Auto merge of #123089 - Philippe-Cholet:vecdeque_pop_assume_cap, r=<try>

Add invariant to VecDeque::pop_* that len < cap if pop successful

Similar to #114370 for VecDeque instead of Vec.

I initially come from rust-itertools/itertools#899 where we noticed that `pop_front;push_back;` was slower than expected so `@scottmcm` suggested I file an issue which lead to https://internals.rust-lang.org/t/vecdeque-pop-front-push-back/20483 where **kornel** mentionned #114334 (fixed by #114370).

This is my first time with codegen tests, I based the test on what was done for Vec.

Author: bors

library/alloc/src/collections/vec_deque/mod.rs

@@ -1614,6 +1614,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
             let old_head = self.head;
             self.head = self.to_physical_idx(1);
             self.len -= 1;
+            unsafe {
+                core::intrinsics::assume(self.len < self.capacity());
+            }
             Some(unsafe { self.buffer_read(old_head) })
         }
     }
@@ -1638,6 +1641,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
             None
         } else {
             self.len -= 1;
+            unsafe {
+                core::intrinsics::assume(self.len < self.capacity());
+            }
             Some(unsafe { self.buffer_read(self.to_physical_idx(self.len)) })
         }
     }

tests/codegen/vecdeque_pop_push.rs

@@ -0,0 +1,75 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+use std::collections::VecDeque;
+
+#[no_mangle]
+// CHECK-LABEL: @noop_back(
+pub fn noop_back(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_back() {
+        v.push_back(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @noop_front(
+pub fn noop_front(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_front() {
+        v.push_front(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @move_byte_front_to_back(
+pub fn move_byte_front_to_back(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_front() {
+        v.push_back(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @move_byte_back_to_front(
+pub fn move_byte_back_to_front(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_back() {
+        v.push_front(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @push_back_byte(
+pub fn push_back_byte(v: &mut VecDeque<u8>) {
+    // CHECK: call {{.*}}reserve_for_push
+    v.push_back(3);
+}
+
+#[no_mangle]
+// CHECK-LABEL: @push_front_byte(
+pub fn push_front_byte(v: &mut VecDeque<u8>) {
+    // CHECK: call {{.*}}reserve_for_push
+    v.push_front(3);
+}