v2.1: ed25519_verify_cpu: remove allocation in hot path (backport of …

…#3941) (#3945)

ed25519_verify_cpu: remove allocation in hot path (#3941)

The old code was a remnant from a time when we tried to "balance" the
amount of work across threads by re-batching and therefore allocating.

This is a hot path. Allocations are bad. Let work stealing work its
magic.

(cherry picked from commit a53a876)

Co-authored-by: Alessandro Decina <[email protected]>

perf/src/sigverify.rs

@@ -517,18 +517,11 @@ pub fn shrink_batches(batches: &mut Vec<PacketBatch>) {
 pub fn ed25519_verify_cpu(batches: &mut [PacketBatch], reject_non_vote: bool, packet_count: usize) {
     debug!("CPU ECDSA for {}", packet_count);
     PAR_THREAD_POOL.install(|| {
-        batches
-            .par_iter_mut()
-            .flatten()
-            .collect::<Vec<&mut Packet>>()
-            .par_chunks_mut(VERIFY_PACKET_CHUNK_SIZE)
-            .for_each(|packets| {
-                for packet in packets.iter_mut() {
-                    if !packet.meta().discard() && !verify_packet(packet, reject_non_vote) {
-                        packet.meta_mut().set_discard(true);
-                    }
-                }
-            });
+        batches.par_iter_mut().flatten().for_each(|packet| {
+            if !packet.meta().discard() && !verify_packet(packet, reject_non_vote) {
+                packet.meta_mut().set_discard(true);
+            }
+        });
     });
 }