Introduce debuginfo to statements in MIR #142771

dianqk · 2025-06-20T07:16:46Z

The PR introduces support for debug information within dead statements. Currently, only the reference statement is supported, which is sufficient to fix #128081.

I don't modify Stable MIR, as I don't think we need debug information when using it.

This PR represents the debug information for the dead reference statement via #dbg_value. For example, let _foo_b = &foo.b becomes #dbg_value(ptr %foo, !22, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value), !26). You can see this here: https://rust.godbolt.org/z/d43js6adv.

The general principle for handling debug information is to never provide less debug information than the optimized LLVM IR.

The current rules for dropping debug information in this PR are:

If the LLVM IR cannot represent a reference address, it's replaced with poison or simply dropped. For example, see: https://rust.godbolt.org/z/shGqPec8W. I'm using poison in all such cases now.
All debuginfos is dropped when merging multiple successor BBs. An example is available here: https://rust.godbolt.org/z/TE1q3Wq6M.

I haven't drop debuginfos in MatchBranchSimplification, because LLVM also pick one branch for it.

For the perf result:

I expected this to introduce some regressions; however, the results mixed the effects of inlining. Looking at the doc profile, this is a clear optimization. One potential regression I'm investigating is serde-1.0.219-debug-full.

compiler/rustc_middle/src/mir/mod.rs

dianqk · 2025-06-20T10:52:57Z

@bors2 try @rust-timer queue

rust-bors · 2025-06-20T10:53:00Z

⌛ Trying commit 51576e7 with merge eb83156…

To cancel the try build, run the command @bors2 try cancel.

Introduce debuginfo to statements in MIR Not ready for reviewing. Something known: - [ ] Retain debuginfo when concatenating bbs - [ ] Document about when to drop debuginfos (don't be worse than the optimized LLVM IR) - [ ] Missing tests r? ghost

rust-bors · 2025-06-20T13:23:31Z

☀️ Try build successful (CI)
Build commit: eb83156 (eb83156169ae3bbdd1385d498455dbc44283f5ff, parent: 18491d5be00eb3ed2f1ccee2ac5b792694f2a7a0)

rust-timer · 2025-06-20T23:32:50Z

Finished benchmarking commit (eb83156): 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)	0.9%	[0.2%, 14.5%]	51
Regressions ❌ (secondary)	0.8%	[0.1%, 1.6%]	52
Improvements ✅ (primary)	-0.4%	[-2.1%, -0.2%]	35
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.2%]	39
All ❌✅ (primary)	0.4%	[-2.1%, 14.5%]	86

Max RSS (memory usage)

Results (primary 2.3%, secondary 3.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[0.8%, 4.7%]	54
Regressions ❌ (secondary)	3.6%	[0.7%, 7.4%]	38
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-3.3%, -3.3%]	1
All ❌✅ (primary)	2.3%	[0.8%, 4.7%]	54

Cycles

Results (primary 3.2%, secondary 2.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.9%	[1.1%, 14.4%]	7
Regressions ❌ (secondary)	2.5%	[2.0%, 3.3%]	3
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.2%	[-2.0%, 14.4%]	8

Binary size

Results (primary 0.5%, secondary 0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.0%, 2.2%]	123
Regressions ❌ (secondary)	1.1%	[0.0%, 3.5%]	59
Improvements ✅ (primary)	-0.9%	[-6.5%, -0.1%]	11
Improvements ✅ (secondary)	-0.5%	[-13.0%, -0.1%]	55
All ❌✅ (primary)	0.5%	[-6.5%, 2.2%]	134

Bootstrap: 691.482s -> 692.445s (0.14%)
Artifact size: 371.94 MiB -> 372.12 MiB (0.05%)

dianqk · 2025-06-21T02:59:42Z

@bors2 try @rust-timer queue

rust-bors · 2025-06-21T02:59:46Z

⌛ Trying commit e72c3ae with merge 77d5c6a…

To cancel the try build, run the command @bors2 try cancel.

Introduce debuginfo to statements in MIR Not ready for reviewing. Something known: - [ ] Retain debuginfo when concatenating bbs - [ ] Document about when to drop debuginfos (don't be worse than the optimized LLVM IR) - [ ] Missing tests r? ghost

rust-bors · 2025-06-21T05:28:30Z

☀️ Try build successful (CI)
Build commit: 77d5c6a (77d5c6a20a77fabdee3790a412618b82178e9ab4, parent: 15c701fbc995eb6c5b3a86021c18185f8eee020d)

rust-timer · 2025-06-21T09:26:06Z

Finished benchmarking commit (77d5c6a): 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)	0.9%	[0.1%, 14.5%]	53
Regressions ❌ (secondary)	0.8%	[0.1%, 1.6%]	50
Improvements ✅ (primary)	-0.4%	[-3.0%, -0.2%]	46
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.2%]	41
All ❌✅ (primary)	0.3%	[-3.0%, 14.5%]	99

Max RSS (memory usage)

