Merge #13204: Faster sigcache nonce

152e8baf08 Use salted hasher instead of nonce in sigcache (Jeremy Rubin)
5495fa5850 Add Hash Padding Microbenchmarks (Jeremy Rubin)

Pull request description:

  This PR replaces nonces in two places with pre-salted hashers.

  The nonce is chosen to be 64 bytes long so that it forces the SHA256 hasher to process the chunk. This leaves the next 64 (or 56 depending if final chunk) open for data. In the case of the script execution cache, this does not make a big performance improvement because the nonce was already properly padded to fit into one buffer, but does make the code a little simpler. In the case of the sig cache, this should reduce the hashing overhead slightly because we are less likely to need an additional processing step.

  I haven't benchmarked this, but back of the envelope it should reduce the hashing by one buffer for all combinations except compressed public keys with compact signatures.

ACKs for top commit:
  ryanofsky:
    Code review ACK 152e8baf08. No code changes, just rebase since last review and expanded commit message

Tree-SHA512: b133e902fd595cfe3b54ad8814b823f4d132cb2c358c89158842ae27daee56ab5f70cde2585078deb46f77a6e7b35b4cc6bba47b65302b7befc2cff254bad93d

src/script/sigcache.cpp

@@ -23,7 +23,7 @@ class CSignatureCache
 {
 private:
      //! Entries are SHA256(nonce || signature hash || public key || signature):
-    uint256 nonce;
+    CSHA256 m_salted_hasher;
     typedef CuckooCache::cache<uint256, SignatureCacheHasher> map_type;
     map_type setValid;
     boost::shared_mutex cs_sigcache;
@@ -31,13 +31,19 @@ private:
 public:
     CSignatureCache()
     {
-        GetRandBytes(nonce.begin(), 32);
+        uint256 nonce = GetRandHash();
+        // We want the nonce to be 64 bytes long to force the hasher to process
+        // this chunk, which makes later hash computations more efficient. We
+        // just write our 32-byte entropy twice to fill the 64 bytes.
+        m_salted_hasher.Write(nonce.begin(), 32);
+        m_salted_hasher.Write(nonce.begin(), 32);
     }
 
     void
     ComputeEntry(uint256& entry, const uint256 &hash, const std::vector<unsigned char>& vchSig, const CPubKey& pubkey)
     {
-        CSHA256().Write(nonce.begin(), 32).Write(hash.begin(), 32).Write(&pubkey[0], pubkey.size()).Write(&vchSig[0], vchSig.size()).Finalize(entry.begin());
+        CSHA256 hasher = m_salted_hasher;
+        hasher.Write(hash.begin(), 32).Write(&pubkey[0], pubkey.size()).Write(&vchSig[0], vchSig.size()).Finalize(entry.begin());
     }
 
     bool