Auto merge of #101313 - SparrowLii:mk_attr_id, r=cjgillot

make `mk_attr_id` part of `ParseSess`

Updates #48685

The current `mk_attr_id` uses the `AtomicU32` type, which is not very efficient and adds a lot of lock contention in a parallel environment.

This PR refers to the task list in #48685, uses `mk_attr_id` as a method of the `AttrIdGenerator` struct, and adds a new field `attr_id_generator` to `ParseSess`.

`AttrIdGenerator` uses the `WorkerLocal`, which has two advantages: 1. `Cell` is more efficient than `AtomicU32`, and does not increase any lock contention. 2. We put the index of the work thread in the first few bits of the generated `AttrId`, so that the `AttrId` generated in different threads can be easily guaranteed to be unique.

cc `@cjgillot`

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2524,8 +2524,8 @@ impl<S: Encoder> Encodable<S> for AttrId {
 }
 
 impl<D: Decoder> Decodable<D> for AttrId {
-    fn decode(_: &mut D) -> AttrId {
-        crate::attr::mk_attr_id()
+    default fn decode(_: &mut D) -> AttrId {
+        panic!("cannot decode `AttrId` with `{}`", std::any::type_name::<D>());
     }
 }
 

compiler/rustc_ast/src/attr/mod.rs

@@ -11,12 +11,18 @@ use crate::tokenstream::{DelimSpan, Spacing, TokenTree};
 use crate::tokenstream::{LazyAttrTokenStream, TokenStream};
 use crate::util::comments;
 
+use rustc_data_structures::sync::WorkerLocal;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_span::source_map::BytePos;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
 
+use std::cell::Cell;
 use std::iter;
+#[cfg(debug_assertions)]
+use std::ops::BitXor;
+#[cfg(debug_assertions)]
+use std::sync::atomic::{AtomicU32, Ordering};
 
 pub struct MarkedAttrs(GrowableBitSet<AttrId>);
 
@@ -346,52 +352,86 @@ pub fn mk_nested_word_item(ident: Ident) -> NestedMetaItem {
     NestedMetaItem::MetaItem(mk_word_item(ident))
 }
 
-pub(crate) fn mk_attr_id() -> AttrId {
-    use std::sync::atomic::AtomicU32;
-    use std::sync::atomic::Ordering;
+pub struct AttrIdGenerator(WorkerLocal<Cell<u32>>);
 
-    static NEXT_ATTR_ID: AtomicU32 = AtomicU32::new(0);
+#[cfg(debug_assertions)]
+static MAX_ATTR_ID: AtomicU32 = AtomicU32::new(u32::MAX);
 
-    let id = NEXT_ATTR_ID.fetch_add(1, Ordering::SeqCst);
-    assert!(id != u32::MAX);
-    AttrId::from_u32(id)
+impl AttrIdGenerator {
+    pub fn new() -> Self {
+        // We use `(index as u32).reverse_bits()` to initialize the
+        // starting value of AttrId in each worker thread.
+        // The `index` is the index of the worker thread.
+        // This ensures that the AttrId generated in each thread is unique.
+        AttrIdGenerator(WorkerLocal::new(|index| {
+            let index: u32 = index.try_into().unwrap();
+
+            #[cfg(debug_assertions)]
+            {
+                let max_id = ((index + 1).next_power_of_two() - 1).bitxor(u32::MAX).reverse_bits();
+                MAX_ATTR_ID.fetch_min(max_id, Ordering::Release);
+            }
+
+            Cell::new(index.reverse_bits())
+        }))
+    }
+
+    pub fn mk_attr_id(&self) -> AttrId {
+        let id = self.0.get();
+
+        // Ensure the assigned attr_id does not overlap the bits
+        // representing the number of threads.
+        #[cfg(debug_assertions)]
+        assert!(id <= MAX_ATTR_ID.load(Ordering::Acquire));
+
+        self.0.set(id + 1);
+        AttrId::from_u32(id)
+    }
 }
 
