Implement opt-bisect-limit for MIR by sgasho · Pull Request #152474 · rust-lang/rust

sgasho · 2026-02-11T09:11:36Z

Enable bisecting MIR optimization passes to enhance debuggability.

discussions on zulip: https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/MIR.20dump.20the.20pass.20names/with/573219207

Check it works

Sample code

fn abs(num: isize) -> usize {
    if num < 0 { -num as usize } else { num as usize }
}

fn main() {
    println!("{}", abs(-10));
}

Output

rustc +mir -Zmir-opt-bisect-limit=30 src/main.rs
BISECT: running pass (1) CheckAlignment on main[89d5]::main
BISECT: running pass (2) CheckNull on main[89d5]::main
BISECT: running pass (3) CheckEnums on main[89d5]::main
BISECT: running pass (4) LowerSliceLenCalls on main[89d5]::main
BISECT: running pass (5) InstSimplify-before-inline on main[89d5]::main
BISECT: running pass (6) ForceInline on main[89d5]::main
BISECT: running pass (7) RemoveStorageMarkers on main[89d5]::main
BISECT: running pass (8) RemoveZsts on main[89d5]::main
BISECT: running pass (9) RemoveUnneededDrops on main[89d5]::main
BISECT: running pass (10) UnreachableEnumBranching on main[89d5]::main
BISECT: running pass (11) SimplifyCfg-after-unreachable-enum-branching on main[89d5]::main
BISECT: running pass (12) InstSimplify-after-simplifycfg on main[89d5]::main
BISECT: running pass (13) SimplifyConstCondition-after-inst-simplify on main[89d5]::main
BISECT: running pass (14) SimplifyLocals-before-const-prop on main[89d5]::main
BISECT: running pass (15) SimplifyLocals-after-value-numbering on main[89d5]::main
BISECT: running pass (16) MatchBranchSimplification on main[89d5]::main
BISECT: running pass (17) SingleUseConsts on main[89d5]::main
BISECT: running pass (18) SimplifyConstCondition-after-const-prop on main[89d5]::main
BISECT: running pass (19) SimplifyConstCondition-final on main[89d5]::main
BISECT: running pass (20) RemoveNoopLandingPads on main[89d5]::main
BISECT: running pass (21) SimplifyCfg-final on main[89d5]::main
BISECT: running pass (22) CopyProp on main[89d5]::main
BISECT: running pass (23) SimplifyLocals-final on main[89d5]::main
BISECT: running pass (24) AddCallGuards on main[89d5]::main
BISECT: running pass (25) PreCodegen on main[89d5]::main
BISECT: running pass (26) CheckAlignment on main[89d5]::abs
BISECT: running pass (27) CheckNull on main[89d5]::abs
BISECT: running pass (28) CheckEnums on main[89d5]::abs
BISECT: running pass (29) LowerSliceLenCalls on main[89d5]::abs
BISECT: running pass (30) InstSimplify-before-inline on main[89d5]::abs
BISECT: NOT running pass (31) ForceInline on main[89d5]::abs
BISECT: NOT running pass (32) RemoveStorageMarkers on main[89d5]::abs
BISECT: NOT running pass (33) RemoveZsts on main[89d5]::abs
BISECT: NOT running pass (34) RemoveUnneededDrops on main[89d5]::abs
BISECT: NOT running pass (35) UnreachableEnumBranching on main[89d5]::abs
BISECT: NOT running pass (36) SimplifyCfg-after-unreachable-enum-branching on main[89d5]::abs
BISECT: NOT running pass (37) InstSimplify-after-simplifycfg on main[89d5]::abs
BISECT: NOT running pass (38) SimplifyConstCondition-after-inst-simplify on main[89d5]::abs
BISECT: NOT running pass (39) SimplifyLocals-before-const-prop on main[89d5]::abs
BISECT: NOT running pass (40) SimplifyLocals-after-value-numbering on main[89d5]::abs
BISECT: NOT running pass (41) MatchBranchSimplification on main[89d5]::abs
BISECT: NOT running pass (42) SingleUseConsts on main[89d5]::abs
BISECT: NOT running pass (43) SimplifyConstCondition-after-const-prop on main[89d5]::abs
BISECT: NOT running pass (44) SimplifyConstCondition-final on main[89d5]::abs
BISECT: NOT running pass (45) RemoveNoopLandingPads on main[89d5]::abs
BISECT: NOT running pass (46) SimplifyCfg-final on main[89d5]::abs
BISECT: NOT running pass (47) CopyProp on main[89d5]::abs
BISECT: NOT running pass (48) SimplifyLocals-final on main[89d5]::abs
BISECT: NOT running pass (49) AddCallGuards on main[89d5]::abs
BISECT: NOT running pass (50) PreCodegen on main[89d5]::abs

r? @saethlin

rustbot · 2026-02-11T09:11:41Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

bjorn3 · 2026-02-11T09:27:14Z

compiler/rustc_session/src/session.rs

+    ///
+    /// Used by `-Zmir-opt-bisect-limit` to assign an index to each
+    /// optimization-pass execution candidate during this compilation.
+    pub mir_opt_bisect_eval_count: AtomicUsize,


