Represent MIR composite debuginfo as projections instead of aggregates

[cjgillot]

Composite debuginfo for MIR is currently represented as

debug name => Type { projection1 => place1, projection2 => place2 };

ie. a single VarDebugInfo object with that name, and its value a VarDebugInfoContents::Composite.

This PR proposes to reverse the representation to be

debug name.projection1 => place1;
debug name.projection2 => place2;

ie. multiple VarDebugInfo objects with each their projection.

This simplifies the handling of composite debuginfo by the compiler by avoiding weird nesting.

Based on #115139

[rustbot]

r? @davidtwco

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[davidtwco]

@bors r+

[bors]

📌 Commit 1cac0ae5e259e288045675553ff5d54526892d30 has been approved by davidtwco

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1cac0ae5e259e288045675553ff5d54526892d30 with merge 94aacbc28dfbcef5b8cd22ed74718ea99015cfb8...

[bors]

💔 Test failed - checks-actions

[cjgillot]

No idea what failed.
@bors retry

[bors]

⌛ Testing commit 1cac0ae5e259e288045675553ff5d54526892d30 with merge 94c65b155804f7dbc7889935b6dea74e1318b08a...

[davidtwco]

Are you waiting on another review?

[cjgillot]

I changed the implementation a little to simplify formatting. See the first commit. Could you take a second look?
For the test failures, they appear to come from 2 different builders are 2 different places, so I have no reason to suspect them to be real.

[davidtwco]

@bors r+

[bors]

📌 Commit 26c48e6 has been approved by davidtwco

It is now in the queue for this repository.

[bors]

⌛ Testing commit 26c48e6 with merge a5b2ac6...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing a5b2ac6 to master...

[rust-timer]

Finished benchmarking commit (a5b2ac6): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.9%	[3.9%, 3.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	2
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.3%]	4
All ❌✅ (primary)	1.1%	[-0.3%, 3.9%]	3

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.2%	[0.5%, 3.3%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-8.1%	[-12.0%, -4.3%]	2
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-1.9%	[-12.0%, 3.3%]	5

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.9%	[-0.9%, -0.9%]	1
All ❌✅ (primary)	3.0%	[3.0%, 3.0%]	1

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.6%]	71
Regressions ❌ (secondary)	0.2%	[0.0%, 1.7%]	36
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.0%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.6%, 0.6%]	76

Bootstrap: 627.776s -> 629.849s (0.33%)
Artifact size: 317.73 MiB -> 317.80 MiB (0.02%)

[nnethercote]

@cjgillot: this change regressed binary size by a small amount across many benchmarks. Was that expected? Could it be avoided?

[cjgillot]

From the stats, I understand that the regression is mostly in metadata size. That can be explained by the data structure change. The representation is more general, but unfortunately less compact: it repeats the type several times. I'm not sure how to shrink it back though.