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Auto merge of #118770 - saethlin:fix-inline-never-uses, r=nnethercote

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Fix cases where std accidentally relied on inline(never)

This PR increases the power of `-Zcross-crate-inline-threshold=always` so that it applies through `#[inline(never)]`. Note that though this is called "cross-crate-inlining" in this case especially it is _just_ lazy per-CGU codegen. The MIR inliner and LLVM still respect the attribute as much as they ever have.

Trying to bootstrap with the new `-Zcross-crate-inline-threshold=always` change revealed two bugs:

We have special intrinsics `assert_inhabited`, `assert_zero_valid`, and `assert_mem_uniniitalized_valid` which codegen backends will lower to nothing or a call to `panic_nounwind`.  Since we may not have any call to `panic_nounwind` in MIR but emit one anyway, we need to specially tell `MirUsedCollector` about this situation.

`#[lang = "start"]` is special-cased already so that `MirUsedCollector` will collect it, but then when we make it cross-crate-inlinable it is only assigned to a CGU based on whether `MirUsedCollector` saw a call to it, which of course we didn't.

---

I started looking into this because https://github.com/rust-lang/rust/pull/118683 revealed a case where we were accidentally relying on a function being `#[inline(never)]`, and cranking up cross-crate-inlinability seems like a way to find other situations like that.

r? `@nnethercote` because I don't like what I'm doing to the CGU partitioning code here but I can't come up with something much better
This commit is contained in:
bors 2023-12-15 04:54:14 +00:00
commit 1559dd2dbf
4 changed files with 37 additions and 11 deletions
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19
compiler/rustc_mir_transform/src/cross_crate_inline.rs
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@ -22,6 +22,18 @@ fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
return false;
}
// This just reproduces the logic from Instance::requires_inline.
match tcx.def_kind(def_id) {
DefKind::Ctor(..) | DefKind::Closure => return true,
DefKind::Fn | DefKind::AssocFn => {}
_ => return false,
}
// From this point on, it is valid to return true or false.
if tcx.sess.opts.unstable_opts.cross_crate_inline_threshold == InliningThreshold::Always {
return true;
}
// Obey source annotations first; this is important because it means we can use
// #[inline(never)] to force code generation.
match codegen_fn_attrs.inline {
@ -30,13 +42,6 @@ fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
_ => {}
}
// This just reproduces the logic from Instance::requires_inline.
match tcx.def_kind(def_id) {
DefKind::Ctor(..) | DefKind::Closure => return true,
DefKind::Fn | DefKind::AssocFn => {}
_ => return false,
}
// Don't do any inference when incremental compilation is enabled; the additional inlining that
// inference permits also creates more work for small edits.
if tcx.sess.opts.incremental.is_some() {