We used to have optimization fuel in the session, but removed it in #115293

bjorn3 · 2026-02-11T12:53:16Z

compiler/rustc_mir_transform/src/pass_manager.rs

-    limit: usize,
-    pass: &P,
-) -> bool
+static MIR_OPT_BISECT_COUNT: AtomicUsize = AtomicUsize::new(0);


This is actually worse. This now not only breaks incr comp, but also breaks running multiple rustc sessions in the same process. Optimization fuel is not really compatible with incr comp. Perhaps we should just warn against enabling incr comp when optimization fuel is used?

We used to have optimization fuel in the session, but removed it in #115293

So, with this comment, should we remove the counter from Session while not making this feature broken when running multiple sessions in the same process?

Perhaps we should just warn against enabling incr comp when optimization fuel is used?

Thanks, this makes sense. I'll also add a warning when -Zmir-opt-bisect-limit is used together with incremental compilation.

should we remove the counter from Session while not making this feature broken when running multiple sessions in the same process?

If so, I would like to know some possible ways to achieve it.

It should definitively not be a static. Putting it in the Session breaks incr comp, but I don't think there is any easy way around that.

saethlin · 2026-02-13T22:21:31Z

This might have weird interactions with -Zthreads, but I don't see a coherent way to resolve that, apart from mandating that the parallel compiler runs everything in a deterministic order. I think the current approach is good enough for a debugging tool.

squash/fixup the commits into one and then I'll approve :)

hanna-kruppe · 2026-02-15T21:15:34Z

This might have weird interactions with -Zthreads, but I don't see a coherent way to resolve that, apart from mandating that the parallel compiler runs everything in a deterministic order. I think the current approach is good enough for a debugging tool.

FWIW, an interesting alternative that avoids this problem is hash based bisect. It's harder to implement than a binary search over a single integer, both for the program being bisected (here: rustc) and for doing the bisection (really annoying without automation, maybe the Go tool can be reused), but in return it's order/threading-independent and enables minimizing more precisely than the "fuel" approach.

saethlin · 2026-02-15T21:46:47Z

🤦 I missed that you actually squashed this down until I got the notification from the merge conflict

saethlin · 2026-02-15T21:47:45Z

This should prevent it from happening again:

@bors delegate=sgasho

rust-bors · 2026-02-15T21:47:47Z

Invalid command: Invalid delegation type sgasho. Possible values are try/review. Run @bors help to see available commands.

saethlin · 2026-02-15T22:21:25Z

@bors delegate

rust-bors · 2026-02-15T22:21:27Z

✌️ @sgasho, you can now approve this pull request!

If @saethlin told you to "r=me" after making some further change, then please make that change and post @bors r=saethlin.

rustbot · 2026-02-15T23:59:35Z

This PR was rebased onto a different main 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.

sgasho · 2026-02-16T01:32:49Z

@bors r=saethlin rollup

rust-bors · 2026-02-16T01:32:51Z

📌 Commit 25c2191 has been approved by saethlin

It is now in the queue for this repository.