-pub fn mk_attr(style: AttrStyle, path: Path, args: MacArgs, span: Span) -> Attribute {
-    mk_attr_from_item(AttrItem { path, args, tokens: None }, None, style, span)
+pub fn mk_attr(
+    g: &AttrIdGenerator,
+    style: AttrStyle,
+    path: Path,
+    args: MacArgs,
+    span: Span,
+) -> Attribute {
+    mk_attr_from_item(g, AttrItem { path, args, tokens: None }, None, style, span)
 }
 
 pub fn mk_attr_from_item(
+    g: &AttrIdGenerator,
     item: AttrItem,
     tokens: Option<LazyAttrTokenStream>,
     style: AttrStyle,
     span: Span,
 ) -> Attribute {
     Attribute {
         kind: AttrKind::Normal(P(ast::NormalAttr { item, tokens })),
-        id: mk_attr_id(),
+        id: g.mk_attr_id(),
         style,
         span,
     }
 }
 
 /// Returns an inner attribute with the given value and span.
-pub fn mk_attr_inner(item: MetaItem) -> Attribute {
-    mk_attr(AttrStyle::Inner, item.path, item.kind.mac_args(item.span), item.span)
+pub fn mk_attr_inner(g: &AttrIdGenerator, item: MetaItem) -> Attribute {
+    mk_attr(g, AttrStyle::Inner, item.path, item.kind.mac_args(item.span), item.span)
 }
 
 /// Returns an outer attribute with the given value and span.
-pub fn mk_attr_outer(item: MetaItem) -> Attribute {
-    mk_attr(AttrStyle::Outer, item.path, item.kind.mac_args(item.span), item.span)
+pub fn mk_attr_outer(g: &AttrIdGenerator, item: MetaItem) -> Attribute {
+    mk_attr(g, AttrStyle::Outer, item.path, item.kind.mac_args(item.span), item.span)
 }
 
 pub fn mk_doc_comment(
+    g: &AttrIdGenerator,
     comment_kind: CommentKind,
     style: AttrStyle,
     data: Symbol,
     span: Span,
 ) -> Attribute {
-    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: mk_attr_id(), style, span }
+    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: g.mk_attr_id(), style, span }
 }
 
 pub fn list_contains_name(items: &[NestedMetaItem], name: Symbol) -> bool {

compiler/rustc_ast_lowering/src/expr.rs

@@ -1552,7 +1552,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let uc_nested = attr::mk_nested_word_item(uc_ident);
                 attr::mk_list_item(allow_ident, vec![uc_nested])
             };
-            attr::mk_attr_outer(allow)
+            attr::mk_attr_outer(&self.tcx.sess.parse_sess.attr_id_generator, allow)
         };
         let attrs: AttrVec = thin_vec![attr];
 

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -22,6 +22,7 @@ use rustc_span::source_map::{SourceMap, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, IdentPrinter, Symbol};
 use rustc_span::{BytePos, FileName, Span};
 
+use rustc_ast::attr::AttrIdGenerator;
 use std::borrow::Cow;
 
 pub use self::delimited::IterDelimited;
@@ -107,6 +108,7 @@ pub fn print_crate<'a>(
     ann: &'a dyn PpAnn,
     is_expanded: bool,
     edition: Edition,
+    g: &AttrIdGenerator,
 ) -> String {
     let mut s =
         State { s: pp::Printer::new(), comments: Some(Comments::new(sm, filename, input)), ann };
@@ -120,15 +122,15 @@ pub fn print_crate<'a>(
         // `#![feature(prelude_import)]`
         let pi_nested = attr::mk_nested_word_item(Ident::with_dummy_span(sym::prelude_import));
         let list = attr::mk_list_item(Ident::with_dummy_span(sym::feature), vec![pi_nested]);
-        let fake_attr = attr::mk_attr_inner(list);
+        let fake_attr = attr::mk_attr_inner(g, list);
         s.print_attribute(&fake_attr);
 
