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Allow MIR debuginfo to point to a variable's address #111440

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merged 8 commits into from
May 14, 2023

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cjgillot
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MIR optimizations currently do not to operate on borrowed locals.

When enabling #106285, many borrows will be left as-is because they are used in debuginfo. This pass allows to replace this pattern directly in MIR debuginfo:

a => _1
_1 = &raw? mut? _2

becomes

a => &_2
// No statement to borrow _2.

This pass is implemented as a drive-by in ReferencePropagation MIR pass.

This transformation allows following following MIR opts to treat _2 as an unborrowed local, and optimize it as such, even in builds with debuginfo.

In codegen, when encountering a => &..&_2, we create a list of allocas:

store ptr %_2.dbg.spill, ptr %a.ref0.dbg.spill
store ptr %a.ref0.dbg.spill, ptr %a.ref1.dbg.spill
...
call void @llvm.dbg.declare(metadata ptr %a.ref{n}.dbg.spill, /* ... */)

Caveat: this transformation looses the exact type, we do not differentiate a as a immutable, mutable reference or a raw pointer. Everything is declared to *mut to codegen. I'm not convinced this is a blocker.

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rustbot commented May 10, 2023

r? @jackh726

(rustbot has picked a reviewer for you, use r? to override)

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 10, 2023
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rustbot commented May 10, 2023

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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r? mir-opt

@rustbot rustbot assigned oli-obk and unassigned jackh726 May 11, 2023
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oli-obk commented May 12, 2023

r=me with the debuginfo test extended to have a case where references is 2 or more.

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bors commented May 12, 2023

☔ The latest upstream changes (presumably #111493) made this pull request unmergeable. Please resolve the merge conflicts.

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@oli-obk to simplify triggering the case with 2 or more references, I added 1965237 too. Is the debuginfo test sufficient, or do you need me to check more than 2?

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oli-obk commented May 13, 2023

That's perfect, thanks!

@bors r+

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bors commented May 13, 2023

📌 Commit 8fb888d has been approved by oli-obk

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 13, 2023
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bors commented May 14, 2023

⌛ Testing commit 8fb888d with merge bc88895...

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bors commented May 14, 2023

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing bc88895 to master...

@bors bors added the merged-by-bors This PR was explicitly merged by bors. label May 14, 2023
@bors bors merged commit bc88895 into rust-lang:master May 14, 2023
@rustbot rustbot added this to the 1.71.0 milestone May 14, 2023
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Finished benchmarking commit (bc88895): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

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)
- - 0
Regressions ❌
(secondary)
- - 0
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
-2.6% [-2.6%, -2.6%] 1
All ❌✅ (primary) - - 0

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)
4.7% [4.2%, 5.5%] 3
Regressions ❌
(secondary)
2.3% [2.1%, 2.5%] 2
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
-2.0% [-2.0%, -2.0%] 1
All ❌✅ (primary) 4.7% [4.2%, 5.5%] 3

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)
- - 0
Regressions ❌
(secondary)
- - 0
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
-3.2% [-3.2%, -3.2%] 1
All ❌✅ (primary) - - 0

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%, 0.2%] 59
Regressions ❌
(secondary)
0.1% [0.0%, 0.2%] 32
Improvements ✅
(primary)
- - 0
Improvements ✅
(secondary)
- - 0
All ❌✅ (primary) 0.1% [0.0%, 0.2%] 59

Bootstrap: 661.349s -> 660.613s (-0.11%)

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6 participants