proc_macro: use crossbeam channels for the proc_macro cross-thread br…

…idge

This is done by having the crossbeam dependency inserted into the
proc_macro server code from the server side, to avoid adding a
dependency to proc_macro.

In addition, this introduces a -Z command-line option which will switch
rustc to run proc-macros using this cross-thread executor. With the
changes to the bridge in #98186, #98187, #98188 and #98189, the
performance of the executor should be much closer to same-thread
execution.

In local testing, the crossbeam executor was substantially more
performant than either of the two existing CrossThread strategies, so
they have been removed to keep things simple.

Cargo.lock

@@ -3885,6 +3885,7 @@ dependencies = [
 name = "rustc_expand"
 version = "0.0.0"
 dependencies = [
+ "crossbeam-channel",
  "rustc_ast",
  "rustc_ast_passes",
  "rustc_ast_pretty",

compiler/rustc_expand/Cargo.toml

@@ -24,3 +24,4 @@ rustc_parse = { path = "../rustc_parse" }
 rustc_session = { path = "../rustc_session" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 rustc_ast = { path = "../rustc_ast" }
+crossbeam-channel = "0.5.0"

compiler/rustc_expand/src/proc_macro.rs

@@ -8,10 +8,37 @@ use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorGuaranteed;
 use rustc_parse::parser::ForceCollect;
+use rustc_session::config::ProcMacroExecutionStrategy;
 use rustc_span::profiling::SpannedEventArgRecorder;
 use rustc_span::{Span, DUMMY_SP};
 
-const EXEC_STRATEGY: pm::bridge::server::SameThread = pm::bridge::server::SameThread;
+struct CrossbeamMessagePipe<T> {
+    tx: crossbeam_channel::Sender<T>,
+    rx: crossbeam_channel::Receiver<T>,
+}
+
+impl<T> pm::bridge::server::MessagePipe<T> for CrossbeamMessagePipe<T> {
+    fn new() -> (Self, Self) {
+        let (tx1, rx1) = crossbeam_channel::bounded(1);
+        let (tx2, rx2) = crossbeam_channel::bounded(1);
+        (CrossbeamMessagePipe { tx: tx1, rx: rx2 }, CrossbeamMessagePipe { tx: tx2, rx: rx1 })
+    }
+
+    fn send(&mut self, value: T) {
+        self.tx.send(value).unwrap();
+    }
+
+    fn recv(&mut self) -> Option<T> {
+        self.rx.recv().ok()
+    }
+}
+
+fn exec_strategy(ecx: &ExtCtxt<'_>) -> impl pm::bridge::server::ExecutionStrategy {
+    pm::bridge::server::MaybeCrossThread::<CrossbeamMessagePipe<_>>::new(
+        ecx.sess.opts.unstable_opts.proc_macro_execution_strategy
+            == ProcMacroExecutionStrategy::CrossThread,
+    )
+}
 
 pub struct BangProcMacro {
     pub client: pm::bridge::client::Client<pm::TokenStream, pm::TokenStream>,
@@ -30,8 +57,9 @@ impl base::BangProcMacro for BangProcMacro {
             });
 
         let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
+        let strategy = exec_strategy(ecx);
         let server = proc_macro_server::Rustc::new(ecx);
-        self.client.run(&EXEC_STRATEGY, server, input, proc_macro_backtrace).map_err(|e| {
+        self.client.run(&strategy, server, input, proc_macro_backtrace).map_err(|e| {
             let mut err = ecx.struct_span_err(span, "proc macro panicked");
             if let Some(s) = e.as_str() {
                 err.help(&format!("message: {}", s));
@@ -59,16 +87,17 @@ impl base::AttrProcMacro for AttrProcMacro {
             });
 
         let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
+        let strategy = exec_strategy(ecx);
         let server = proc_macro_server::Rustc::new(ecx);
-        self.client
-            .run(&EXEC_STRATEGY, server, annotation, annotated, proc_macro_backtrace)
-            .map_err(|e| {
+        self.client.run(&strategy, server, annotation, annotated, proc_macro_backtrace).map_err(
+            |e| {
                 let mut err = ecx.struct_span_err(span, "custom attribute panicked");
                 if let Some(s) = e.as_str() {
                     err.help(&format!("message: {}", s));
                 }
                 err.emit()
-            })
+            },
+        )
     }
 }
 
