One more question: do you think it's *better* to let dataToTag# float and then fix it up later, or better to mark it can_fail? Unlike reallyUnsafePtrEquality#, dataToTag# is used all over the place, so it is important that it interacts as well as possible with the optimizer, whatever that entails. On Wednesday, January 18, 2017 2:45:04 PM EST David Feuer wrote:
I opened up https://phabricator.haskell.org/D2987 to mark reallyUnsafePtrEquality# can_fail, but in the process I realized a couple things.
1. Part of https://phabricator.haskell.org/rGHC5a9a1738023a may actually not have been such a hot idea after all (although it certainly sounded good at the time). Are there actually any primops with lifted arguments where we *want* speculation? Perhaps the most important primop to consider is seq#, which is (mysteriously?) marked neither can_fail nor has_side_effects, but another to look at is unpackClosure#, which seems likely to give different results before and after forcing. Most other primops with lifted arguments are marked has_side_effects, and therefore won't be floated out anyway.
2. If dataToTag# is marked can_fail (an aspect of https:// phabricator.haskell.org/rGHC5a9a1738023a), is it still possible for it to end up being applied to an unevaluated argument? If not, perhaps the CorePrep specials can be removed altogether.
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