Unroll a couple loops in Poseidon code (#215)

* Unroll a couple loops in Poseidon code (super hacky)

* Comments

src/hash/poseidon.rs

@@ -137,7 +137,7 @@ where
     const FAST_PARTIAL_ROUND_W_HATS: [[u64; WIDTH - 1]; N_PARTIAL_ROUNDS];
     const FAST_PARTIAL_ROUND_INITIAL_MATRIX: [[u64; WIDTH - 1]; WIDTH - 1];
 
-    #[inline]
+    #[inline(always)]
     #[unroll_for_loops]
     fn mds_row_shf(r: usize, v: &[u64; WIDTH]) -> u128 {
         debug_assert!(r < WIDTH);
@@ -148,9 +148,15 @@ where
         // NB: Unrolling this, calculating each term independently, and
         // summing at the end, didn't improve performance for me.
         let mut res = 0u128;
-        for i in 0..WIDTH {
-            res += (v[(i + r) % WIDTH] as u128) << Self::MDS_MATRIX_EXPS[i];
+
+        // This is a hacky way of fully unrolling the loop.
+        assert!(WIDTH <= 12);
+        for i in 0..12 {
+            if i < WIDTH {
+                res += (v[(i + r) % WIDTH] as u128) << Self::MDS_MATRIX_EXPS[i];
+            }
         }
+
         res
     }
 
@@ -164,9 +170,14 @@ where
             state[r] = state_[r].to_noncanonical_u64();
         }
 
-        for r in 0..WIDTH {
-            result[r] = Self::from_noncanonical_u128(Self::mds_row_shf(r, &state));
+        // This is a hacky way of fully unrolling the loop.
+        assert!(WIDTH <= 12);
+        for r in 0..12 {
+            if r < WIDTH {
+                result[r] = Self::from_noncanonical_u128(Self::mds_row_shf(r, &state));
+            }
         }
+
         result
     }