Use DefPathHash instead of HirId to break inlining cycles. #85321

cjgillot · 2021-05-15T09:59:38Z

The DefPathHash is stable across incremental compilation sessions, so provides a total order on LocalDefId. Using it instead of HirId ensures the MIR inliner has the same behaviour for incremental and non-incremental compilation.

A downside is that the cycle tie break is not as predictable is with HirId.

rust-highfive · 2021-05-15T09:59:41Z

r? @jackh726

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jackh726

I don't feel really comfortable reviewing this, since I'm not familiar with the intricacies here. I also don't know who would be best to review.

jackh726 · 2021-05-15T16:19:29Z

compiler/rustc_mir/src/transform/inline.rs

-            // So don't do it if that is enabled.
-            if !self.tcx.dep_graph.is_fully_enabled() && self.hir_id < callee_hir_id {
+            // a lower `DefPathHash` than the callee. This ensures that the callee will
+            // not inline us. This trick only works even with incremental compilation,


- This trick only works even with + This trick works even with

bjorn3 · 2021-05-17T14:03:25Z

compiler/rustc_mir/src/transform/inline.rs

+            // since `DefPathHash` is stable.
+            if self.tcx.def_path_hash(caller_def_id)
+                < self.tcx.def_path_hash(callee_def_id.to_def_id())
+            {
                return Ok(());
            }


Why does this still exist now that we have mir_callgraph_reachable?

Two advantages:

it halves the amount of calls to mir_callgraph_reachable,

it allows for inlining even in the presence of a cycle.

But this completely omits half of all inlining candidates. Also with hir id comparisons this would give a consistent result. Now any rustc version increment would cause the result to change as the def path hash contains the -Cmetadata calculated by cargo based on the compiler version, which would break the tests and seemingly randomly regress/improve performance when enabling mir inlining.

But this completely omits half of all inlining candidates.

To the contrary, this branch allows half of all candidates without further examination.

Also with hir id comparisons this would give a consistent result. Now any rustc version increment would cause the result to change as the def path hash contains the -Cmetadata calculated by cargo based on the compiler version, which would break the tests and seemingly randomly regress/improve performance when enabling mir inlining.

I will change the test to only consider the local part of the DefPathHash, removing the influence of -Cmetadata.

I would prefer to avoid optimization decisions being nondeterministic or otherwise uniform - we already see a lot of pain due to CGU partitioning, for example. This seems likely to be even less explainable, right?

The mir_callgraph_reachable is relatively expensive. If we can avoid calling it half the time that seems like a nice win!

The unpredictable nature of the check is a disadvantage, but the condition is effectively: caller_hash < callee_hash || !reachable(callee, caller), which means that it becomes unpredictable only in a presence of a cycle, since otherwise MIR would be available for inlining anyway. I think this is a reasonable trade-off to make.

BTW. Does comparing local_hash help? It seems to incorporate complete DefPathHash of a parent:

rust/compiler/rustc_hir/src/definitions.rs

Lines 143 to 156 in fa72878

parent.hash(&mut hasher);

let DisambiguatedDefPathData { ref data, disambiguator } = self.disambiguated_data;

std::mem::discriminant(data).hash(&mut hasher);

if let Some(name) = data.get_opt_name() {

// Get a stable hash by considering the symbol chars rather than

// the symbol index.

name.as_str().hash(&mut hasher);

}

disambiguator.hash(&mut hasher);

let local_hash: u64 = hasher.finish();

which means that it becomes unpredictable only in a presence of a cycle, since otherwise MIR would be available for inlining anyway. I think this is a reasonable trade-off to make.

I see. In that case I think it is fine.

jackh726 · 2021-06-01T02:06:10Z

@cjgillot any idea on a better reviewer?

jackh726 · 2021-06-24T14:45:05Z

Let's r? @bjorn3 😉

bjorn3 · 2021-06-24T14:59:12Z

The inliner isn't enabled by default. Any idea how to do a perf run for this?

