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Fix #[inline(always)] on closures with target feature 1.1 #111836

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Jul 23, 2023
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Fix #[inline(always)] on closures with target feature 1.1 #111836

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ddb16e2
Fix #[inline(always)] on closures with target feature 1.1
calebzulawski May 22, 2023
cdb9de7
Add codegen test ensuring always-inline closures don't bypass target …
calebzulawski Jul 17, 2023
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17 changes: 16 additions & 1 deletion compiler/rustc_codegen_ssa/src/codegen_attrs.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -493,7 +493,22 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
});

// #73631: closures inherit `#[target_feature]` annotations
if tcx.features().target_feature_11 && tcx.is_closure(did.to_def_id()) {
//
// If this closure is marked `#[inline(always)]`, simply skip adding `#[target_feature]`.
//
// At this point, `unsafe` has already been checked and `#[target_feature]` only affects codegen.
// Emitting both `#[inline(always)]` and `#[target_feature]` can potentially result in an
// ICE, because LLVM errors when the function fails to be inlined due to a target feature
// mismatch.
//
// Using `#[inline(always)]` implies that this closure will most likely be inlined into
// its parent function, which effectively inherits the features anyway. Boxing this closure
// would result in this closure being compiled without the inherited target features, but this
// is probably a poor usage of `#[inline(always)]` and easily avoided by not using the attribute.
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Boxing seems like a waste, yes, but now that I am thinking about it, this seems like it could result in confusing behavior in the "escaping closure" case, when that would result, instead of the IIFE? Does that even make sense?

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Inlining with closures is unfortunately always confusing. Box<dyn FnOnce()>, for example, implements FnOnce itself:

rust/library/alloc/src/boxed.rs

Lines 2003 to 2009 in c4083fa

impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
type Output = <F as FnOnce<Args>>::Output;
extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
<F as FnOnce<Args>>::call_once(*self, args)
}
}

This call_once doesn't have any inline attribute at all! Therefore, the boxed closure's call_once inlines into this call_once, and then it's up in the air after that.

if tcx.features().target_feature_11
&& tcx.is_closure(did.to_def_id())
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...apparently is_closure will return true if this is a generator, also. I frankly have no idea how that should work, but dropping the features should remain safe in that case, at least...

&& codegen_fn_attrs.inline != InlineAttr::Always
{
let owner_id = tcx.parent(did.to_def_id());
if tcx.def_kind(owner_id).has_codegen_attrs() {
codegen_fn_attrs
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18 changes: 18 additions & 0 deletions tests/ui/rfcs/rfc-2396-target_feature-11/issue-108655-inline-always-closure.rs
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@@ -0,0 +1,18 @@
// Tests #108655: closures in `#[target_feature]` functions can still be marked #[inline(always)]

// check-pass
// revisions: mir thir
// [thir]compile-flags: -Z thir-unsafeck
// only-x86_64

#![feature(target_feature_11)]

#[target_feature(enable = "avx")]
pub unsafe fn test() {
({
#[inline(always)]
move || {}
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Thinking about this forced me to check if you can annotate closures with target_feature(enable). (You cannot, fortunately.)

})();
}

fn main() {}