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.

mir: Do not modify NonUse in `super_projection_elem` Split from rust-lang#142771. r? cjgillot

Rollup merge of #144998 - dianqk:visit-no-use-proj, r=cjgillot mir: Do not modify NonUse in `super_projection_elem` Split from #142771. r? cjgillot

Rollup merge of #144996 - dianqk:simplifycfg-collapse_goto_chain-changed, r=cjgillot simplifycfg: Mark as changed when start is modified in collapse goto chain Split from #142771. r? cjgillot

cjgillot

This is looking very good thanks! I'm not certain I'm the best reviewer for the codegen part, so I just left a few questions.

cjgillot · 2025-08-10T16:28:43Z

compiler/rustc_middle/src/mir/visit.rs

@@ -312,8 +321,11 @@ macro_rules! make_mir_visitor {
                    index += 1;
                }

+                let location = Location { block, statement_index: index };
+                for debuginfo in after_last_stmt_debuginfos as & $($mutability)? [_] {


in &$(mutability)? after_last_stmt_debuginfos?

Dereferencing does not happen with this syntax.

cjgillot · 2025-08-10T16:50:49Z

compiler/rustc_mir_dataflow/src/impls/liveness.rs

+    ) -> Option<Place<'tcx>> {
+        // Compute the place that we are storing to, if any
+        let destination = match stmt_kind {
+            StatementKind::Assign(box (place, rvalue)) => (rvalue.is_safe_to_remove()


Nit, easier for me to understand:

StatementKind::Assign(box (_, rvalue)) if !rvalue.is_safe_to_remove() => None, StatementKind::Assign(box (place, _)) if stmt_kind.as_debuginfo().is_some() => Some(*place),

and then proceed to reasoning according to debuginfo_locals.

This is a little scary for me, their order can be accidentally re-ordered.

cjgillot · 2025-08-10T17:11:26Z

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -160,19 +160,23 @@ impl<'ll> DebugInfoBuilderMethods for Builder<'_, 'll, '_> {
        &mut self,
        dbg_var: &'ll DIVariable,
        dbg_loc: &'ll DILocation,
-        variable_alloca: Self::Value,
+        is_declared: bool,
+        val: Self::Value,


Should we just have 2 methods? a dbg_var_addr and a dbg_var_value?

cjgillot · 2025-08-10T17:17:46Z

compiler/rustc_codegen_ssa/src/mir/statement.rs

+    pub(crate) fn codegen_stmt_debuginfo(&mut self, bx: &mut Bx, debuginfo: &StmtDebugInfo<'tcx>) {
+        match debuginfo {
+            StmtDebugInfo::AssignRef(dest, place) => {
+                let place_ref = match self.locals[place.local] {


This local is poorly named. It represents place.local, not the while place. local_ref?

cjgillot · 2025-08-10T17:19:55Z

compiler/rustc_codegen_ssa/src/mir/statement.rs

+                        (Some(place_ref), false) => {
+                            (place_ref.val, place_ref.layout, place.projection.as_slice())
+                        }
+                        (Some(place_ref), true) => {
+                            let projected_ty =
+                                place_ref.layout.ty.builtin_deref(true).unwrap_or_else(|| {
+                                    bug!("deref of non-pointer {:?}", place_ref)
+                                });
+                            let layout = bx.cx().layout_of(projected_ty);
+                            (place_ref.val, layout, &place.projection[1..])
+                        }


I don't understand the logic here. If this is an indirect place, we change the type but we don't change the val?

The layout is used to calculate offset with the projection. The function name should be debug_new_ref_to_local , not debug_new_value_to_local. I'm looking a better logic.

fcd691f and 48dff89.

cjgillot · 2025-08-10T17:32:49Z

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

+        }
+    }
+
+    fn debug_new_value_to_local_as_var(


Should this be inlined? This should allow to compute offsets only once outside the loop.

cjgillot · 2025-08-10T17:36:25Z

tests/codegen-llvm/debuginfo-dse.rs

+    let ref_v1 = &ref_foo.1;
+    let ref_v2 = &ref_foo.2;
+    ref_foo.0
+}


Do you have an example where we take the address of an Index projection?

cjgillot · 2025-08-10T17:37:44Z

compiler/rustc_mir_transform/src/simplify.rs

+                terminator.successors_mut(|successor| {
+                    self.collapse_goto_chain(successor, &mut changed);
+                });
+                if changed && let Some(identical_succ) = identical_succ {


Should we introduce a more localized chain_changed variable?