Dylan-DPC-zz · 2021-07-12T17:54:00Z

The inliner isn't enabled by default. Any idea how to do a perf run for this?

cc @Mark-Simulacrum

Mark-Simulacrum · 2021-07-12T19:07:27Z

Generally speaking there's no trivial way - my recommendation is to edit a PR to change the defaults and then run a try build with that, then revert anything on the PR and run a try build with that. You want to make sure the two try builds have the same parent commit (from master). After that, perf runs for each of the try builds and compare the try build hashes in the UI.

bors · 2021-09-09T00:00:11Z

☔ The latest upstream changes (presumably #80522) made this pull request unmergeable. Please resolve the merge conflicts.

bjorn3 · 2021-09-16T14:37:16Z

@cjgillot I would like to see a perf run first before merging this. #85321 (comment) suggests how to do this.

JohnCSimon · 2021-10-03T06:40:05Z

triage: merge conflicts

bors · 2021-12-23T02:13:28Z

☔ The latest upstream changes (presumably #90408) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2022-02-18T22:02:50Z

☔ The latest upstream changes (presumably #94121) made this pull request unmergeable. Please resolve the merge conflicts.

Dylan-DPC · 2022-02-19T02:06:51Z

Marking as blocked on #82280

@cjgillot the blocked experimental PR has been closed, so is this still blocked?

pierwill · 2022-03-16T19:29:00Z

Marking as blocked on #82280

@cjgillot the blocked experimental PR has been closed, so is this still blocked?

Friendly, non-urgent ping for @cjgillot, since some work on #90317 is blocked here. :)

cjgillot · 2022-04-02T17:12:32Z

I recall that this PR was blocked on a perf run with inliner enabled. Enabling the inliner by default is not possible yet, because of 2 unresolved type normalization bugs.

#92233 will remove this inlining tie breaker. I see two options:

merge Remove ordering traits from HirId #92233, and merge this PR when the inliner is on by default;
merge this PR and adapt Remove ordering traits from HirId #92233.

compiler/rustc_mir_transform/src/inline.rs

bjorn3 · 2022-04-02T18:33:29Z

I think merging this PR first is best so as to not break MIR inlining even more. r=me with or without the nit fixed.

bjorn3 · 2022-04-03T07:49:32Z

@bors r+

bors · 2022-04-03T07:49:33Z

📌 Commit 297dde9 has been approved by bjorn3

bors · 2022-04-03T07:53:13Z

⌛ Testing commit 297dde9 with merge ec7b753...

bors · 2022-04-03T10:33:46Z

☀️ Test successful - checks-actions
Approved by: bjorn3
Pushing ec7b753 to master...

rust-timer · 2022-04-03T11:47:14Z

Finished benchmarking commit (ec7b753): comparison url.

Summary:

Primary benchmarks: no relevant changes found. 1 results were found to be statistically significant but too small to be relevant.
Secondary benchmarks: no relevant changes found. 1 results were found to be statistically significant but too small to be relevant.

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count¹	0	0	1	1	1
mean²	N/A	N/A	-0.3%	-3.3%	-0.3%
max	N/A	N/A	-0.3%	-3.3%	0.0%

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

number of relevant changes ↩
the arithmetic mean of the percent change ↩

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cjgillot force-pushed the mir-cycle branch from bf23f4d to 5a2c4fe Compare October 22, 2021 21:30

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Remove ordering traits from HirId #92233

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compiler/rustc_mir_transform/src/inline.rs Outdated Show resolved Hide resolved

cjgillot added 3 commits April 2, 2022 23:23

Use DefPathHash instead of HirId to break cycles.

e1b36f5

Use only local hash.

2d3d9b2

Less manipulation of the callee_def_id.

297dde9

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Add query to check for impossible predicates, use it to skip NoopMethodCall and Inline #95398

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Use DefPathHash instead of HirId to break inlining cycles. #85321

Use DefPathHash instead of HirId to break inlining cycles. #85321

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	parent.hash(&mut hasher);

	let DisambiguatedDefPathData { ref data, disambiguator } = self.disambiguated_data;

	std::mem::discriminant(data).hash(&mut hasher);
	if let Some(name) = data.get_opt_name() {
	// Get a stable hash by considering the symbol chars rather than
	// the symbol index.
	name.as_str().hash(&mut hasher);
	}

	disambiguator.hash(&mut hasher);

	let local_hash: u64 = hasher.finish();

Use DefPathHash instead of HirId to break inlining cycles. #85321

Use DefPathHash instead of HirId to break inlining cycles. #85321

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