Enable -Zshare-generics for inline(never) functions

This avoids inlining cross-crate generic items when possible that are
already marked inline(never), implying that the author is not intending
for the function to be inlined by callers. As such, having a local copy
may make it easier for LLVM to optimize but mostly just adds to binary
bloat and codegen time. In practice our benchmarks indicate this is
indeed a win for larger compilations, where the extra cost in dynamic
linking to these symbols is diminished compared to the advantages in
fewer copies that need optimizing in each binary.

It might also make sense it expand this with other heuristics (e.g.,
`#[cold]`) in the future, but this seems like a good starting point.

Cargo.lock

@@ -4361,6 +4361,7 @@ dependencies = [
 name = "rustc_monomorphize"
 version = "0.0.0"
 dependencies = [
+ "rustc_attr",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_fluent_macro",

compiler/rustc_codegen_llvm/src/callee.rs

@@ -112,7 +112,9 @@ pub fn get_fn<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) ->
                 // This is a monomorphization. Its expected visibility depends
                 // on whether we are in share-generics mode.
 
-                if cx.tcx.sess.opts.share_generics() {
+                if cx.tcx.sess.opts.share_generics()
+                    || tcx.codegen_fn_attrs(instance_def_id).inline == rustc_attr::InlineAttr::Never
+                {
                     // We are in share_generics mode.
 
                     if let Some(instance_def_id) = instance_def_id.as_local() {

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -306,7 +306,7 @@ fn exported_symbols_provider_local(
         ));
     }
 
-    if tcx.sess.opts.share_generics() && tcx.local_crate_exports_generics() {
+    if tcx.local_crate_exports_generics() {
         use rustc_middle::mir::mono::{Linkage, MonoItem, Visibility};
         use rustc_middle::ty::InstanceDef;
 
@@ -334,6 +334,16 @@ fn exported_symbols_provider_local(
                 continue;
             }
 
+            if !tcx.sess.opts.share_generics() {
+                if tcx.codegen_fn_attrs(mono_item.def_id()).inline == rustc_attr::InlineAttr::Never
+                {
+                    // this is OK, we explicitly allow sharing inline(never) across crates even
+                    // without share-generics.
+                } else {
+                    continue;
+                }
+            }
+
             match *mono_item {
                 MonoItem::Fn(Instance { def: InstanceDef::Item(def), args }) => {
                     if args.non_erasable_generics(tcx, def).next().is_some() {

compiler/rustc_middle/src/mir/mono.rs

@@ -119,6 +119,13 @@ impl<'tcx> MonoItem<'tcx> {
                     return InstantiationMode::GloballyShared { may_conflict: false };
                 }
 
+                if let InlineAttr::Never = tcx.codegen_fn_attrs(instance.def_id()).inline
+                    && self.is_generic_fn(tcx)
+                {
+                    // Upgrade inline(never) to a globally shared instance.
+                    return InstantiationMode::GloballyShared { may_conflict: true };
+                }
+
                 // At this point we don't have explicit linkage and we're an
                 // inlined function. If we're inlining into all CGUs then we'll
                 // be creating a local copy per CGU.

compiler/rustc_middle/src/ty/context.rs

@@ -1301,8 +1301,6 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn local_crate_exports_generics(self) -> bool {
-        debug_assert!(self.sess.opts.share_generics());
-
         self.crate_types().iter().any(|crate_type| {
             match crate_type {
                 CrateType::Executable

compiler/rustc_middle/src/ty/instance.rs

@@ -157,19 +157,23 @@ impl<'tcx> Instance<'tcx> {
     /// This method already takes into account the global `-Zshare-generics`
     /// setting, always returning `None` if `share-generics` is off.
     pub fn upstream_monomorphization(&self, tcx: TyCtxt<'tcx>) -> Option<CrateNum> {
-        // If we are not in share generics mode, we don't link to upstream
-        // monomorphizations but always instantiate our own internal versions
-        // instead.
-        if !tcx.sess.opts.share_generics() {
-            return None;
-        }
-
         // If this is an item that is defined in the local crate, no upstream
         // crate can know about it/provide a monomorphization.
         if self.def_id().is_local() {
             return None;
         }
 
