Automatically enable cross-crate inlining for small functions

compiler/rustc_mir_transform/src/cross_crate_inline.rs

@@ -0,0 +1,119 @@
+use rustc_attr::InlineAttr;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::*;
+use rustc_middle::query::Providers;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::config::OptLevel;
+
+pub fn provide(providers: &mut Providers) {
+    providers.cross_crate_inlinable = cross_crate_inlinable;
+}
+
+fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
+    let codegen_fn_attrs = tcx.codegen_fn_attrs(def_id);
+    // If this has an extern indicator, then this function is globally shared and thus will not
+    // generate cgu-internal copies which would make it cross-crate inlinable.
+    if codegen_fn_attrs.contains_extern_indicator() {
+        return false;
+    }
+
+    // Obey source annotations first; this is important because it means we can use
+    // #[inline(never)] to force code generation.
+    match codegen_fn_attrs.inline {
+        InlineAttr::Never => return false,
+        InlineAttr::Hint | InlineAttr::Always => return true,
+        _ => {}
+    }
+
+    // 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() {
+        return false;
+    }
+
+    // Don't do any inference unless optimizations are enabled.
+    if matches!(tcx.sess.opts.optimize, OptLevel::No) {
+        return false;
+    }
+
+    if !tcx.is_mir_available(def_id) {
+        return false;
+    }
+
+    let mir = tcx.optimized_mir(def_id);
+    let mut checker =
+        CostChecker { tcx, callee_body: mir, calls: 0, statements: 0, landing_pads: 0, resumes: 0 };
+    checker.visit_body(mir);
+    checker.calls == 0
+        && checker.resumes == 0
+        && checker.landing_pads == 0
+        && checker.statements
+            <= tcx.sess.opts.unstable_opts.cross_crate_inline_threshold.unwrap_or(100)
+}
+
+struct CostChecker<'b, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    callee_body: &'b Body<'tcx>,
+    calls: usize,
+    statements: usize,
+    landing_pads: usize,
+    resumes: usize,
+}
+
+impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
+    fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
+        // Don't count StorageLive/StorageDead in the inlining cost.
+        match statement.kind {
+            StatementKind::StorageLive(_)
+            | StatementKind::StorageDead(_)
+            | StatementKind::Deinit(_)
+            | StatementKind::Nop => {}
+            _ => self.statements += 1,
+        }
+    }
+
+    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, _: Location) {
+        let tcx = self.tcx;
+        match terminator.kind {
+            TerminatorKind::Drop { ref place, unwind, .. } => {
+                let ty = place.ty(self.callee_body, tcx).ty;
+                if !ty.is_trivially_pure_clone_copy() {
+                    self.calls += 1;
+                    if let UnwindAction::Cleanup(_) = unwind {
+                        self.landing_pads += 1;
+                    }
+                }
+            }
+            TerminatorKind::Call { unwind, .. } => {
+                self.calls += 1;
+                if let UnwindAction::Cleanup(_) = unwind {
+                    self.landing_pads += 1;
+                }
+            }
+            TerminatorKind::Assert { unwind, .. } => {
+                self.calls += 1;
+                if let UnwindAction::Cleanup(_) = unwind {
+                    self.landing_pads += 1;
+                }
+            }
+            TerminatorKind::UnwindResume => self.resumes += 1,
+            TerminatorKind::InlineAsm { unwind, .. } => {
+                self.statements += 1;
+                if let UnwindAction::Cleanup(_) = unwind {
+                    self.landing_pads += 1;
+                }
+            }
+            TerminatorKind::Return => {}
+            _ => self.statements += 1,
+        }
+    }
+}

compiler/rustc_mir_transform/src/inline.rs

@@ -169,8 +169,11 @@ impl<'tcx> Inliner<'tcx> {
         caller_body: &mut Body<'tcx>,
         callsite: &CallSite<'tcx>,
     ) -> Result<std::ops::Range<BasicBlock>, &'static str> {
+        self.check_mir_is_available(caller_body, &callsite.callee)?;
+
         let callee_attrs = self.tcx.codegen_fn_attrs(callsite.callee.def_id());
-        self.check_codegen_attributes(callsite, callee_attrs)?;
+        let cross_crate_inlinable = self.tcx.cross_crate_inlinable(callsite.callee.def_id());
+        self.check_codegen_attributes(callsite, callee_attrs, cross_crate_inlinable)?;
 
         let terminator = caller_body[callsite.block].terminator.as_ref().unwrap();
         let TerminatorKind::Call { args, destination, .. } = &terminator.kind else { bug!() };
@@ -183,9 +186,8 @@ impl<'tcx> Inliner<'tcx> {
             }
         }
 
-        self.check_mir_is_available(caller_body, &callsite.callee)?;
         let callee_body = try_instance_mir(self.tcx, callsite.callee.def)?;
-        self.check_mir_body(callsite, callee_body, callee_attrs)?;
+        self.check_mir_body(callsite, callee_body, callee_attrs, cross_crate_inlinable)?;
 
         if !self.tcx.consider_optimizing(|| {
             format!("Inline {:?} into {:?}", callsite.callee, caller_body.source)
@@ -401,6 +403,7 @@ impl<'tcx> Inliner<'tcx> {
         &self,
         callsite: &CallSite<'tcx>,
         callee_attrs: &CodegenFnAttrs,
+        cross_crate_inlinable: bool,
     ) -> Result<(), &'static str> {
         if let InlineAttr::Never = callee_attrs.inline {
             return Err("never inline hint");
@@ -414,7 +417,7 @@ impl<'tcx> Inliner<'tcx> {
             .non_erasable_generics(self.tcx, callsite.callee.def_id())
             .next()
             .is_some();
-        if !is_generic && !callee_attrs.requests_inline() {
+        if !is_generic && !cross_crate_inlinable {
             return Err("not exported");
         }
 
@@ -456,10 +459,11 @@ impl<'tcx> Inliner<'tcx> {
         callsite: &CallSite<'tcx>,
         callee_body: &Body<'tcx>,
         callee_attrs: &CodegenFnAttrs,
+        cross_crate_inlinable: bool,
     ) -> Result<(), &'static str> {
         let tcx = self.tcx;
 
-        let mut threshold = if callee_attrs.requests_inline() {
+        let mut threshold = if cross_crate_inlinable {
             self.tcx.sess.opts.unstable_opts.inline_mir_hint_threshold.unwrap_or(100)
         } else {
             self.tcx.sess.opts.unstable_opts.inline_mir_threshold.unwrap_or(50)

compiler/rustc_mir_transform/src/lib.rs

@@ -62,6 +62,7 @@ mod const_prop;
 mod const_prop_lint;
 mod copy_prop;
 mod coverage;
+mod cross_crate_inline;
 mod ctfe_limit;
 mod dataflow_const_prop;
 mod dead_store_elimination;
@@ -123,6 +124,7 @@ pub fn provide(providers: &mut Providers) {
     coverage::query::provide(providers);
     ffi_unwind_calls::provide(providers);
     shim::provide(providers);
+    cross_crate_inline::provide(providers);
     *providers = Providers {
         mir_keys,
         mir_const,