The code has been removed in 8702b70.

cjgillot · 2025-08-10T17:48:25Z

compiler/rustc_mir_transform/src/simplify.rs

+                let mut succ_debuginfos =
+                    self.basic_blocks[succ].after_last_stmt_debuginfos.clone();
+                self.basic_blocks[current].after_last_stmt_debuginfos.extend(&mut succ_debuginfos);
+            }


Is there a way to look at pred_count to avoid a clone?

8702b70. I don't think I should move debuginfos before the terminator. The new change should be clearer and easier to maintain, and it also fixes a case where debuginfos was lost.

cjgillot · 2025-08-10T17:53:33Z

compiler/rustc_mir_transform/src/dead_store_elimination.rs

 use crate::util::is_within_packed;

 /// Performs the optimization on the body
 ///
 /// The `borrowed` set must be a `DenseBitSet` of all the locals that are ever borrowed in this
 /// body. It can be generated via the [`borrowed_locals`] function.
-fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> bool {


Could you doc-comment what this bool is?

dianqk · 2025-08-17T14:28:45Z

Sorry I haven't replied to all of them. A little busy...

rustbot · 2025-08-28T14:28:46Z

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.

Rename `place_ref` to `local_ref`.

Add `debug_new_val_to_local` Inline `debug_new_value_to_local_as_var`

`&_1` is only valid if the pass mode is indirect.

Move all debuginfos to the last basic block if each bb has only one predecessor.

Add a test case for the index projection.

Add comments

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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bjorn3 reviewed Jun 20, 2025

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

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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 removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 21, 2025

GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this pull request Aug 6, 2025

Rollup merge of rust-lang#144998 - dianqk:visit-no-use-proj, r=cjgillot

c152aa8

mir: Do not modify NonUse in `super_projection_elem` Split from rust-lang#142771. r? cjgillot

rust-timer added a commit that referenced this pull request Aug 7, 2025

Unrolled build for #144998

41b645e

Rollup merge of #144998 - dianqk:visit-no-use-proj, r=cjgillot mir: Do not modify NonUse in `super_projection_elem` Split from #142771. r? cjgillot

dianqk force-pushed the mir-stmt-debuginfo branch 2 times, most recently from 7a0c75b to 458ad93 Compare August 10, 2025 14:48

cjgillot reviewed Aug 10, 2025

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dianqk added 5 commits August 28, 2025 22:16

mir-opt: Eliminate dead ref statements

83c20f3

codegen: Generate dbg_value for the ref statement

678cd04

simplifycfg: Preserve debuginfos when merging bbs

97ddf91

mir-opt: Eliminate trivial unnecessary storage annotations

e3ecfa3

mir-opt: Eliminate dead statements even if they are used by debuginfos

ecf4538

dianqk force-pushed the mir-stmt-debuginfo branch from a4ce010 to b0d20dd Compare August 28, 2025 14:28

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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 Aug 28, 2025

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dianqk added 3 commits August 29, 2025 08:44

fixup! codegen: Generate dbg_value for the ref statement

de65379

fixup! codegen: Generate dbg_value for the ref statement

635ae63

Rename `place_ref` to `local_ref`.

fixup! codegen: Generate dbg_value for the ref statement

0a0b3c1

Add `debug_new_val_to_local` Inline `debug_new_value_to_local_as_var`

dianqk force-pushed the mir-stmt-debuginfo branch from 39c5bf2 to 450c929 Compare August 29, 2025 00:45

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dianqk added 4 commits August 29, 2025 21:35

fixup! codegen: Generate dbg_value for the ref statement

1e903d1

`&_1` is only valid if the pass mode is indirect.

fixup! simplifycfg: Preserve debuginfos when merging bbs

6e997fa

Move all debuginfos to the last basic block if each bb has only one predecessor.

fixup! codegen: Generate dbg_value for the ref statement

0e8c053

Add a test case for the index projection.

fixup! mir-opt: Eliminate trivial unnecessary storage annotations

84d42cf

Add comments

dianqk force-pushed the mir-stmt-debuginfo branch from 450c929 to 84d42cf Compare August 29, 2025 13:38

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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dianqk commented Jun 20, 2025 •

edited by rustbot

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