Unroll one more loop

I also tried unrolling the 256-iteration loop further below, but it actually caused a slowdown (my guess is either branch prediction stopped kicking in or the instruction cache was being maculated).
mkvtvseries · Jul 19, 2017 · 2cc1c92 · 2cc1c92
1 parent d51958d
commit 2cc1c92
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Showing 2 changed files with 19 additions and 12 deletions.
diff --git a/Cargo.lock b/Cargo.lock
diff --git a/ethash/src/compute.rs b/ethash/src/compute.rs
@@ -298,7 +298,7 @@ fn hash_compute(light: &Light, full_size: usize, header_hash: &H256, nonce: u64)
 
 				let index = fnv_hash(
 					f_mix.get_unchecked(0).as_words().get_unchecked(0) ^ i,
-					*mix.get_unchecked(0).as_words().get_unchecked(i_usize % MIX_WORDS)
+					*mix.get_unchecked(0).as_words().get_unchecked(i as usize % MIX_WORDS)
 				) % num_full_pages;
 
 				unroll! {
@@ -324,14 +324,18 @@ fn hash_compute(light: &Light, full_size: usize, header_hash: &H256, nonce: u64)
 			}
 		}
 
+		debug_assert_eq!(MIX_WORDS / 4, 8);
+
 		// compress mix
-		for i in 0..(MIX_WORDS / 4) {
-			let w = i * 4;
-			let mut reduction = *mix.get_unchecked(0).as_words().get_unchecked(w + 0);
-			reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 1);
-			reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 2);
-			reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 3);
-			*mix.get_unchecked_mut(0).as_words_mut().get_unchecked_mut(i) = reduction;
+		unroll! {
+			for i in 0..8 {
+				let w = i * 4;
+				let mut reduction = *mix.get_unchecked(0).as_words().get_unchecked(w + 0);
+				reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 1);
+				reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 2);
+				reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 3);
+				*mix.get_unchecked_mut(0).as_words_mut().get_unchecked_mut(i) = reduction;
+			}
 		}
 
 		let mut mix_hash = [0u8; 32];
@@ -356,13 +360,15 @@ fn calculate_dag_item(node_index: u32, cache: &[Node]) -> Node {
 		*ret.as_words_mut().get_unchecked_mut(0) ^= node_index;
 		sha3::sha3_512(ret.bytes.as_mut_ptr(), ret.bytes.len(), ret.bytes.as_ptr(), ret.bytes.len());
 
-		for i in 0..ETHASH_DATASET_PARENTS {
+		for i_usize in 0..ETHASH_DATASET_PARENTS {
+			let i = i_usize as u32;
 			let parent_index = fnv_hash(node_index ^ i, *ret.as_words().get_unchecked(i as usize % NODE_WORDS)) % num_parent_nodes as u32;
 			let parent = cache.get_unchecked(parent_index as usize);
 			for w in 0..NODE_WORDS {
 				*ret.as_words_mut().get_unchecked_mut(w) = fnv_hash(*ret.as_words().get_unchecked(w), *parent.as_words().get_unchecked(w));
 			}
 		}
+
 		sha3::sha3_512(ret.bytes.as_mut_ptr(), ret.bytes.len(), ret.bytes.as_ptr(), ret.bytes.len());
 		ret
 	}
@@ -386,6 +392,7 @@ fn light_new(block_number: u64) -> Light {
 			sha3::sha3_512(nodes.get_unchecked_mut(i).bytes.as_mut_ptr(), NODE_BYTES, nodes.get_unchecked(i - 1).bytes.as_ptr(), NODE_BYTES);
 		}
 
+		// This _should_ get unrolled by the compiler, since it's not using the loop variable.
 		for _ in 0..ETHASH_CACHE_ROUNDS {
 			for i in 0..num_nodes {
 				let idx = *nodes.get_unchecked_mut(i).as_words().get_unchecked(0) as usize % num_nodes;