[PATCH] D76923: [NFC] Improve performances of Value::getSingleUndroppableUse

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

I'm seeing a 0.1-0.2% regression <http://llvm-compile-time-tracker.com/compare.php?from=17e4c38739aa78638c783dac6c149858d1c0a550&to=01c5f915086420379e1b3d5b80cb098d3beb9940&stat=instructions> with this change. Probably the added cost of the pointer tagging outweighs the benefit we see for the single user of `getSingleUndroppableUse()` right now. If this API is going to be used more heavily in the future, the picture will of course change. Hard for me to say whether it makes sense to make this change at this time.

I do wonder whether there's a middle ground here: Rather than adding a tag to `Use`, we could use a bit of subclass data on `CallInst` to indicate whether the call is droppable. That would reduce `User->isDroppable()` to a compare and mask check, and we could at least inline it.



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Comment at: llvm/include/llvm/IR/User.h:227
+  /// This will mark every use as droppable if the user is droppable.
+  /// This should be called from 
+  void maybeMarkUsesAsDroppable();
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