Use SetLenOnDrop in Vec::truncate()

This avoids a redundant length check in some cases when calling
`Vec::truncate` or `Vec::clear`.

Fixes #51802

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index 5efe1e23309a7d7a84e96796149be522763511c9..cc913dfbb4b01e140abb47403998f7a65c224f3a 100644 (file)
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -690,14 +690,20 @@ pub fn into_boxed_slice(mut self) -> Box<[T]> {
     /// [`drain`]: #method.drain
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn truncate(&mut self, len: usize) {
+        let current_len = self.len;
         unsafe {
+            let mut ptr = self.as_mut_ptr().offset(self.len as isize);
+            // Set the final length at the end, keeping in mind that
+            // dropping an element might panic. Works around a missed
+            // optimization, as seen in the following issue:
+            // https://github.com/rust-lang/rust/issues/51802
+            let mut local_len = SetLenOnDrop::new(&mut self.len);
+
             // drop any extra elements
-            while len < self.len {
-                // decrement len before the drop_in_place(), so a panic on Drop
-                // doesn't re-drop the just-failed value.
-                self.len -= 1;
-                let len = self.len;
-                ptr::drop_in_place(self.get_unchecked_mut(len));
+            for _ in len..current_len {
+                local_len.decrement_len(1);
+                ptr = ptr.offset(-1);
+                ptr::drop_in_place(ptr);
             }
         }
     }
@@ -1512,6 +1518,11 @@ fn new(len: &'a mut usize) -> Self {
     fn increment_len(&mut self, increment: usize) {
         self.local_len += increment;
     }
+
+    #[inline]
+    fn decrement_len(&mut self, decrement: usize) {
+        self.local_len -= decrement;
+    }
 }
 
 impl<'a> Drop for SetLenOnDrop<'a> {

diff --git a/src/test/codegen/vec-clear.rs b/src/test/codegen/vec-clear.rs
new file mode 100644 (file)
index 0000000..a73dd07
--- /dev/null
+++ b/src/test/codegen/vec-clear.rs
@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @vec_clear
+#[no_mangle]
+pub fn vec_clear(x: &mut Vec<u32>) {
+    // CHECK-NOT: load
+    // CHECK-NOT: icmp
+    x.clear()
+}