@@ -105,8 +134,9 @@ impl MultiItemModifier for DeriveProcMacro {
                     recorder.record_arg_with_span(ecx.expansion_descr(), span);
                 });
             let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
+            let strategy = exec_strategy(ecx);
             let server = proc_macro_server::Rustc::new(ecx);
-            match self.client.run(&EXEC_STRATEGY, server, input, proc_macro_backtrace) {
+            match self.client.run(&strategy, server, input, proc_macro_backtrace) {
                 Ok(stream) => stream,
                 Err(e) => {
                     let mut err = ecx.struct_span_err(span, "proc-macro derive panicked");

compiler/rustc_interface/src/tests.rs

@@ -11,7 +11,7 @@ use rustc_session::config::{
 };
 use rustc_session::config::{
     BranchProtection, Externs, OomStrategy, OutputType, OutputTypes, PAuthKey, PacRet,
-    SymbolManglingVersion, WasiExecModel,
+    ProcMacroExecutionStrategy, SymbolManglingVersion, WasiExecModel,
 };
 use rustc_session::config::{CFGuard, ExternEntry, LinkerPluginLto, LtoCli, SwitchWithOptPath};
 use rustc_session::lint::Level;
@@ -685,6 +685,7 @@ fn test_unstable_options_tracking_hash() {
     untracked!(print_mono_items, Some(String::from("abc")));
     untracked!(print_type_sizes, true);
     untracked!(proc_macro_backtrace, true);
+    untracked!(proc_macro_execution_strategy, ProcMacroExecutionStrategy::CrossThread);
     untracked!(query_dep_graph, true);
     untracked!(save_analysis, true);
     untracked!(self_profile, SwitchWithOptPath::Enabled(None));

compiler/rustc_session/src/config.rs

@@ -2959,3 +2959,13 @@ impl OomStrategy {
         }
     }
 }
+
+/// How to run proc-macro code when building this crate
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum ProcMacroExecutionStrategy {
+    /// Run the proc-macro code on the same thread as the server.
+    SameThread,
+
+    /// Run the proc-macro code on a different thread.
+    CrossThread,
+}

compiler/rustc_session/src/options.rs

@@ -415,6 +415,8 @@ mod desc {
         "one of (`none` (default), `basic`, `strong`, or `all`)";
     pub const parse_branch_protection: &str =
         "a `,` separated combination of `bti`, `b-key`, `pac-ret`, or `leaf`";
+    pub const parse_proc_macro_execution_strategy: &str =
+        "one of supported execution strategies (`same-thread`, or `cross-thread`)";
 }
 
 mod parse {
@@ -1062,6 +1064,18 @@ mod parse {
         }
         true
     }
+
+    pub(crate) fn parse_proc_macro_execution_strategy(
+        slot: &mut ProcMacroExecutionStrategy,
+        v: Option<&str>,
+    ) -> bool {
+        *slot = match v {
+            Some("same-thread") => ProcMacroExecutionStrategy::SameThread,
+            Some("cross-thread") => ProcMacroExecutionStrategy::CrossThread,
+            _ => return false,
+        };
+        true
+    }
 }
 
 options! {
@@ -1457,6 +1471,9 @@ options! {
         "print layout information for each type encountered (default: no)"),
     proc_macro_backtrace: bool = (false, parse_bool, [UNTRACKED],
          "show backtraces for panics during proc-macro execution (default: no)"),
+    proc_macro_execution_strategy: ProcMacroExecutionStrategy = (ProcMacroExecutionStrategy::SameThread,
+        parse_proc_macro_execution_strategy, [UNTRACKED],
+        "how to run proc-macro code (default: same-thread)"),
     profile: bool = (false, parse_bool, [TRACKED],
         "insert profiling code (default: no)"),
     profile_closures: bool = (false, parse_no_flag, [UNTRACKED],

library/proc_macro/src/bridge/server.rs

@@ -2,6 +2,8 @@
 
 use super::*;
 
+use std::marker::PhantomData;
+
 // FIXME(eddyb) generate the definition of `HandleStore` in `server.rs`.
 use super::client::HandleStore;
 
@@ -143,6 +145,41 @@ pub trait ExecutionStrategy {
     ) -> Buffer;
 }
 
+pub struct MaybeCrossThread<P> {
+    cross_thread: bool,
+    marker: PhantomData<P>,
+}
+
+impl<P> MaybeCrossThread<P> {
+    pub const fn new(cross_thread: bool) -> Self {
+        MaybeCrossThread { cross_thread, marker: PhantomData }
+    }
+}
+
+impl<P> ExecutionStrategy for MaybeCrossThread<P>
+where
+    P: MessagePipe<Buffer> + Send + 'static,
+{
+    fn run_bridge_and_client(
+        &self,
+        dispatcher: &mut impl DispatcherTrait,
+        input: Buffer,
+        run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
+        force_show_panics: bool,
+    ) -> Buffer {
+        if self.cross_thread {
+            <CrossThread<P>>::new().run_bridge_and_client(
+                dispatcher,
+                input,
+                run_client,
+                force_show_panics,
+            )
+        } else {
+            SameThread.run_bridge_and_client(dispatcher, input, run_client, force_show_panics)
+        }
+    }
+}
+
 pub struct SameThread;
 
 impl ExecutionStrategy for SameThread {
@@ -164,28 +201,31 @@ impl ExecutionStrategy for SameThread {
     }
 }
 
-// NOTE(eddyb) Two implementations are provided, the second one is a bit
-// faster but neither is anywhere near as fast as same-thread execution.
+pub struct CrossThread<P>(PhantomData<P>);
 
-pub struct CrossThread1;
+impl<P> CrossThread<P> {
+    pub const fn new() -> Self {
+        CrossThread(PhantomData)
+    }
+}
 
-impl ExecutionStrategy for CrossThread1 {
+impl<P> ExecutionStrategy for CrossThread<P>
+where
+    P: MessagePipe<Buffer> + Send + 'static,
+{
     fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
         input: Buffer,
         run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
         force_show_panics: bool,
     ) -> Buffer {
-        use std::sync::mpsc::channel;
-
-        let (req_tx, req_rx) = channel();
-        let (res_tx, res_rx) = channel();
+        let (mut server, mut client) = P::new();
 
         let join_handle = thread::spawn(move || {
-            let mut dispatch = |buf| {
-                req_tx.send(buf).unwrap();
-                res_rx.recv().unwrap()
+            let mut dispatch = |b: Buffer| -> Buffer {
+                client.send(b);
+                client.recv().expect("server died while client waiting for reply")
             };
 
             run_client(BridgeConfig {
@@ -196,75 +236,27 @@ impl ExecutionStrategy for CrossThread1 {
             })
         });
 
-        for b in req_rx {
-            res_tx.send(dispatcher.dispatch(b)).unwrap();
+        while let Some(b) = server.recv() {
+            server.send(dispatcher.dispatch(b));
         }
 
         join_handle.join().unwrap()
     }
 }
 
-pub struct CrossThread2;
+/// A message pipe used for communicating between server and client threads.
+pub trait MessagePipe<T>: Sized {
+    /// Create a new pair of endpoints for the message pipe.
+    fn new() -> (Self, Self);
 
-impl ExecutionStrategy for CrossThread2 {
-    fn run_bridge_and_client(
-        &self,
-        dispatcher: &mut impl DispatcherTrait,
-        input: Buffer,
-        run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
-        force_show_panics: bool,
-    ) -> Buffer {
-        use std::sync::{Arc, Mutex};
-
-        enum State<T> {
-            Req(T),
-            Res(T),
-        }
-
-        let mut state = Arc::new(Mutex::new(State::Res(Buffer::new())));
-
-        let server_thread = thread::current();
-        let state2 = state.clone();
-        let join_handle = thread::spawn(move || {
-            let mut dispatch = |b| {
-                *state2.lock().unwrap() = State::Req(b);
-                server_thread.unpark();
-                loop {
-                    thread::park();
-                    if let State::Res(b) = &mut *state2.lock().unwrap() {
-                        break b.take();
-                    }
-                }
-            };
-
-            let r = run_client(BridgeConfig {
-                input,
-                dispatch: (&mut dispatch).into(),
-                force_show_panics,
-                _marker: marker::PhantomData,
-            });
-
-            // Wake up the server so it can exit the dispatch loop.
-            drop(state2);
-            server_thread.unpark();
-
-            r
-        });
-
-        // Check whether `state2` was dropped, to know when to stop.
-        while Arc::get_mut(&mut state).is_none() {
-            thread::park();
-            let mut b = match &mut *state.lock().unwrap() {
-                State::Req(b) => b.take(),
-                _ => continue,
-            };
-            b = dispatcher.dispatch(b.take());
-            *state.lock().unwrap() = State::Res(b);
-            join_handle.thread().unpark();
-        }
+    /// Send a message to the other endpoint of this pipe.
+    fn send(&mut self, value: T);
 
-        join_handle.join().unwrap()
-    }
+    /// Receive a message from the other endpoint of this pipe.
+    ///
+    /// Returns `None` if the other end of the pipe has been destroyed, and no
+    /// message was received.
+    fn recv(&mut self) -> Option<T>;
 }
 
 fn run_server<

src/test/rustdoc-ui/z-help.stdout

@@ -114,6 +114,7 @@
     -Z                        print-mono-items=val -- print the result of the monomorphization collection pass
     -Z                        print-type-sizes=val -- print layout information for each type encountered (default: no)
     -Z                    proc-macro-backtrace=val -- show backtraces for panics during proc-macro execution (default: no)
+    -Z           proc-macro-execution-strategy=val -- how to run proc-macro code (default: same-thread)
     -Z                                 profile=val -- insert profiling code (default: no)
     -Z                        profile-closures=val -- profile size of closures
     -Z                            profile-emit=val -- file path to emit profiling data at runtime when using 'profile' (default based on relative source path)