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<p class="MsoNormal">Hello,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Currently, llvm::isSafeToSpeculativelyExecute() always returns false for Call instructions.
<o:p></o:p></p>
<p class="MsoNormal">This has actual performance implications, because loop-invariant code motion makes this check, and will never hoist instructions that are not safe to speculatively execute.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Unfortunately, there is currently no way to signal to LICM that a function is safe to speculatively execute. The implementation of isSafeToSpeculativelyExecute() explicitly states that marking a function nounwind and readnone are not sufficient.
The rationale, if I understand it correctly, is that even readnone nounwind functions can contain undefined behavior, so hoisting them may be unsafe. A trivial example of this are functions with infinite loops (By the way, I saw an earlier discussion about
a “halting” attribute – I understand this never made it into the trunk?)<o:p></o:p></p>
<p class="MsoNormal">A comment in the implementation also says a readnone nounwind function may have side effects, but this, to the best of my understanding, contradicts the language reference manual which explicitly states that a readnone function may not
“access any mutable state visible to caller functions”.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Does adding a new attribute to signal that a function is speculation-safe sound reasonable? If not, anyone has any other ideas on how this can be resolved? It doesn’t seem like LICM can safely make a weaker query than isSafeToSpeculativelyExecute().<o:p></o:p></p>
<p class="MsoNormal">If you think the attribute is a good idea, name suggestions for it are also welcome: “pure” and “const” are already used in gcc in a slightly different meaning (equivalent to LLVM “readonly” and “readnone”, I think), “nosideeffects” risks
confusion with mayHaveSideEffects(), and anything that has the word “speculatively” in it is probably too long.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal"> Michael<o:p></o:p></p>
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