Results (primary 2.4%, secondary 3.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[0.5%, 5.7%]	38
Regressions ❌ (secondary)	4.2%	[1.3%, 6.2%]	25
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.7%	[-3.0%, -1.1%]	4
All ❌✅ (primary)	2.4%	[0.5%, 5.7%]	38

Cycles

Results (primary 2.1%, secondary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[0.8%, 14.1%]	10
Regressions ❌ (secondary)	3.3%	[2.3%, 4.3%]	2
Improvements ✅ (primary)	-2.3%	[-2.9%, -1.7%]	2
Improvements ✅ (secondary)	-2.2%	[-2.6%, -1.9%]	2
All ❌✅ (primary)	2.1%	[-2.9%, 14.1%]	12

Binary size

Results (primary 0.5%, secondary 0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.0%, 3.0%]	122
Regressions ❌ (secondary)	1.1%	[0.0%, 3.5%]	59
Improvements ✅ (primary)	-0.7%	[-4.9%, -0.0%]	15
Improvements ✅ (secondary)	-0.5%	[-12.7%, -0.0%]	55
All ❌✅ (primary)	0.5%	[-4.9%, 3.0%]	137

Bootstrap: 690.617s -> 691.47s (0.12%)
Artifact size: 371.83 MiB -> 372.01 MiB (0.05%)

bors · 2025-06-25T18:27:45Z

☔ The latest upstream changes (presumably #142870) made this pull request unmergeable. Please resolve the merge conflicts.

rust-bors · 2025-07-14T03:36:18Z

⌛ Trying commit 910cac7 with merge 91b1a3e…

To cancel the try build, run the command @bors2 try cancel.

rust-bors · 2025-07-14T05:52:04Z

☀️ Try build successful (CI)
Build commit: 91b1a3e (91b1a3e3ba6355bd6e26493f0e529bec9b45c616, parent: 9c3064e131f4939cc95a29bb11413c49bbda1491)

rust-timer · 2025-07-14T07:38:08Z

Finished benchmarking commit (91b1a3e): 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)	0.6%	[0.2%, 3.2%]	58
Regressions ❌ (secondary)	0.7%	[0.1%, 1.8%]	67
Improvements ✅ (primary)	-0.3%	[-1.9%, -0.1%]	65
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.0%]	67
All ❌✅ (primary)	0.1%	[-1.9%, 3.2%]	123

Max RSS (memory usage)

Results (primary 2.1%, secondary 2.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[0.6%, 4.7%]	55
Regressions ❌ (secondary)	3.7%	[1.3%, 6.1%]	33
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-3.5%, -1.1%]	5
All ❌✅ (primary)	2.1%	[0.6%, 4.7%]	55

Cycles

Results (primary 2.6%, secondary 5.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	5.8%	[2.6%, 10.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Binary size

Results (primary 0.5%, secondary 0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.0%, 2.0%]	116
Regressions ❌ (secondary)	1.1%	[0.0%, 3.5%]	57
Improvements ✅ (primary)	-0.7%	[-6.3%, -0.1%]	15
Improvements ✅ (secondary)	-0.5%	[-12.7%, -0.1%]	57
All ❌✅ (primary)	0.5%	[-6.3%, 2.0%]	131

Bootstrap: 462.57s -> 466.721s (0.90%)
Artifact size: 374.65 MiB -> 374.70 MiB (0.01%)

dianqk · 2025-07-16T01:23:42Z

@bors2 try @rust-timer queue

rust-bors · 2025-07-16T01:23:46Z

⌛ Trying commit 3a1ab06 with merge 9f0ec68…

To cancel the try build, run the command @bors2 try cancel.

Introduce debuginfo to statements in MIR The PR introduces support for debug information within dead statements. Currently, only the reference statement is supported, which is sufficient to fix #128081. I don't modify Stable MIR, as I don't think we need debug information when using it. This PR represents the debug information for the dead reference statement via `#dbg_value`. For example, `let _foo_b = &foo.b` becomes `#dbg_value(ptr %foo, !22, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value), !26)`. You can see this here: https://rust.godbolt.org/z/d43js6adv. The general principle for handling debug information is to never provide less debug information than the optimized LLVM IR. The current rules for dropping debug information in this PR are: - If the LLVM IR cannot represent a reference address, it's replaced with poison or simply dropped. For example, see: https://rust.godbolt.org/z/shGqPec8W. I'm using poison in all such cases now. - All debuginfos is dropped when merging multiple successor BBs. An example is available here: https://rust.godbolt.org/z/TE1q3Wq6M. > I haven't drop debuginfos in `MatchBranchSimplification`, because LLVM also pick one branch for it. For [the perf result](#142771 (comment)): I expected this to introduce some regressions; however, the results mixed the effects of inlining. Looking at the doc profile, this is a clear optimization. One potential regression I'm investigating is `serde-1.0.219-debug-full`.

