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<p class="MsoNormal"><span lang="EN-US">Hello everyone,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I have a proposal to introduce a function attribute to solve a dead code elimination problem for singleton-get-like functions. Imagine the following code:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">MyObj *Singleton = nullptr;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">MyObj *getUnique() {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> if (Singleton)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> return Singleton;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> Singleton = <idempotent initialization><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Here getUnique is the only accessor of Singleton variable, which cannot be otherwise read or written.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">This function is formally writing memory, but it has 2 important properties:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">- This function is idempotent, meaning that its result only depends on parameters (which is empty in this case);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">- If the result of the call is not used, it can safely be deleted without causing any problems.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In other words, if we have code like<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">for (int I = 0; I < 1000; I++) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> getUnique();<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">We can consider this loop dead and just delete it. Note that currently, because getUnique may write external memory, we cannot do so (even if we inline it and cleanse the remaining code, we’ll still have a store to Singleton).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In Java world, motivation for this is Object identity hash code. It requires to return the same value for the same object every time, it may write memory (usually it
<a href="https://srvaroa.github.io/jvm/java/openjdk/biased-locking/2017/01/30/hashCode.html">
computes hash on 1<sup>st</sup> invocation</a> basing on current address and stores it to the object). But if the hash code result is not used, it’s completely fine to skip this invocation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In current function attributes, I could not find anything that could express “this is a mem-writing idempotent function which is OK to delete if trivially dead”. Optimization prospects for this are great: we can simply
DCE them.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Do you think it’s work having such attribute?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Best regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Max<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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