MIR: split Operand::Consume into Copy and Move. by eddyb · Pull Request #46142 · rust-lang/rust

eddyb · 2017-11-21T00:33:18Z

By encoding the choice of leaving the source untouched (Copy) and invalidating it (Move) in MIR, we can express moves of copyable values and have MIR borrow-checking enforce them, including ownership transfer of stack locals in calls (when the ABI passes by indirection).

Optimizations could turn a "last-use" Copy into a Move, and the MIR borrow-checker, at least within the confines of safe code, could even do this when the underlying lvalue was borrowed.
(However, that last part would be the first time lifetime inference affects code generation, AFAIK).

Furthermore, as Moves invalidate borrows as well, for any local that is initialized only once, we can ignore borrows that happened before a Move and safely reuse/replace its memory storage.
This will allow us to perform NRVO in the presence of short-lived borrows, unlike LLVM (currently), and even compute optimal StorageLive...StorageDead ranges instead of discarding them.

rust-highfive · 2017-11-21T00:33:21Z

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

eddyb · 2017-11-21T00:33:33Z

r? @nikomatsakis

nikomatsakis

r=me modulo nits

nikomatsakis · 2017-11-21T13:24:28Z

Can we leave some comments explaining what these mean, in terms of the expectations they provide? I think something like this:
Copy: The value must be available for use afterwards. This implies that the type of the lvalue must be Copy; this is true by construction during build, but checked by the MIR type checker.
Move: The value will not be used again. Safe for values of all types (modulo future developments towards ?Move). Enforced by borrow checker. Sometimes Copy is converted to Move to enable "last-use" optimizations.

nikomatsakis · 2017-11-21T13:29:16Z

Personally, I'd rather this exhaustive (and perhaps move the if on the arm above inside the arm. But I rate the probability of overlooking this match after some future change to Operand as fairly low, so it's not that important.

scottmcm · 2017-11-21T22:59:28Z

FYI: If PR #46093 goes in first, I think it will conflict with this in a way that git won't notice. This line will need to become a Move: https://github.com/rust-lang/rust/pull/46093/files#diff-69bf2a484158ebf56322b7ffb03601cdR108

bors · 2017-11-22T01:56:05Z

☔ The latest upstream changes (presumably #45879) made this pull request unmergeable. Please resolve the merge conflicts.

nikomatsakis · 2017-11-27T21:42:58Z

@bors r+

bors · 2017-11-27T21:42:59Z

📌 Commit cd99774 has been approved by nikomatsakis

scottmcm · 2017-11-27T22:53:45Z

Looks like this might be real?

[00:19:46] error[E0599]: no associated item named `Consume` found for type `rustc::mir::Operand<'_>` in the current scope
[00:19:46] --> /checkout/src/librustc_mir/transform/add_moves_for_packed_drops.rs:135:34[00:19:46] |
[00:19:46]135 | Rvalue::Use(Operand::Consume(location.clone())));[00:19:46] | ^^^^^^^^^^^^^^^^

eddyb · 2017-11-28T00:08:16Z

@bors r=nikomatsakis

bors · 2017-11-28T00:08:17Z

📌 Commit 56fb59c has been approved by nikomatsakis

bors · 2017-11-28T02:09:17Z

☔ The latest upstream changes (presumably #46312) made this pull request unmergeable. Please resolve the merge conflicts.

…value.

eddyb · 2017-11-28T02:18:55Z

@bors r=nikomatsakis

bors · 2017-11-28T02:18:55Z

📌 Commit 919ed40 has been approved by nikomatsakis

bors · 2017-11-28T05:38:26Z

⌛ Testing commit 919ed40 with merge 7745a7a...

MIR: split Operand::Consume into Copy and Move. By encoding the choice of leaving the source untouched (`Copy`) and invalidating it (`Move`) in MIR, we can express moves of copyable values and have MIR borrow-checking enforce them, *including* ownership transfer of stack locals in calls (when the ABI passes by indirection). Optimizations could turn a "last-use" `Copy` into a `Move`, and the MIR borrow-checker, at least within the confines of safe code, could even do this when the underlying lvalue was borrowed. (However, that last part would be the first time lifetime inference affects code generation, AFAIK). Furthermore, as `Move`s invalidate borrows as well, for any local that is initialized only once, we can ignore borrows that happened before a `Move` and safely reuse/replace its memory storage. This will allow us to perform NRVO in the presence of short-lived borrows, unlike LLVM (currently), and even compute optimal `StorageLive...StorageDead` ranges instead of discarding them.

rustc_mir: implement an "lvalue reuse" optimization (aka destination propagation aka NRVO). Replaces a chain of moves, such as `a = ...; ... b = move a; ... f(&mut b) ... c = move b`, with the final destination, i.e. only `c = ...; ... f(&mut c); ...` remains (note that borrowing is allowed). **DO NOT MERGE** only for testing atm. Based on #46142.

bors · 2017-11-28T08:04:52Z

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 7745a7a to master...

rust-highfive assigned arielb1 Nov 21, 2017

rust-highfive assigned nikomatsakis and unassigned arielb1 Nov 21, 2017

kennytm added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 21, 2017

nikomatsakis requested changes Nov 21, 2017
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kennytm added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 23, 2017

eddyb force-pushed the even-mirer-2 branch from 8149186 to cd99774Compare November 27, 2017 21:38

nikomatsakis approved these changes Nov 27, 2017
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eddyb force-pushed the even-mirer-2 branch from cd99774 to 56fb59cCompare November 27, 2017 23:20

eddyb added 5 commits November 28, 2017 04:11

rustc_mir: move storage_dead_or_drop_error_reported logic to access_l…
0477319
…value.

rustc_mir: enforce that arguments are replaced with Local's only.
73f5bab

MIR: split Operand::Consume into Copy and Move.
c42a118

rustc_mir: require that Copy(L) satisfies typeof L: Copy.
170b88d

tests: update to include move annotations in MIR.
919ed40

eddyb force-pushed the even-mirer-2 branch from 56fb59c to 919ed40Compare November 28, 2017 02:18

eddyb mentioned this pull request Nov 28, 2017
rustc_mir: implement an "lvalue reuse" optimization (aka destination propagation aka NRVO). #46321
Closed

bors merged commit 919ed40 into rust-lang:masterNov 28, 2017

This was referenced Nov 28, 2017
Update compiler-builtins and use it in the 128-bit lowering MIR test #46290
Merged
Always unwind through a Resume and other fixes #46320
Merged

eddyb deleted the even-mirer-2 branch November 28, 2017 11:31

dwrensha added a commit to dwrensha/seer that referenced this pull request Nov 29, 2017
fix breakage fromrust-lang/rust#46142
ede519b

nikomatsakis mentioned this pull request Nov 29, 2017
Lifetime bounds in Copy impls are ignored #29149
Closed

eddyb mentioned this pull request Jan 30, 2018
MIR optimization tracking issue. #44285
Closed
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