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Auto merge of #122568 - RalfJung:mentioned-items, r=oli-obk
recursively evaluate the constants in everything that is 'mentioned' This is another attempt at fixing #107503. The previous attempt at #112879 seems stuck in figuring out where the [perf regression](https://perf.rust-lang.org/compare.html?start=c55d1ee8d4e3162187214692229a63c2cc5e0f31&end=ec8de1ebe0d698b109beeaaac83e60f4ef8bb7d1&stat=instructions:u) comes from. In #122258 I learned some things, which informed the approach this PR is taking. Quoting from the new collector docs, which explain the high-level idea: ```rust //! One important role of collection is to evaluate all constants that are used by all the items //! which are being collected. Codegen can then rely on only encountering constants that evaluate //! successfully, and if a constant fails to evaluate, the collector has much better context to be //! able to show where this constant comes up. //! //! However, the exact set of "used" items (collected as described above), and therefore the exact //! set of used constants, can depend on optimizations. Optimizing away dead code may optimize away //! a function call that uses a failing constant, so an unoptimized build may fail where an //! optimized build succeeds. This is undesirable. //! //! To fix this, the collector has the concept of "mentioned" items. Some time during the MIR //! pipeline, before any optimization-level-dependent optimizations, we compute a list of all items //! that syntactically appear in the code. These are considered "mentioned", and even if they are in //! dead code and get optimized away (which makes them no longer "used"), they are still //! "mentioned". For every used item, the collector ensures that all mentioned items, recursively, //! do not use a failing constant. This is reflected via the [`CollectionMode`], which determines //! whether we are visiting a used item or merely a mentioned item. //! //! The collector and "mentioned items" gathering (which lives in `rustc_mir_transform::mentioned_items`) //! need to stay in sync in the following sense: //! //! - For every item that the collector gather that could eventually lead to build failure (most //! likely due to containing a constant that fails to evaluate), a corresponding mentioned item //! must be added. This should use the exact same strategy as the ecollector to make sure they are //! in sync. However, while the collector works on monomorphized types, mentioned items are //! collected on generic MIR -- so any time the collector checks for a particular type (such as //! `ty::FnDef`), we have to just onconditionally add this as a mentioned item. //! - In `visit_mentioned_item`, we then do with that mentioned item exactly what the collector //! would have done during regular MIR visiting. Basically you can think of the collector having //! two stages, a pre-monomorphization stage and a post-monomorphization stage (usually quite //! literally separated by a call to `self.monomorphize`); the pre-monomorphizationn stage is //! duplicated in mentioned items gathering and the post-monomorphization stage is duplicated in //! `visit_mentioned_item`. //! - Finally, as a performance optimization, the collector should fill `used_mentioned_item` during //! its MIR traversal with exactly what mentioned item gathering would have added in the same //! situation. This detects mentioned items that have *not* been optimized away and hence don't //! need a dedicated traversal. enum CollectionMode { /// Collect items that are used, i.e., actually needed for codegen. /// /// Which items are used can depend on optimization levels, as MIR optimizations can remove /// uses. UsedItems, /// Collect items that are mentioned. The goal of this mode is that it is independent of /// optimizations: the set of "mentioned" items is computed before optimizations are run. /// /// The exact contents of this set are *not* a stable guarantee. (For instance, it is currently /// computed after drop-elaboration. If we ever do some optimizations even in debug builds, we /// might decide to run them before computing mentioned items.) The key property of this set is /// that it is optimization-independent. MentionedItems, } ``` And the `mentioned_items` MIR body field docs: ```rust /// Further items that were mentioned in this function and hence *may* become monomorphized, /// depending on optimizations. We use this to avoid optimization-dependent compile errors: the /// collector recursively traverses all "mentioned" items and evaluates all their /// `required_consts`. /// /// This is *not* soundness-critical and the contents of this list are *not* a stable guarantee. /// All that's relevant is that this set is optimization-level-independent, and that it includes /// everything that the collector would consider "used". (For example, we currently compute this /// set after drop elaboration, so some drop calls that can never be reached are not considered /// "mentioned".) See the documentation of `CollectionMode` in /// `compiler/rustc_monomorphize/src/collector.rs` for more context. pub mentioned_items: Vec<Spanned<MentionedItem<'tcx>>>, ``` Fixes #107503
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,117 @@ | ||
| use rustc_middle::mir::visit::Visitor; | ||
| use rustc_middle::mir::{self, Location, MentionedItem, MirPass}; | ||
| use rustc_middle::ty::{self, adjustment::PointerCoercion, TyCtxt}; | ||
| use rustc_session::Session; | ||
| use rustc_span::source_map::Spanned; | ||
|
|
||
| pub struct MentionedItems; | ||
|
|
||
| struct MentionedItemsVisitor<'a, 'tcx> { | ||
| tcx: TyCtxt<'tcx>, | ||
| body: &'a mir::Body<'tcx>, | ||
| mentioned_items: &'a mut Vec<Spanned<MentionedItem<'tcx>>>, | ||
| } | ||
|
|
||
| impl<'tcx> MirPass<'tcx> for MentionedItems { | ||
| fn is_enabled(&self, _sess: &Session) -> bool { | ||
| // If this pass is skipped the collector assume that nothing got mentioned! We could | ||
| // potentially skip it in opt-level 0 if we are sure that opt-level will never *remove* uses | ||
| // of anything, but that still seems fragile. Furthermore, even debug builds use level 1, so | ||
| // special-casing level 0 is just not worth it. | ||
| true | ||
| } | ||
|
|
||
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) { | ||
| debug_assert!(body.mentioned_items.is_empty()); | ||
| let mut mentioned_items = Vec::new(); | ||
| MentionedItemsVisitor { tcx, body, mentioned_items: &mut mentioned_items }.visit_body(body); | ||
| body.mentioned_items = mentioned_items; | ||
| } | ||
| } | ||
|
|
||
| // This visitor is carefully in sync with the one in `rustc_monomorphize::collector`. We are | ||
| // visiting the exact same places but then instead of monomorphizing and creating `MonoItems`, we | ||
| // have to remain generic and just recording the relevant information in `mentioned_items`, where it | ||
| // will then be monomorphized later during "mentioned items" collection. | ||
| impl<'tcx> Visitor<'tcx> for MentionedItemsVisitor<'_, 'tcx> { | ||
| fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) { | ||
| self.super_terminator(terminator, location); | ||
| let span = || self.body.source_info(location).span; | ||
| match &terminator.kind { | ||
| mir::TerminatorKind::Call { func, .. } => { | ||
| let callee_ty = func.ty(self.body, self.tcx); | ||
| self.mentioned_items | ||
| .push(Spanned { node: MentionedItem::Fn(callee_ty), span: span() }); | ||
| } | ||
| mir::TerminatorKind::Drop { place, .. } => { | ||
| let ty = place.ty(self.body, self.tcx).ty; | ||
| self.mentioned_items.push(Spanned { node: MentionedItem::Drop(ty), span: span() }); | ||
| } | ||
| mir::TerminatorKind::InlineAsm { ref operands, .. } => { | ||
| for op in operands { | ||
| match *op { | ||
| mir::InlineAsmOperand::SymFn { ref value } => { | ||
| self.mentioned_items.push(Spanned { | ||
| node: MentionedItem::Fn(value.const_.ty()), | ||
| span: span(), | ||
| }); | ||
| } | ||
| _ => {} | ||
| } | ||
| } | ||
| } | ||
| _ => {} | ||
| } | ||
| } | ||
|
|
||
| fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) { | ||
| self.super_rvalue(rvalue, location); | ||
| let span = || self.body.source_info(location).span; | ||
| match *rvalue { | ||
| // We need to detect unsizing casts that required vtables. | ||
| mir::Rvalue::Cast( | ||
| mir::CastKind::PointerCoercion(PointerCoercion::Unsize), | ||
| ref operand, | ||
| target_ty, | ||
| ) | ||
| | mir::Rvalue::Cast(mir::CastKind::DynStar, ref operand, target_ty) => { | ||
| // This isn't monomorphized yet so we can't tell what the actual types are -- just | ||
| // add everything that may involve a vtable. | ||
| let source_ty = operand.ty(self.body, self.tcx); | ||
| let may_involve_vtable = match ( | ||
| source_ty.builtin_deref(true).map(|t| t.ty.kind()), | ||
| target_ty.builtin_deref(true).map(|t| t.ty.kind()), | ||
| ) { | ||
| (Some(ty::Array(..)), Some(ty::Str | ty::Slice(..))) => false, // &str/&[T] unsizing | ||
| _ => true, | ||
| }; | ||
| if may_involve_vtable { | ||
| self.mentioned_items.push(Spanned { | ||
| node: MentionedItem::UnsizeCast { source_ty, target_ty }, | ||
| span: span(), | ||
| }); | ||
| } | ||
| } | ||
| // Similarly, record closures that are turned into function pointers. | ||
| mir::Rvalue::Cast( | ||
| mir::CastKind::PointerCoercion(PointerCoercion::ClosureFnPointer(_)), | ||
| ref operand, | ||
| _, | ||
| ) => { | ||
| let source_ty = operand.ty(self.body, self.tcx); | ||
| self.mentioned_items | ||
| .push(Spanned { node: MentionedItem::Closure(source_ty), span: span() }); | ||
| } | ||
| // And finally, function pointer reification casts. | ||
| mir::Rvalue::Cast( | ||
| mir::CastKind::PointerCoercion(PointerCoercion::ReifyFnPointer), | ||
| ref operand, | ||
| _, | ||
| ) => { | ||
| let fn_ty = operand.ty(self.body, self.tcx); | ||
| self.mentioned_items.push(Spanned { node: MentionedItem::Fn(fn_ty), span: span() }); | ||
| } | ||
| _ => {} | ||
| } | ||
| } | ||
| } |
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