rust-bors · 2025-07-16T03:41:39Z

☀️ Try build successful (CI)
Build commit: 9f0ec68 (9f0ec689777f59dbfda5167d583b5785427ad06e, parent: 3014e79f9c8d5510ea7b3a3b70d171d0948b1e96)

rust-timer · 2025-07-16T04:52:33Z

Finished benchmarking commit (9f0ec68): 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)	0.6%	[0.2%, 3.3%]	57
Regressions ❌ (secondary)	0.8%	[0.1%, 2.0%]	60
Improvements ✅ (primary)	-0.4%	[-1.2%, -0.1%]	79
Improvements ✅ (secondary)	-0.6%	[-2.0%, -0.0%]	73
All ❌✅ (primary)	0.0%	[-1.2%, 3.3%]	136

Max RSS (memory usage)

Results (primary 2.3%, secondary 3.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[0.7%, 8.2%]	68
Regressions ❌ (secondary)	3.7%	[1.1%, 6.3%]	33
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-3.7%, -1.7%]	4
All ❌✅ (primary)	2.3%	[0.7%, 8.2%]	68

Cycles

Results (primary -0.3%, secondary -2.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-3.3%	[-3.3%, -3.3%]	1
Improvements ✅ (secondary)	-5.5%	[-6.1%, -5.0%]	2
All ❌✅ (primary)	-0.3%	[-3.3%, 2.8%]	2

Binary size

Results (primary 0.5%, secondary 0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.1%, 2.0%]	112
Regressions ❌ (secondary)	1.1%	[0.0%, 3.5%]	58
Improvements ✅ (primary)	-0.5%	[-6.5%, -0.1%]	20
Improvements ✅ (secondary)	-0.5%	[-12.7%, -0.1%]	56
All ❌✅ (primary)	0.5%	[-6.5%, 2.0%]	132

Bootstrap: 465.928s -> 464.642s (-0.28%)
Artifact size: 374.79 MiB -> 374.37 MiB (-0.11%)

dianqk · 2025-07-17T12:14:29Z

@bors2 try @rust-timer queue

rust-timer · 2025-07-17T12:14:32Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

rust-bors · 2025-07-17T12:14:34Z

⌛ Trying commit d520046 with merge 63e6243…

To cancel the try build, run the command @bors2 try cancel.

Introduce debuginfo to statements in MIR The PR introduces support for debug information within dead statements. Currently, only the reference statement is supported, which is sufficient to fix #128081. I don't modify Stable MIR, as I don't think we need debug information when using it. This PR represents the debug information for the dead reference statement via `#dbg_value`. For example, `let _foo_b = &foo.b` becomes `#dbg_value(ptr %foo, !22, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value), !26)`. You can see this here: https://rust.godbolt.org/z/d43js6adv. The general principle for handling debug information is to never provide less debug information than the optimized LLVM IR. The current rules for dropping debug information in this PR are: - If the LLVM IR cannot represent a reference address, it's replaced with poison or simply dropped. For example, see: https://rust.godbolt.org/z/shGqPec8W. I'm using poison in all such cases now. - All debuginfos is dropped when merging multiple successor BBs. An example is available here: https://rust.godbolt.org/z/TE1q3Wq6M. > I haven't drop debuginfos in `MatchBranchSimplification`, because LLVM also pick one branch for it. For [the perf result](#142771 (comment)): I expected this to introduce some regressions; however, the results mixed the effects of inlining. Looking at the doc profile, this is a clear optimization. One potential regression I'm investigating is `serde-1.0.219-debug-full`.

rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 20, 2025

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dianqk force-pushed the mir-stmt-debuginfo branch from 15c968a to 6b013d4 Compare June 20, 2025 08:01

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bjorn3 reviewed Jun 20, 2025

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compiler/rustc_middle/src/mir/mod.rs Show resolved Hide resolved

dianqk force-pushed the mir-stmt-debuginfo branch from 6b013d4 to 51576e7 Compare June 20, 2025 09:23

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 20, 2025

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 20, 2025

dianqk force-pushed the mir-stmt-debuginfo branch from 51576e7 to e72c3ae Compare June 21, 2025 02:31

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 14, 2025

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dianqk 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 Jul 14, 2025

dianqk added 4 commits July 15, 2025 13:16

mir-opt: Eliminate dead ref statements

ef38bc4

codegen: Generate dbg_value for the ref statement

1d27c0d

simplifycfg: Preserve debuginfos when merging bbs

a6dccd7

mir-opt: Eliminate trivial unnecessary storage annotations

3a1ab06

dianqk force-pushed the mir-stmt-debuginfo branch from 910cac7 to 3a1ab06 Compare July 16, 2025 00:03

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test: Visit debuginfos when needed

d520046

dianqk force-pushed the mir-stmt-debuginfo branch from ac6c6e9 to d520046 Compare July 17, 2025 10:35

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 17, 2025

Introduce debuginfo to statements in MIR #142771

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Introduce debuginfo to statements in MIR #142771

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

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

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

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

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