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Promoted constants lead to poor MIR from comparisons #139093
Description
Follow-along at https://rust.godbolt.org/z/Trzrobvo5
Consider these two functions that ought to be equivalent:
use std::cmp::Ordering;
#[no_mangle]
pub fn direct(e: Option<Ordering>) -> bool {
e == Some(Ordering::Equal)
}
#[no_mangle]
pub fn with_let(e: Option<Ordering>) -> bool {
let eq = Ordering::Equal;
e == Some(eq)
}The latter ends up with about the MIR you'd expect: it checks for Some, then for Equal
bb0: {
StorageLive(_2);
_2 = discriminant(_1);
switchInt(move _2) -> [0: bb1, 1: bb2, otherwise: bb4];
}
bb1: {
_0 = const false;
goto -> bb3;
}
bb2: {
StorageLive(_3);
_3 = discriminant(((_1 as Some).0: std::cmp::Ordering));
_0 = Eq(copy _3, const 0_i8);
StorageDead(_3);
goto -> bb3;
}
bb3: {
StorageDead(_2);
return;
}
bb4: {
unreachable;
}The former, though, ends up with way more MIR because of the promoted const:
bb0: {
_2 = const direct::promoted[0];
StorageLive(_5);
StorageLive(_4);
StorageLive(_3);
_3 = discriminant(_1);
switchInt(move _3) -> [0: bb1, 1: bb2, otherwise: bb6];
}
bb1: {
_4 = discriminant((*_2));
_0 = Eq(copy _4, const 0_isize);
goto -> bb5;
}
bb2: {
_5 = discriminant((*_2));
switchInt(move _5) -> [0: bb3, 1: bb4, otherwise: bb6];
}
bb3: {
_0 = const false;
goto -> bb5;
}
bb4: {
StorageLive(_6);
StorageLive(_7);
_6 = discriminant(((_1 as Some).0: std::cmp::Ordering));
_7 = discriminant((((*_2) as Some).0: std::cmp::Ordering));
_0 = Eq(copy _6, copy _7);
StorageDead(_7);
StorageDead(_6);
goto -> bb5;
}
bb5: {
StorageDead(_3);
StorageDead(_4);
StorageDead(_5);
return;
}
bb6: {
unreachable;
}Apparently we can't even tell what the discriminant is of that const direct::promoted[0], so we end up emitting even LLVM IR checks on the value of the constant, which it then has to turn around and optimize away again.
Can we maybe just unilaterally un-promote simple (Copy + NoCell + Scalar/ScalarPair) constants like this?
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Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.htmlArea: MIR optimizationsCategory: An issue highlighting optimization opportunities or PRs implementing suchCall for participation: An issue has been fixed and does not reproduce, but no test has been added.Relevant to the compiler team, which will review and decide on the PR/issue.Working group: MIR optimizations
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cjgillot commentedon Jul 20, 2025
Current MIR is identical for both versions.
borsakv commentedon Aug 6, 2025
@rustbot claim
Adding a regression test for issue rust-lang#139093 (const promotion …
Rollup merge of rust-lang#145651 - borsakv:139093, r=cjgillot
Unrolled build for #145651
Rollup merge of #145651 - borsakv:139093, r=cjgillot
Rollup merge of #145651 - borsakv:139093, r=cjgillot
Rollup merge of rust-lang#145651 - borsakv:139093, r=cjgillot
Rollup merge of rust-lang#145651 - borsakv:139093, r=cjgillot
Rollup merge of #145651 - borsakv:139093, r=cjgillot
Rollup merge of #145651 - borsakv:139093, r=cjgillot
Rollup merge of #145651 - borsakv:139093, r=cjgillot