NLL: experiment with inverting liveness #53314
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@bors try |
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☀️ Test successful - status-travis |
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@rust-timer build 0d2e1bf |
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Success: Queued 0d2e1bf with parent d5a448b, comparison URL. |
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Looks nice, except for |
Curious. I suspect that is not meaningful, though, since NLL is not enabled in that configuration. Still...weird. |
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OK, well, this seems to have enough widespread benefits to be worth landing. |
nikomatsakis
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Aug 14, 2018
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| /// A little data structure that makes it more efficient to find the | ||
| /// predecessors of each point. | ||
| crate struct PointIndexMap<'me, 'tcx> { |
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oh, hmm, I had thought about removing this code and merging this into RegionValueElements. Maybe I should do that before landing.
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pnkfelix
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Aug 15, 2018
| /// read or modified. e.g., `y` is used in `x = y` but not `x`. | ||
| first_use_at: IndexVec<LocalWithRegion, Option<AppearanceIndex>>, | ||
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| /// Head of a linked list of **uses** of each variable -- use in |
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this comment seems like it needs a sed -e 's/use/drop/g, no?
pnkfelix
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Aug 15, 2018
| /// read or modified. e.g., `y` is used in `x = y` but not `x`. | ||
| first_drop_at: IndexVec<LocalWithRegion, Option<AppearanceIndex>>, | ||
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| appearances: IndexVec<AppearanceIndex, Appearance>, |
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silly question: there is an invariant that the linked lists formed in this store of Appearances never share tails, right? At first I had thought there might be some reason that you used a single datatype for all three of the lists above, but now I'm figuring it must just be expedience, right?
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Just expedience. Originally I thought I would have to do more forms of inter-linking -- i.e., maybe I would want to iterate not just "all places that a local is used" but also "all locals used at this place" -- but that did not turn out to be true. If that had been true, then I think a single type might have been more useful.
pnkfelix
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Aug 15, 2018
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
| // option. This file may not be copied, modified, or distributed | ||
| // except according to those terms. | ||
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my gut reaction is: "Is this somehow reinventing a dataflow analysis that we don't currently support because our dataflow infrastructure only supports forward-flows?" Maybe I'm missing something crucially different that a hypothetical generalized dataflow could not provide. (Or maybe its simpler to just write some one-off code like this for now until we have at least two analyses that require a backward flow...) |
No, this is one iteration past that. In particular, But what this code does is to rewrite that. In particular, instead of computing the "set of live variables at a given block" we now answer the question "for which blocks is a variable live". This turns out to be a better question to answer efficiency wise (though not by that much) -- in part because the resulting bitset is exactly what our regions want as input (whereas with the older style, we had to "invert" the result). |
Ah yes. That is a pretty important (and succinct) insight. |
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@pnkfelix I added the comments you requested. I think I will (a) rebase and then (b) try to do one more "cleanup" pass on the code. I guess your final comment means that you at least agree with the general concept of this PR? :) |
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@nikomatsakis hmm I thought I had put a comment on here with an r=me once xyz is addressed. Well I'll try to double-check the commit series tomorrow to make sure I'm not mixing this up with a different PR. |
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@bors r=pnkfelix |
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📌 Commit 09feec6 has been approved by |
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⌛ Testing commit 09feec6 with merge b51bd0babd32f4e50d7ee895d9b1dded660b376d... |
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💔 Test failed - status-travis |
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@bors r+ |
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📌 Commit 8d231ec has been approved by |
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This was referenced Aug 28, 2018
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I got inspired to see what would happen here.
Fixes #52460
r? @pnkfelix