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Implement opt-bisect-limit for MIR #152474

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Implement opt-bisect-limit for MIR #152474

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@sgasho sgasho commented Feb 11, 2026 •
edited by rustbot
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closes: #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

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

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rustbot commented Feb 11, 2026

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

@rustbot 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
@sgasho sgasho changed the title Implement opt-bisect-limit for mir Implement opt-bisect-limit for MIR Feb 11, 2026
bjorn3
bjorn3 reviewed Feb 11, 2026
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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,
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@bjorn3 bjorn3 Feb 11, 2026

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

bjorn3
bjorn3 reviewed Feb 11, 2026
View reviewed changes
compiler/rustc_mir_transform/src/pass_manager.rs Outdated
limit: usize,
pass: &P,
) -> bool
static MIR_OPT_BISECT_COUNT: AtomicUsize = AtomicUsize::new(0);
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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?

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@sgasho sgasho Feb 11, 2026

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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.

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@sgasho sgasho Feb 11, 2026

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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.

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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.

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-Zmir-enable-passes=+SimplifyCfg doesn't enable any passes and doesn't warn

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