@saethlin

…thlin Implement opt-bisect-limit for MIR closes: rust-lang#150910 Enable bisecting MIR optimization passes to enhance debuggability. discussions on zulip: https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/MIR.20dump.20the.20pass.20names/with/573219207 ### Check it works #### Sample code ```rust fn abs(num: isize) -> usize { if num < 0 { -num as usize } else { num as usize } } fn main() { println!("{}", abs(-10)); } ``` #### Output ```shell rustc +mir -Zmir-opt-bisect-limit=30 src/main.rs BISECT: running pass (1) CheckAlignment on main[89d5]::main BISECT: running pass (2) CheckNull on main[89d5]::main BISECT: running pass (3) CheckEnums on main[89d5]::main BISECT: running pass (4) LowerSliceLenCalls on main[89d5]::main BISECT: running pass (5) InstSimplify-before-inline on main[89d5]::main BISECT: running pass (6) ForceInline on main[89d5]::main BISECT: running pass (7) RemoveStorageMarkers on main[89d5]::main BISECT: running pass (8) RemoveZsts on main[89d5]::main BISECT: running pass (9) RemoveUnneededDrops on main[89d5]::main BISECT: running pass (10) UnreachableEnumBranching on main[89d5]::main BISECT: running pass (11) SimplifyCfg-after-unreachable-enum-branching on main[89d5]::main BISECT: running pass (12) InstSimplify-after-simplifycfg on main[89d5]::main BISECT: running pass (13) SimplifyConstCondition-after-inst-simplify on main[89d5]::main BISECT: running pass (14) SimplifyLocals-before-const-prop on main[89d5]::main BISECT: running pass (15) SimplifyLocals-after-value-numbering on main[89d5]::main BISECT: running pass (16) MatchBranchSimplification on main[89d5]::main BISECT: running pass (17) SingleUseConsts on main[89d5]::main BISECT: running pass (18) SimplifyConstCondition-after-const-prop on main[89d5]::main BISECT: running pass (19) SimplifyConstCondition-final on main[89d5]::main BISECT: running pass (20) RemoveNoopLandingPads on main[89d5]::main BISECT: running pass (21) SimplifyCfg-final on main[89d5]::main BISECT: running pass (22) CopyProp on main[89d5]::main BISECT: running pass (23) SimplifyLocals-final on main[89d5]::main BISECT: running pass (24) AddCallGuards on main[89d5]::main BISECT: running pass (25) PreCodegen on main[89d5]::main BISECT: running pass (26) CheckAlignment on main[89d5]::abs BISECT: running pass (27) CheckNull on main[89d5]::abs BISECT: running pass (28) CheckEnums on main[89d5]::abs BISECT: running pass (29) LowerSliceLenCalls on main[89d5]::abs BISECT: running pass (30) InstSimplify-before-inline on main[89d5]::abs BISECT: NOT running pass (31) ForceInline on main[89d5]::abs BISECT: NOT running pass (32) RemoveStorageMarkers on main[89d5]::abs BISECT: NOT running pass (33) RemoveZsts on main[89d5]::abs BISECT: NOT running pass (34) RemoveUnneededDrops on main[89d5]::abs BISECT: NOT running pass (35) UnreachableEnumBranching on main[89d5]::abs BISECT: NOT running pass (36) SimplifyCfg-after-unreachable-enum-branching on main[89d5]::abs BISECT: NOT running pass (37) InstSimplify-after-simplifycfg on main[89d5]::abs BISECT: NOT running pass (38) SimplifyConstCondition-after-inst-simplify on main[89d5]::abs BISECT: NOT running pass (39) SimplifyLocals-before-const-prop on main[89d5]::abs BISECT: NOT running pass (40) SimplifyLocals-after-value-numbering on main[89d5]::abs BISECT: NOT running pass (41) MatchBranchSimplification on main[89d5]::abs BISECT: NOT running pass (42) SingleUseConsts on main[89d5]::abs BISECT: NOT running pass (43) SimplifyConstCondition-after-const-prop on main[89d5]::abs BISECT: NOT running pass (44) SimplifyConstCondition-final on main[89d5]::abs BISECT: NOT running pass (45) RemoveNoopLandingPads on main[89d5]::abs BISECT: NOT running pass (46) SimplifyCfg-final on main[89d5]::abs BISECT: NOT running pass (47) CopyProp on main[89d5]::abs BISECT: NOT running pass (48) SimplifyLocals-final on main[89d5]::abs BISECT: NOT running pass (49) AddCallGuards on main[89d5]::abs BISECT: NOT running pass (50) PreCodegen on main[89d5]::abs ``` r? @saethlin

Rollup of 4 pull requests Successful merges: - #152103 (Consider captures to be used by closures that unwind) - #152474 (Implement opt-bisect-limit for MIR) - #152648 (Remove timing assertion from `oneshot::send_before_recv_timeout`) - #152686 (bootstrap: Inline the `is_tool` check for setting `-Zforce-unstable-if-unmarked`)

jhpratt · 2026-02-16T09:28:19Z

@bors r-

#152691 (comment)

rust-bors · 2026-02-16T09:28:25Z

Commit 25c2191 has been unapproved.

This PR was contained in a rollup (#152691), which was also unapproved.

sgasho · 2026-02-16T13:49:16Z

@bors try jobs=dist-various-1

rust-bors · 2026-02-16T13:49:21Z

⌛ Trying commit 805b4a8 with merge 77ac94f…

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

Workflow: https://github.com/rust-lang/rust/actions/runs/22065265525

Implement opt-bisect-limit for MIR try-job: dist-various-1

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs 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 Feb 11, 2026

rustbot assigned saethlin Feb 11, 2026

sgasho changed the title ~~Implement opt-bisect-limit for mir~~ Implement opt-bisect-limit for MIR Feb 11, 2026

bjorn3 reviewed Feb 11, 2026

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sgasho force-pushed the opt-bisect-limit-mir branch from 8922a59 to 6a9357a Compare February 14, 2026 01:24

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Implement opt-bisect-limit for mir

25c2191

sgasho force-pushed the opt-bisect-limit-mir branch from 6a9357a to 25c2191 Compare February 15, 2026 23:59

rust-bors bot 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 Feb 16, 2026

jhpratt mentioned this pull request Feb 16, 2026

Rollup of 4 pull requests #152691

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rust-bors bot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Feb 16, 2026

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fix mir-opt-bisect-limit test

805b4a8

rust-bors bot pushed a commit that referenced this pull request Feb 16, 2026

Auto merge of #152474 - sgasho:opt-bisect-limit-mir, r=<try>

77ac94f

Implement opt-bisect-limit for MIR try-job: dist-various-1

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