+        // If we are not in share generics mode, we don't link to upstream
+        // monomorphizations but always instantiate our own internal versions
+        // instead.
+        if !tcx.sess.opts.share_generics()
+            // However, if the def_id is marked inline(never), then it's fine to just reuse the
+            // upstream monomorphization.
+            && tcx.codegen_fn_attrs(self.def_id()).inline != rustc_attr::InlineAttr::Never
+        {
+            return None;
+        }
+
         // If this a non-generic instance, it cannot be a shared monomorphization.
         self.args.non_erasable_generics(tcx, self.def_id()).next()?;
 

compiler/rustc_monomorphize/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
+rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_fluent_macro = { path = "../rustc_fluent_macro" }

compiler/rustc_monomorphize/src/partitioning.rs

@@ -203,8 +203,8 @@ where
     // available to downstream crates. This depends on whether we are in
     // share-generics mode and whether the current crate can even have
     // downstream crates.
-    let export_generics =
-        cx.tcx.sess.opts.share_generics() && cx.tcx.local_crate_exports_generics();
+    let can_export_generics = cx.tcx.local_crate_exports_generics();
+    let always_export_generics = can_export_generics && cx.tcx.sess.opts.share_generics();
 
     let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx);
     let cgu_name_cache = &mut FxHashMap::default();
@@ -244,7 +244,8 @@ where
             cx.tcx,
             &mono_item,
             &mut can_be_internalized,
-            export_generics,
+            can_export_generics,
+            always_export_generics,
         );
         if visibility == Visibility::Hidden && can_be_internalized {
             internalization_candidates.insert(mono_item);
@@ -727,12 +728,19 @@ fn mono_item_linkage_and_visibility<'tcx>(
     tcx: TyCtxt<'tcx>,
     mono_item: &MonoItem<'tcx>,
     can_be_internalized: &mut bool,
-    export_generics: bool,
+    can_export_generics: bool,
+    always_export_generics: bool,
 ) -> (Linkage, Visibility) {
     if let Some(explicit_linkage) = mono_item.explicit_linkage(tcx) {
         return (explicit_linkage, Visibility::Default);
     }
-    let vis = mono_item_visibility(tcx, mono_item, can_be_internalized, export_generics);
+    let vis = mono_item_visibility(
+        tcx,
+        mono_item,
+        can_be_internalized,
+        can_export_generics,
+        always_export_generics,
+    );
     (Linkage::External, vis)
 }
 
@@ -755,7 +763,8 @@ fn mono_item_visibility<'tcx>(
     tcx: TyCtxt<'tcx>,
     mono_item: &MonoItem<'tcx>,
     can_be_internalized: &mut bool,
-    export_generics: bool,
+    can_export_generics: bool,
+    always_export_generics: bool,
 ) -> Visibility {
     let instance = match mono_item {
         // This is pretty complicated; see below.
@@ -812,7 +821,11 @@ fn mono_item_visibility<'tcx>(
 
     // Upstream `DefId` instances get different handling than local ones.
     let Some(def_id) = def_id.as_local() else {
-        return if export_generics && is_generic {
+        return if is_generic
+            && (always_export_generics
+                || (can_export_generics
+                    && tcx.codegen_fn_attrs(def_id).inline == rustc_attr::InlineAttr::Never))
+        {
             // If it is an upstream monomorphization and we export generics, we must make
             // it available to downstream crates.
             *can_be_internalized = false;
@@ -823,7 +836,12 @@ fn mono_item_visibility<'tcx>(
     };
 
     if is_generic {
-        if export_generics {
+        // An inline(never) function won't get inlined in upstream crates anyway, so we might as
+        // well export it.
+        if always_export_generics
+            || (can_export_generics
+                && tcx.codegen_fn_attrs(def_id).inline == rustc_attr::InlineAttr::Never)
+        {
             if tcx.is_unreachable_local_definition(def_id) {
                 // This instance cannot be used from another crate.
                 Visibility::Hidden

tests/codegen-units/partitioning/auxiliary/cgu_generic_function.rs

@@ -11,10 +11,21 @@ pub fn foo<T>(x: T) -> (T, u32, i8) {
 #[inline(never)]
 fn bar<T>(x: T) -> (T, Struct) {
     let _ = not_exported_and_not_generic(0);
+    exported_and_generic::<u32>(0);
     (x, Struct(1))
 }
 
+pub static F: fn(u32) -> u32 = exported_and_generic::<u32>;
+
 // These should not contribute to the codegen items of other crates.
+
+// This is generic, but it's only instantiated with a u32 argument and that instantiation is present
+// in the local crate (see F above).
+#[inline(never)]
+pub fn exported_and_generic<T>(x: T) -> T {
+    x
+}
+
 #[inline(never)]
 pub fn exported_but_not_generic(x: i32) -> i64 {
     x as i64