         // Currently, in Rust 2018 we don't have `extern crate std;` at the crate
         // root, so this is not needed, and actually breaks things.
         if edition == Edition::Edition2015 {
             // `#![no_std]`
             let no_std_meta = attr::mk_word_item(Ident::with_dummy_span(sym::no_std));
-            let fake_attr = attr::mk_attr_inner(no_std_meta);
+            let fake_attr = attr::mk_attr_inner(g, no_std_meta);
             s.print_attribute(&fake_attr);
         }
     }

compiler/rustc_builtin_macros/src/cmdline_attrs.rs

@@ -28,7 +28,13 @@ pub fn inject(mut krate: ast::Crate, parse_sess: &ParseSess, attrs: &[String]) -
             continue;
         }
 
-        krate.attrs.push(mk_attr(AttrStyle::Inner, path, args, start_span.to(end_span)));
+        krate.attrs.push(mk_attr(
+            &parse_sess.attr_id_generator,
+            AttrStyle::Inner,
+            path,
+            args,
+            start_span.to(end_span),
+        ));
     }
 
     krate

compiler/rustc_builtin_macros/src/derive.rs

@@ -32,7 +32,8 @@ impl MultiItemModifier for Expander {
             ecx.resolver.resolve_derives(ecx.current_expansion.id, ecx.force_mode, &|| {
                 let template =
                     AttributeTemplate { list: Some("Trait1, Trait2, ..."), ..Default::default() };
-                let attr = attr::mk_attr_outer(meta_item.clone());
+                let attr =
+                    attr::mk_attr_outer(&sess.parse_sess.attr_id_generator, meta_item.clone());
                 validate_attr::check_builtin_attribute(
                     &sess.parse_sess,
                     &attr,

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -187,7 +187,10 @@ impl<'a> MutVisitor for EntryPointCleaner<'a> {
                     let dc_nested =
                         attr::mk_nested_word_item(Ident::new(sym::dead_code, self.def_site));
                     let allow_dead_code_item = attr::mk_list_item(allow_ident, vec![dc_nested]);
-                    let allow_dead_code = attr::mk_attr_outer(allow_dead_code_item);
+                    let allow_dead_code = attr::mk_attr_outer(
+                        &self.sess.parse_sess.attr_id_generator,
+                        allow_dead_code_item,
+                    );
                     let attrs = attrs
                         .into_iter()
                         .filter(|attr| {

compiler/rustc_driver/src/pretty.rs

@@ -396,6 +396,7 @@ pub fn print_after_parsing(
                     annotation.pp_ann(),
                     false,
                     parse.edition,
+                    &sess.parse_sess.attr_id_generator,
                 )
             })
         }
@@ -438,6 +439,7 @@ pub fn print_after_hir_lowering<'tcx>(
                     annotation.pp_ann(),
                     true,
                     parse.edition,
+                    &sess.parse_sess.attr_id_generator,
                 )
             })
         }

compiler/rustc_expand/src/build.rs

@@ -616,7 +616,7 @@ impl<'a> ExtCtxt<'a> {
     }
 
     pub fn attribute(&self, mi: ast::MetaItem) -> ast::Attribute {
-        attr::mk_attr_outer(mi)
+        attr::mk_attr_outer(&self.sess.parse_sess.attr_id_generator, mi)
     }
 
     pub fn meta_word(&self, sp: Span, w: Symbol) -> ast::MetaItem {

compiler/rustc_expand/src/config.rs

@@ -424,7 +424,13 @@ impl<'a> StripUnconfigured<'a> {
         );
         trees.push(bracket_group);
         let tokens = Some(LazyAttrTokenStream::new(AttrTokenStream::new(trees)));
-        let attr = attr::mk_attr_from_item(item, tokens, attr.style, item_span);
+        let attr = attr::mk_attr_from_item(
+            &self.sess.parse_sess.attr_id_generator,
+            item,
+            tokens,
+            attr.style,
+            item_span,
+        );
         if attr.has_name(sym::crate_type) {
             self.sess.parse_sess.buffer_lint(
                 rustc_lint_defs::builtin::DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,