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JumpThreading: compute place and value indices on-demand #142881

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@cjgillot cjgillot commented Jun 22, 2025 •
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Profiling JumpThreading reveals that a large part of the runtime happens constructing the place and value Map. This is unfortunate, as jump-threading may end up not even doing anything.

The cause for this large up-front cost is following: Map attempts to create a PlaceIndex for each place that may hold a relevant value. This means all places that appear in MIR, but also all places whose value is accessed by a projection of a copy of a larger place.

This PR refactors the creation of Map to happen on-demand: place and value indices are created when threading computation happens.

The up-front mode is still relevant for DataflowConstProp, so is not touched.

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jun 22, 2025
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@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 22, 2025
bors added a commit that referenced this pull request Jun 22, 2025
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Auto merge of #142881 - cjgillot:minimap, r=<try>
1587bef 
JumpThreading: compute place and value indices on-demand

Perf experiment

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bors commented Jun 22, 2025

⌛ Trying commit ba3a8c5 with merge 1587bef...

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bors commented Jun 22, 2025

☀️ Try build successful - checks-actions
Build commit: 1587bef (1587bef3b2c2aea88de9b552efbf7febe7cd5fc4)

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Finished benchmarking commit (1587bef): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

mean range count
Regressions ❌
(primary)		 1.0% [0.7%, 1.3%] 2
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 -0.3% [-1.4%, -0.1%] 142
Improvements ✅
(secondary)		 -0.4% [-1.7%, -0.1%] 93
All ❌✅ (primary) -0.3% [-1.4%, 1.3%] 144

Max RSS (memory usage)

Results (primary 3.6%, secondary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 3.6% [3.4%, 3.8%] 2
Regressions ❌
(secondary)		 2.0% [2.0%, 2.0%] 1
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 3.6% [3.4%, 3.8%] 2

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary -0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 0.2% [0.0%, 0.4%] 12
Regressions ❌
(secondary)		 0.2% [0.0%, 0.4%] 42
Improvements ✅
(primary)		 -0.3% [-1.4%, -0.0%] 20
Improvements ✅
(secondary)		 -0.3% [-0.9%, -0.0%] 5
All ❌✅ (primary) -0.1% [-1.4%, 0.4%] 32

Bootstrap: 689.082s -> 684.233s (-0.70%)
Artifact size: 371.89 MiB -> 372.00 MiB (0.03%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 23, 2025
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@cjgillot cjgillot marked this pull request as ready for review June 23, 2025 09:47
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rustbot commented Jun 23, 2025

r? @eholk

rustbot has assigned @eholk.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 23, 2025
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rustbot commented Jun 23, 2025

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

@cjgillot cjgillot added the A-mir-opt Area: MIR optimizations label Jun 23, 2025
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rustbot commented Jun 23, 2025

Some changes occurred in coverage tests.

cc @Zalathar

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r? compiler

@rustbot rustbot assigned oli-obk and unassigned eholk Jul 20, 2025
@cjgillot cjgillot force-pushed the minimap branch 2 times, most recently from d08b44c to f2018e5 Compare July 26, 2025 01:22
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bors commented Jul 28, 2025

☔ The latest upstream changes (presumably #144469) made this pull request unmergeable. Please resolve the merge conflicts.

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r? oli-obk

@rustbot rustbot assigned oli-obk and unassigned fee1-dead Sep 1, 2025
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rustbot commented Sep 1, 2025

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

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r? saethlin

@rustbot rustbot assigned saethlin and unassigned oli-obk Sep 18, 2025
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The PR description sounds like this is just a perf change, but I see that some of the tests were perturbed. Do you know why?

@saethlin saethlin 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 Sep 20, 2025
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cjgillot commented Sep 21, 2025
View reviewed changes
tests/mir-opt/jump_threading.rs
// CHECK: [[tmp:_.*]] = copy [[c]];
// CHECK: [[eq:_.*]] = Eq(move [[tmp]], const 2_i32);
// CHECK: switchInt(move [[eq]]) -> [0: bb2, otherwise: bb1];
// CHECK: goto -> bb1;
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@cjgillot cjgillot Sep 21, 2025 •
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Thanks @saethlin for pushing me to investigate this. One of the behaviour changes is a bugfix for this pattern. On master, if we read a twice, the b = a assignment makes us forget what the c = a assignment told us. With this PR, we correctly merge the sets of conditions.

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As for the other behaviour changes, my understanding is that this PR changes the order in which place and value indices are allocated. So changes where the MAX_PLACES cutoff happens.

@saethlin saethlin added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 26, 2025
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bors commented Sep 27, 2025

☔ The latest upstream changes (presumably #146829) made this pull request unmergeable. Please resolve the merge conflicts.

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rustbot commented Oct 4, 2025

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

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