read this page bottom-up (there must be a better link somewhere?) and then read the follow-ups in the thread, you'll see how we arrived at the current rules for the standard FP ops:</div><div><a href="https://lists.llvm.org/pipermail/llvm-dev/2018-March/121481.html">https://lists.llvm.org/pipermail/llvm-dev/2018-March/121481.html</a></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jul 22, 2021 at 6:34 AM Serge Pavlov <<a href="mailto:sepavloff@gmail.com">sepavloff@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">The concept of undefined value has always been obscure and caused many questions. I'd like to share my opinion, however I am not sure if I understand this concept correctly.<br><br>LLVM documentation (<a href="https://llvm.org/docs/LangRef.html#undefined-values" target="_blank">https://llvm.org/docs/LangRef.html#undefined-values</a>) describes undefined values:<br>"Undefined values are useful because they indicate to the compiler that the program is well defined no matter what value is used". So these are values on which the result of program execution does not depend. This is why an undefined value may be replaced by an arbitrary value of proper type and range. The choice of the replacement value is dictated mainly by convenience. If however the produced result depends on this choice, it means the value of `undef` affects results, so the initial supposition is broken and we have undefined behavior.<br><br>I agree with Sanjay that constrained intrinsics should behave in the same way as regular FP operations with respect to `undef`. Control modes (like rounding mode) influence result value, but we know that particular value of `undef` is not important. FP exceptions are a bit more complex. If the value of `undef` may be arbitrary, it is not possible to guarantee that FP exceptions would be the same for all possible values. So we can assume that `undef` operands do not affect FP exceptions. Either such operation is eliminated, because its value is not used, or the operation itself does not use the `undef` argument.<br><br>If any of standard IR FP operations has undef argument, the result may be either `undef` or any FP value. It is convenient to use NaN in such cases. It does not make the program more correct but it can help to detect undefined behavior in some FP environments. However `undef` result seems better choice than NaN, because in this case the user of `undef` value may choose a convenient representation for `undef`.<br><br>I do not see any reason to distinguish between the cases "all operands are undefs" and "only one operand is undef". In both cases we get a value that is not used in the correct program.<br><br>So I would propose transformations:<br><br>%r = call float @llvm.experimental.constrained.fadd.f32(float undef, float undef, metadata !"round.dynamic", metadata !"fpexcept.strict")<br> --><br> %r = undef<br><br>And<br><br>%r = call float @llvm.experimental.constrained.fadd.f32(float undef, float %x, metadata !"round.dynamic", metadata !"fpexcept.strict")<br> --><br> %r = undef<div><br></div><div>What do you think about it?</div><div><br><div><div dir="ltr">Thanks,<br>--Serge<br></div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Jul 21, 2021 at 8:15 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>Can we use the regular FP instructions (fadd, fmul, etc.) as a model?<br></div><div><br></div><div>If both operands to any of the binops are undef, then the result is undef. So for the corresponding constrained intrinsic, if both operands are undef, the result is undef and the exception state is also undef:</div><div><br></div><div> %r = call float @llvm.experimental.constrained.fadd.f32(float undef, float undef, metadata !"round.dynamic", metadata !"fpexcept.strict")</div><div> --></div><div> %r = undef</div><div><br></div><div><div> %r = call float @llvm.experimental.constrained.fadd.f32(float
undef, float undef, metadata !"round.dynamic", metadata
!"fpexcept.maytrap")</div><div> --></div><div> %r = undef</div><div><br></div></div><div><br></div><div>If one operand is undef and the other is regular value, assume that the undef value takes on some encoding of SNaN:</div><div><div><br></div><div> %r = call float @llvm.experimental.constrained.fadd.f32(float
undef, float %x, metadata !"round.dynamic", metadata
!"fpexcept.strict")</div></div><div> --></div><div><div><div> %r = call float @llvm.experimental.constrained.fadd.f32(float SNaN, float %x, metadata !"round.dynamic", metadata
!"fpexcept.strict") ; raise invalid op exception<br></div></div></div><div> (%r could be folded to QNaN here, but we can't get rid of the call, so don't bother?)<br></div><div><br></div><div><div><div> %r = call float @llvm.experimental.constrained.fadd.f32(float
undef, float %x, metadata !"round.dynamic", metadata
!"fpexcept.maytrap")</div></div><div> --></div><div> %r = QNaN ; exception state does not have to be preserved<br></div><div><br></div><div>Does that match the proposed behavior in <a href="https://reviews.llvm.org/D102673" target="_blank">https://reviews.llvm.org/D102673</a> <span><span>(cc @sepavloff)?</span></span></div><div><br></div><div>We could go further (potentially reduce to poison) if we have fast-math-flags on the calls -- just as we partially do with the regular instructions -- but it probably doesn't matter much to real code.<br></div></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Jul 9, 2021 at 12:06 PM Kevin Neal 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">
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<div>How should the constrained FP intrinsics behave when called with an operand that is “undef” and the FP environment is _<i>not</i>_ the default environment? I’m specifically working in the middle end passes if it matters. Let me start with the assumption
that the rounding mode is not relevant. That still leaves the exception handling as a factor:</div>
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<div>With “fpexcept.maytrap” we are allowed to drop instructions that could or would cause a trap at run-time. Does this imply we can fold the entire instruction to a new undef?</div>
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<div>With “fpexcept.strict” we are _<i>not</i>_ allowed to lose or reorder traps. So how does that affect undef? What happens in the backend? Perhaps the middle end should leave the instruction with the undef and let the backend do something reasonable?</div>
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<div>The “maytrap” case is the one I’m most interested in. An earlier version of D103169 would fold away undef constrained intrinsics in the maytrap case. This was removed so it could be handled without affecting the rest of the patch I believe.</div>
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<div>Opinions?</div>
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</span></font><font face="Courier New">Kevin P. Neal<br>
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<div><font size="2"><span style="font-size:10pt">Compute Services</span></font></div>
<div><font size="2"><span style="font-size:10pt">SAS Institute, Inc.</span></font></div>
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