From b6018c0fc478a45ee39e1efa98032006d7be3d74 Mon Sep 17 00:00:00 2001
From: Brian Smith <brian@briansmith.org>
Date: Thu, 12 Jun 2025 10:27:15 -0700
Subject: [PATCH 1/3] bb: Implement `Word::is_zero`.

---
 src/bb/ops.rs | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 src/bb/ops.rs

diff --git a/src/bb/ops.rs b/src/bb/ops.rs
new file mode 100644
index 0000000000..e69de29bb2

From 0898d7a3cbd14b9c885c9d85c011a1a4eb320e65 Mon Sep 17 00:00:00 2001
From: Brian Smith <brian@briansmith.org>
Date: Thu, 12 Jun 2025 11:00:43 -0700
Subject: [PATCH 2/3] bb/aarch64: Implement optimized `WordOps::is_zero`.

---
 src/bb/word/aarch64.rs | 18 ++++++++++++++++++
 src/bb/word/mod.rs     |  9 ++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 src/bb/word/aarch64.rs

diff --git a/src/bb/word/aarch64.rs b/src/bb/word/aarch64.rs
new file mode 100644
index 0000000000..e5ff3e31dc
--- /dev/null
+++ b/src/bb/word/aarch64.rs
@@ -0,0 +1,18 @@
+use super::{super::BoolMask, Word, WordOps};
+use core::{arch::asm, mem};
+
+impl WordOps for Word {
+    fn is_zero(self) -> BoolMask {
+        let r: u64;
+        unsafe {
+            asm!(
+                "subs {r}, {a}, #1",
+                "sbc  {r}, {r}, {r}", // r - r - carry = 0 - carry = -carry.
+                a = in(reg) self,
+                r = lateout(reg) r,
+                options(nomem, nostack, pure)
+            );
+        }
+        unsafe { mem::transmute::<u64, BoolMask>(r) }
+    }
+}
diff --git a/src/bb/word/mod.rs b/src/bb/word/mod.rs
index 1cfe429085..89dea9e57d 100644
--- a/src/bb/word/mod.rs
+++ b/src/bb/word/mod.rs
@@ -13,6 +13,7 @@
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 use super::{BoolMask, LeakyWord};
+use cfg_if::cfg_if;
 
 /// A native word that may hold a secret.
 ///
@@ -47,4 +48,10 @@ impl From<LeakyWord> for Word {
 }
 */
 
-mod fallback;
+cfg_if! {
+    if #[cfg(target_arch = "aarch64")] {
+        mod aarch64;
+    } else {
+        mod fallback;
+    }
+}

From 16c042160fe2908d4636b5d6c0aaa949ae399f1f Mon Sep 17 00:00:00 2001
From: Brian Smith <brian@briansmith.org>
Date: Thu, 12 Jun 2025 12:24:05 -0700
Subject: [PATCH 3/3] bb: Implement `WordOps::from_u8`.

---
 src/bb/bytes.rs         |  3 +--
 src/bb/word/aarch64.rs  | 14 ++++++++++++++
 src/bb/word/fallback.rs |  5 +++++
 src/bb/word/mod.rs      |  1 +
 4 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/src/bb/bytes.rs b/src/bb/bytes.rs
index 73564019d0..55e05d2d2c 100644
--- a/src/bb/bytes.rs
+++ b/src/bb/bytes.rs
@@ -59,8 +59,7 @@ pub fn bytes_are_equal(a: &[u8], b: &[u8]) -> BoolMask {
         #[allow(clippy::into_iter_on_ref)]
         let rem = a_rem
             .into_iter()
-            .copied()
-            .map(Word::from)
+            .map(Word::from_u8)
             .zip(b_rem.into_iter().copied().map(Word::from))
             .fold(0, |acc, (a, b)| acc | (a ^ b));
         acc |= rem;
diff --git a/src/bb/word/aarch64.rs b/src/bb/word/aarch64.rs
index e5ff3e31dc..d0e09581bb 100644
--- a/src/bb/word/aarch64.rs
+++ b/src/bb/word/aarch64.rs
@@ -1,7 +1,21 @@
 use super::{super::BoolMask, Word, WordOps};
+use crate::polyfill;
 use core::{arch::asm, mem};
 
 impl WordOps for Word {
+    fn from_u8(a: &u8) -> Self {
+        let mut r: Self;
+        unsafe {
+            asm!(
+                "ldrb {r:w}, [{a_ptr}]", // zero-extended to 32-bits, then 64-bits.
+                a_ptr = in(reg) polyfill::ptr::from_ref(a),
+                r = lateout(reg) r,
+                options(nostack, readonly)
+            );
+        }
+        r
+    }
+
     fn is_zero(self) -> BoolMask {
         let r: u64;
         unsafe {
diff --git a/src/bb/word/fallback.rs b/src/bb/word/fallback.rs
index fb8a092257..20ce194edc 100644
--- a/src/bb/word/fallback.rs
+++ b/src/bb/word/fallback.rs
@@ -1,6 +1,11 @@
 use super::{super::BoolMask, Word, WordOps};
 
 impl WordOps for Word {
+    #[inline(always)]
+    fn from_u8(a: &u8) -> Self {
+        Self::from(*a)
+    }
+
     #[inline]
     fn is_zero(self) -> BoolMask {
         use crate::limb::{Limb, LimbMask}; // XXX: Backwards dependency.
diff --git a/src/bb/word/mod.rs b/src/bb/word/mod.rs
index 89dea9e57d..ccbe2d2d39 100644
--- a/src/bb/word/mod.rs
+++ b/src/bb/word/mod.rs
@@ -29,6 +29,7 @@ use cfg_if::cfg_if;
 pub(crate) type Word = LeakyWord;
 
 pub(crate) trait WordOps: Copy {
+    fn from_u8(a: &u8) -> Self;
     fn is_zero(self) -> BoolMask;
 }