Auto merge of #51630 - joshlf:map-split-perf, r=dtolnay

Optimize RefCell refcount tracking

Address the performance concern raised in https://github.com/rust-lang/rust/pull/51466#issuecomment-398255276

cc @dtolnay  @nnethercote @rust-lang/wg-compiler-performance

cc @RalfJung @jhjourdan for soundness concerns

Can somebody kick off a perf run on this? I'm not sure how that's done, but I understand it has to be started manually.

The idea of this change is to switch to representing mutable refcount as values below 0 to eliminate some branching that was required with the old algorithm.

4 files changed, 4 insertions, 4 deletions

diff --git a/src/test/compile-fail/not-panic-safe-2.rs b/src/test/compile-fail/not-panic-safe-2.rs
index d750851b719..5490acf4bd6 100644
--- a/src/test/compile-fail/not-panic-safe-2.rs
+++ b/src/test/compile-fail/not-panic-safe-2.rs
@@ -19,5 +19,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<Rc<RefCell<i32>>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }
diff --git a/src/test/compile-fail/not-panic-safe-3.rs b/src/test/compile-fail/not-panic-safe-3.rs
index cd27b274258..0fac395a115 100644
--- a/src/test/compile-fail/not-panic-safe-3.rs
+++ b/src/test/compile-fail/not-panic-safe-3.rs
@@ -19,5 +19,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<Arc<RefCell<i32>>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }
diff --git a/src/test/compile-fail/not-panic-safe-4.rs b/src/test/compile-fail/not-panic-safe-4.rs
index 956eca432c5..bf0392018b5 100644
--- a/src/test/compile-fail/not-panic-safe-4.rs
+++ b/src/test/compile-fail/not-panic-safe-4.rs
@@ -18,5 +18,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<&RefCell<i32>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }
diff --git a/src/test/compile-fail/not-panic-safe-6.rs b/src/test/compile-fail/not-panic-safe-6.rs
index d0ca1db5212..950f0a0b53a 100644
--- a/src/test/compile-fail/not-panic-safe-6.rs
+++ b/src/test/compile-fail/not-panic-safe-6.rs
@@ -18,5 +18,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<*mut RefCell<i32>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }