Use hardware acceleration for SHA256 (#2426)

## Proposed Changes

Modify the SHA256 implementation in `eth2_hashing` so that it switches between `ring` and `sha2` to take advantage of [x86_64 SHA extensions](https://en.wikipedia.org/wiki/Intel_SHA_extensions). The extensions are available on modern Intel and AMD CPUs, and seem to provide a considerable speed-up: on my Ryzen 5950X it dropped state tree hashing times by about 30% from 35ms to 25ms (on Prater).

## Additional Info

The extensions became available in the `sha2` crate [last year](https://www.reddit.com/r/rust/comments/hf2vcx/ann_rustcryptos_sha1_and_sha2_now_support/), and are not available in Ring, which uses a [pure Rust implementation of sha2](https://github.com/briansmith/ring/blob/main/src/digest/sha2.rs). Ring is faster on CPUs that lack the extensions so I've implemented a runtime switch to use `sha2` only when the extensions are available. The runtime switching seems to impose a miniscule penalty (see the benchmarks linked below).

consensus/tree_hash/src/lib.rs

@@ -7,8 +7,7 @@ pub use merkle_hasher::{Error, MerkleHasher};
 pub use merkleize_padded::merkleize_padded;
 pub use merkleize_standard::merkleize_standard;
 
-use eth2_hashing::{Context, SHA256};
-use eth2_hashing::{ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
+use eth2_hashing::{hash_fixed, ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
 
 pub const BYTES_PER_CHUNK: usize = 32;
 pub const HASHSIZE: usize = 32;
@@ -39,11 +38,7 @@ pub fn merkle_root(bytes: &[u8], minimum_leaf_count: usize) -> Hash256 {
         let mut leaves = [0; HASHSIZE * 2];
         leaves[0..bytes.len()].copy_from_slice(bytes);
 
-        let mut context = Context::new(&SHA256);
-        context.update(&leaves);
-        let digest = context.finish();
-
-        Hash256::from_slice(digest.as_ref())
+        Hash256::from_slice(&hash_fixed(&leaves))
     } else {
         // If there are 3 or more leaves, use `MerkleHasher`.
         let mut hasher = MerkleHasher::with_leaves(leaves);

consensus/tree_hash/src/merkle_hasher.rs

@@ -1,5 +1,5 @@
 use crate::{get_zero_hash, Hash256, HASHSIZE};
-use eth2_hashing::{Context, Digest, SHA256};
+use eth2_hashing::{Context, Sha256Context, HASH_LEN};
 use smallvec::{smallvec, SmallVec};
 use std::mem;
 
@@ -15,7 +15,7 @@ pub enum Error {
 ///
 /// Should be used as a left or right value for some node.
 enum Preimage<'a> {
-    Digest(Digest),
+    Digest([u8; HASH_LEN]),
     Slice(&'a [u8]),
 }
 
@@ -41,7 +41,7 @@ struct HalfNode {
 impl HalfNode {
     /// Create a new half-node from the given `left` value.
     fn new(id: usize, left: Preimage) -> Self {
-        let mut context = Context::new(&SHA256);
+        let mut context = Context::new();
         context.update(left.as_bytes());
 
         Self { context, id }
@@ -49,9 +49,9 @@ impl HalfNode {
 
     /// Complete the half-node by providing a `right` value. Returns a digest of the left and right
     /// nodes.
-    fn finish(mut self, right: Preimage) -> Digest {
+    fn finish(mut self, right: Preimage) -> [u8; HASH_LEN] {
         self.context.update(right.as_bytes());
-        self.context.finish()
+        self.context.finalize()
     }
 }
 
@@ -124,7 +124,7 @@ pub struct MerkleHasher {
     /// Stores the nodes that are half-complete and awaiting a right node.
     ///
     /// A smallvec of size 8 means we can hash a tree with 256 leaves without allocating on the
-    /// heap. Each half-node is 224 bytes, so this smallvec may store 1,792 bytes on the stack.
+    /// heap. Each half-node is 232 bytes, so this smallvec may store 1856 bytes on the stack.
     half_nodes: SmallVec8<HalfNode>,
     /// The depth of the tree that will be produced.
     ///
@@ -368,7 +368,7 @@ mod test {
     fn context_size() {
         assert_eq!(
             mem::size_of::<HalfNode>(),
-            216 + 8,
+            232,
             "Halfnode size should be as expected"
         );
     }

consensus/tree_hash/src/merkleize_padded.rs

@@ -1,5 +1,5 @@
 use super::{get_zero_hash, Hash256, BYTES_PER_CHUNK};
-use eth2_hashing::{hash, hash32_concat};
+use eth2_hashing::{hash32_concat, hash_fixed};
 
 /// Merkleize `bytes` and return the root, optionally padding the tree out to `min_leaves` number of
 /// leaves.
@@ -79,7 +79,7 @@ pub fn merkleize_padded(bytes: &[u8], min_leaves: usize) -> Hash256 {
         // Hash two chunks, creating a parent chunk.
         let hash = match bytes.get(start..start + BYTES_PER_CHUNK * 2) {
             // All bytes are available, hash as usual.
-            Some(slice) => hash(slice),
+            Some(slice) => hash_fixed(slice),
             // Unable to get all the bytes, get a small slice and pad it out.
             None => {
                 let mut preimage = bytes
@@ -87,7 +87,7 @@ pub fn merkleize_padded(bytes: &[u8], min_leaves: usize) -> Hash256 {
                     .expect("`i` can only be larger than zero if there are bytes to read")
                     .to_vec();
                 preimage.resize(BYTES_PER_CHUNK * 2, 0);
-                hash(&preimage)
+                hash_fixed(&preimage)
             }
         };