From https://msdn.microsoft.com/en-us/library/ms235286.aspx: "Leaf functions are functions that do not change any non-volatile registers. .. Leaf functions can be unwound simply by simulating a return, so pdata and xdata are not required." Not emitting these would be good for binary size. Note that such functions can still tail-call others, so they're not really leafs in the call-graph sense. If we hook up a way to determine whether a function is a leaf in this sense, we can also use it to do conditional tail-calls for Win64. Such calls would generally confuse the unwinder, but in leaf functions they are fine.
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