From 9a55d7a8c0dbdc0cf609ec3647c00202bf898a91 Mon Sep 17 00:00:00 2001
From: Kornel <kornel@geekhood.net>
Date: Mon, 9 Oct 2023 15:05:32 +0100
Subject: [PATCH] Hint optimizer about reserved capacity

---
 library/alloc/src/raw_vec.rs               | 22 ++++++++++++++++++----
 tests/ui/hygiene/panic-location.run.stderr |  2 +-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs
index 01b03de6acb5e..bc994694ad7ef 100644
--- a/library/alloc/src/raw_vec.rs
+++ b/library/alloc/src/raw_vec.rs
@@ -291,6 +291,10 @@ impl<T, A: Allocator> RawVec<T, A> {
 
         if self.needs_to_grow(len, additional) {
             do_reserve_and_handle(self, len, additional);
+            unsafe {
+                // Inform the optimizer that the reservation has succeeded
+                core::intrinsics::assume(!self.needs_to_grow(len, additional));
+            }
         }
     }
 
@@ -305,10 +309,13 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// The same as `reserve`, but returns on errors instead of panicking or aborting.
     pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
         if self.needs_to_grow(len, additional) {
-            self.grow_amortized(len, additional)
-        } else {
-            Ok(())
+            self.grow_amortized(len, additional)?;
+            unsafe {
+                // Inform the optimizer that the reservation has succeeded
+                core::intrinsics::assume(!self.needs_to_grow(len, additional));
+            }
         }
+        Ok(())
     }
 
     /// Ensures that the buffer contains at least enough space to hold `len +
@@ -339,7 +346,14 @@ impl<T, A: Allocator> RawVec<T, A> {
         len: usize,
         additional: usize,
     ) -> Result<(), TryReserveError> {
-        if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
+        if self.needs_to_grow(len, additional) {
+            self.grow_exact(len, additional)?;
+            unsafe {
+                // Inform the optimizer that the reservation has succeeded
+                core::intrinsics::assume(!self.needs_to_grow(len, additional));
+            }
+        }
+        Ok(())
     }
 
     /// Shrinks the buffer down to the specified capacity. If the given amount
diff --git a/tests/ui/hygiene/panic-location.run.stderr b/tests/ui/hygiene/panic-location.run.stderr
index 5ed0d9fcf1ef1..25ec67a40dd22 100644
--- a/tests/ui/hygiene/panic-location.run.stderr
+++ b/tests/ui/hygiene/panic-location.run.stderr
@@ -1,3 +1,3 @@
-thread 'main' panicked at library/alloc/src/raw_vec.rs:534:5:
+thread 'main' panicked at library/alloc/src/raw_vec.rs:548:5:
 capacity overflow
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace