Merge bitcoin#19690: util: improve FindByte() performance

72efc26 util: improve streams.h:FindByte() performance (Larry Ruane)
604df63 [bench] add streams findbyte (gzhao408)

Pull request description:

  This PR is strictly a performance improvement; there is no functional change. The `CBufferedFile::FindByte()` method searches for the next occurrence of the given byte in the file. Currently, this is done by explicitly inspecting each byte in turn. This PR takes advantage of `std::find()` to do the same more efficiently, improving its CPU runtime by a factor of about 25 in typical use.

ACKs for top commit:
  achow101:
    re-ACK 72efc26
  stickies-v:
    re-ACK 72efc26

Tree-SHA512: ddf0bff335cc8aa34f911aa4e0558fa77ce35d963d602e4ab1c63090b4a386faf074548daf06ee829c7f2c760d06eed0125cf4c34e981c6129cea1804eb3b719

src/Makefile.bench.include

@@ -47,6 +47,7 @@ bench_bench_bitcoin_SOURCES = \
   bench/rollingbloom.cpp \
   bench/rpc_blockchain.cpp \
   bench/rpc_mempool.cpp \
+  bench/streams_findbyte.cpp \
   bench/strencodings.cpp \
   bench/util_time.cpp \
   bench/verify_script.cpp

src/bench/streams_findbyte.cpp

@@ -0,0 +1,31 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <bench/bench.h>
+
+#include <util/fs.h>
+#include <streams.h>
+
+static void FindByte(benchmark::Bench& bench)
+{
+    // Setup
+    FILE* file = fsbridge::fopen("streams_tmp", "w+b");
+    const size_t file_size = 200;
+    uint8_t data[file_size] = {0};
+    data[file_size-1] = 1;
+    fwrite(&data, sizeof(uint8_t), file_size, file);
+    rewind(file);
+    CBufferedFile bf(file, /*nBufSize=*/file_size + 1, /*nRewindIn=*/file_size, 0, 0);
+
+    bench.run([&] {
+        bf.SetPos(0);
+        bf.FindByte(std::byte(1));
+    });
+
+    // Cleanup
+    bf.fclose();
+    fs::remove("streams_tmp");
+}
+
+BENCHMARK(FindByte, benchmark::PriorityLevel::HIGH);

src/streams.h

@@ -756,15 +756,25 @@ class CBufferedFile
     }
 
     //! search for a given byte in the stream, and remain positioned on it
-    void FindByte(uint8_t ch)
+    void FindByte(std::byte byte)
     {
+        // For best performance, avoid mod operation within the loop.
+        size_t buf_offset{size_t(m_read_pos % uint64_t(vchBuf.size()))};
         while (true) {
-            if (m_read_pos == nSrcPos)
+            if (m_read_pos == nSrcPos) {
+                // No more bytes available; read from the file into the buffer,
+                // setting nSrcPos to one beyond the end of the new data.
+                // Throws exception if end-of-file reached.
                 Fill();
-            if (vchBuf[m_read_pos % vchBuf.size()] == std::byte{ch}) {
-                break;
             }
-            m_read_pos++;
+            const size_t len{std::min<size_t>(vchBuf.size() - buf_offset, nSrcPos - m_read_pos)};
+            const auto it_start{vchBuf.begin() + buf_offset};
+            const auto it_find{std::find(it_start, it_start + len, byte)};
+            const size_t inc{size_t(std::distance(it_start, it_find))};
+            m_read_pos += inc;
+            if (inc < len) break;
+            buf_offset += inc;
+            if (buf_offset >= vchBuf.size()) buf_offset = 0;
         }
     }
 };

src/test/fuzz/buffered_file.cpp

@@ -53,7 +53,7 @@ FUZZ_TARGET(buffered_file)
                         return;
                     }
                     try {
-                        opt_buffered_file->FindByte(fuzzed_data_provider.ConsumeIntegral<uint8_t>());
+                        opt_buffered_file->FindByte(std::byte(fuzzed_data_provider.ConsumeIntegral<uint8_t>()));
                     } catch (const std::ios_base::failure&) {
                     }
                 },

src/test/streams_tests.cpp

@@ -463,7 +463,7 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file_rand)
                 size_t find = currentPos + InsecureRandRange(8);
                 if (find >= fileSize)
                     find = fileSize - 1;
-                bf.FindByte(uint8_t(find));
+                bf.FindByte(std::byte(find));
                 // The value at each offset is the offset.
                 BOOST_CHECK_EQUAL(bf.GetPos(), find);
                 currentPos = find;

src/validation.cpp

@@ -4565,7 +4565,7 @@ void Chainstate::LoadExternalBlockFile(
             try {
                 // locate a header
                 unsigned char buf[CMessageHeader::MESSAGE_START_SIZE];
-                blkdat.FindByte(params.MessageStart()[0]);
+                blkdat.FindByte(std::byte(params.MessageStart()[0]));
                 nRewind = blkdat.GetPos() + 1;
                 blkdat >> buf;
                 if (memcmp(buf, params.MessageStart(), CMessageHeader::MESSAGE_START_SIZE)) {