From 956a84bfe0502e126dcdd173fc84eecc10df5ebc Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Wed, 8 Jun 2022 18:22:48 +0200
Subject: [PATCH 1/3] Optimize `SbTag::eq`

The code before generated really bad code with a branch.
This nudges LLVM towards being smarter and simply comparing
the integers.
---
 src/stacked_borrows.rs | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/stacked_borrows.rs b/src/stacked_borrows.rs
index 0d671ec653..fd33eea790 100644
--- a/src/stacked_borrows.rs
+++ b/src/stacked_borrows.rs
@@ -27,12 +27,27 @@ pub type CallId = NonZeroU64;
 pub type AllocExtra = Stacks;
 
 /// Tracking pointer provenance
-#[derive(Copy, Clone, Hash, PartialEq, Eq)]
+#[derive(Copy, Clone, Hash, Eq)]
 pub enum SbTag {
     Tagged(PtrId),
     Untagged,
 }
 
+impl SbTag {
+    fn as_u64(self) -> u64 {
+        match self {
+            SbTag::Tagged(id) => id.get(),
+            SbTag::Untagged => 0,
+        }
+    }
+}
+
+impl PartialEq for SbTag {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_u64() == other.as_u64()
+    }
+}
+
 impl fmt::Debug for SbTag {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {

From c01bf62ee42f7fa28a95449b374812cb47d38a95 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Wed, 8 Jun 2022 18:43:12 +0200
Subject: [PATCH 2/3] Allow `clippy::derive_hash_xor_eq`

---
 src/lib.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/lib.rs b/src/lib.rs
index 7d8eb92ac5..22869e9b24 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -17,6 +17,7 @@
     clippy::single_match,
     clippy::useless_format,
     clippy::derive_partial_eq_without_eq,
+    clippy::derive_hash_xor_eq,
     clippy::too_many_arguments
 )]
 

From 93db9a6d71a7f66df81b5c8adbac7bb71d182bc1 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Wed, 8 Jun 2022 19:29:54 +0200
Subject: [PATCH 3/3] Add comment to explain manual optimization

---
 src/stacked_borrows.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/stacked_borrows.rs b/src/stacked_borrows.rs
index fd33eea790..88d1b1f105 100644
--- a/src/stacked_borrows.rs
+++ b/src/stacked_borrows.rs
@@ -44,6 +44,9 @@ impl SbTag {
 
 impl PartialEq for SbTag {
     fn eq(&self, other: &Self) -> bool {
+        // The codegen for the derived Partialeq is bad here and includes a branch.
+        // Since this code is extremely hot, this is optimized here.
+        // https://github.com/rust-lang/rust/issues/49892
         self.as_u64() == other.as_u64()
     }
 }