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Commit b15584b

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osiewiczbjorn3
andcommitted
shared-generics: Do not share instantiations that cannot be created outside of the current crate
In Zed shared-generics loading takes up a significant chunk of time in incremental build, as rustc deserializes rmeta of all dependencies of a crate. I've recently realized that shared-generics includes all instantiations of some_generic_function in the following snippet: ```rs pub fn some_generic_function(_: impl Fn()) {} pub fn non_generic_function() { some_generic_function(|| {}); some_generic_function(|| {}); some_generic_function(|| {}); some_generic_function(|| {}); some_generic_function(|| {}); some_generic_function(|| {}); some_generic_function(|| {}); } ``` even though none of these instantiations can actually be created from outside of `non_generic_function`. This PR makes shared-generics account for visibilities of generic arguments; an item is only considered for exporting if it is reachable from the outside or if all of it's arguments are visible outside of the local crate. This PR reduces incremental build time for Zed (touch edito.rs scenario) from 12.4s to 10.4s. Co-authored-by: bjorn3 <17426603+bjorn3@users.noreply.github.com>
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‎compiler/rustc_codegen_ssa/src/back/symbol_export.rs‎

Lines changed: 59 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -164,10 +164,10 @@ fn is_reachable_non_generic_provider_extern(tcx: TyCtxt<'_>, def_id: DefId) -> b
164164
tcx.reachable_non_generics(def_id.krate).contains_key(&def_id)
165165
}
166166

167-
fn exported_symbols_provider_local(
168-
tcx: TyCtxt<'_>,
167+
fn exported_symbols_provider_local<'tcx>(
168+
tcx: TyCtxt<'tcx>,
169169
_: LocalCrate,
170-
) -> &[(ExportedSymbol<'_>, SymbolExportInfo)] {
170+
) -> &'tcx[(ExportedSymbol<'tcx>, SymbolExportInfo)] {
171171
if !tcx.sess.opts.output_types.should_codegen() {
172172
return &[];
173173
}
@@ -321,6 +321,38 @@ fn exported_symbols_provider_local(
321321

322322
let cgus = tcx.collect_and_partition_mono_items(()).codegen_units;
323323

324+
// Do not export symbols that cannot be instantiated by downstream crates.
325+
let reachable_set = tcx.reachable_set(());
326+
let is_local_to_current_crate = |ty: Ty<'_>| {
327+
let no_refs = ty.peel_refs();
328+
let root_def_id = match no_refs.kind() {
329+
ty::Closure(closure, _) => *closure,
330+
ty::FnDef(def_id, _) => *def_id,
331+
ty::Coroutine(def_id, _) => *def_id,
332+
ty::CoroutineClosure(def_id, _) => *def_id,
333+
ty::CoroutineWitness(def_id, _) => *def_id,
334+
_ => return false,
335+
};
336+
let Some(root_def_id) = root_def_id.as_local() else {
337+
return false;
338+
};
339+
340+
let is_local = !reachable_set.contains(&root_def_id);
341+
is_local
342+
};
343+
344+
let is_instantiable_downstream =
345+
|did: Option<DefId>, generic_args: GenericArgsRef<'tcx>| {
346+
generic_args
347+
.types()
348+
.chain(did.into_iter().map(move |did| tcx.type_of(did).skip_binder()))
349+
.all(move |arg| {
350+
arg.walk().all(|ty| {
351+
ty.as_type().map_or(true, |ty| !is_local_to_current_crate(ty))
352+
})
353+
})
354+
};
355+
324356
// The symbols created in this loop are sorted below it
325357
#[allow(rustc::potential_query_instability)]
326358
for (mono_item, data) in cgus.iter().flat_map(|cgu| cgu.items().iter()) {
@@ -349,7 +381,12 @@ fn exported_symbols_provider_local(
349381

350382
match *mono_item {
351383
MonoItem::Fn(Instance { def: InstanceKind::Item(def), args }) => {
352-
if args.non_erasable_generics().next().is_some() {
384+
let has_generics = args.non_erasable_generics().next().is_some();
385+
386+
let should_export =
387+
has_generics && is_instantiable_downstream(Some(def), &args);
388+
389+
if should_export {
353390
let symbol = ExportedSymbol::Generic(def, args);
354391
symbols.push((
355392
symbol,
@@ -364,14 +401,24 @@ fn exported_symbols_provider_local(
364401
MonoItem::Fn(Instance { def: InstanceKind::DropGlue(_, Some(ty)), args }) => {
365402
// A little sanity-check
366403
assert_eq!(args.non_erasable_generics().next(), Some(GenericArgKind::Type(ty)));
367-
symbols.push((
368-
ExportedSymbol::DropGlue(ty),
369-
SymbolExportInfo {
370-
level: SymbolExportLevel::Rust,
371-
kind: SymbolExportKind::Text,
372-
used: false,
373-
},
374-
));
404+
405+
// Drop glue did is always going to be non-local outside of libcore, thus we don't need to check it's locality (which includes invoking `type_of` query).
406+
let should_export = match ty.kind() {
407+
ty::Adt(_, args) => is_instantiable_downstream(None, args),
408+
ty::Closure(_, args) => is_instantiable_downstream(None, args),
409+
_ => true,
410+
};
411+
412+
if should_export {
413+
symbols.push((
414+
ExportedSymbol::DropGlue(ty),
415+
SymbolExportInfo {
416+
level: SymbolExportLevel::Rust,
417+
kind: SymbolExportKind::Text,
418+
used: false,
419+
},
420+
));
421+
}
375422
}
376423
MonoItem::Fn(Instance {
377424
def: InstanceKind::AsyncDropGlueCtorShim(_, Some(ty)),

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