Merge bitcoin/bitcoin#24852: util: optimize HexStr

5e61532e72c1021fda9c7b213bd9cf397cb3a802 util: optimizes HexStr (Martin Leitner-Ankerl)
4e2b99f72a90b956f3050095abed4949aff9b516 bench: Adds a benchmark for HexStr (Martin Leitner-Ankerl)
67c8411c37b483caa2fe3f7f4f40b68ed2a9bcf7 test: Adds a test for HexStr that checks all 256 bytes (Martin Leitner-Ankerl)

Pull request description:

  In my benchmark, this rewrite improves runtime 27% (g++) to 46% (clang++) for the benchmark `HexStrBench`:

  g++ 11.2.0
  |             ns/byte |              byte/s |    err% |        ins/byte |        cyc/byte |    IPC |       bra/byte |   miss% |     total | benchmark
  |--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
  |                0.94 |    1,061,381,310.36 |    0.7% |           12.00 |            3.01 |  3.990 |           1.00 |    0.0% |      0.01 | `HexStrBench` master
  |                0.68 |    1,465,366,544.25 |    1.7% |            6.00 |            2.16 |  2.778 |           1.00 |    0.0% |      0.01 | `HexStrBench` branch

  clang++ 13.0.1
  |             ns/byte |              byte/s |    err% |        ins/byte |        cyc/byte |    IPC |       bra/byte |   miss% |     total | benchmark
  |--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
  |                0.80 |    1,244,713,415.92 |    0.9% |           10.00 |            2.56 |  3.913 |           0.50 |    0.0% |      0.01 | `HexStrBench` master
  |                0.43 |    2,324,188,940.72 |    0.2% |            4.00 |            1.37 |  2.914 |           0.25 |    0.0% |      0.01 | `HexStrBench` branch

  Note that the idea for this change comes from denis2342 in #23364. This is a rewrite so no unaligned accesses occur.

  Also, the lookup table is now calculated at compile time, which hopefully makes the code a bit easier to review.

ACKs for top commit:
  laanwj:
    Code review ACK 5e61532e72c1021fda9c7b213bd9cf397cb3a802
  aureleoules:
    tACK 5e61532e72c1021fda9c7b213bd9cf397cb3a802.
  theStack:
    ACK 5e61532e72c1021fda9c7b213bd9cf397cb3a802 🚤

Tree-SHA512: 40b53d5908332473ef24918d3a80ad1292b60566c02585fa548eb4c3189754971be5a70325f4968fce6d714df898b52d9357aba14d4753a8c70e6ffd273a2319

1 files changed, 18 insertions, 0 deletions

diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index 890b2f997e..3b2aca5887 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -198,6 +198,24 @@ BOOST_AUTO_TEST_CASE(util_HexStr)
         BOOST_CHECK_EQUAL(HexStr(in_s), out_exp);
         BOOST_CHECK_EQUAL(HexStr(in_b), out_exp);
     }
+
+    {
+        auto input = std::string();
+        for (size_t i=0; i<256; ++i) {
+            input.push_back(static_cast<char>(i));
+        }
+
+        auto hex = HexStr(input);
+        BOOST_TEST_REQUIRE(hex.size() == 512);
+        static constexpr auto hexmap = std::string_view("0123456789abcdef");
+        for (size_t i = 0; i < 256; ++i) {
+            auto upper = hexmap.find(hex[i * 2]);
+            auto lower = hexmap.find(hex[i * 2 + 1]);
+            BOOST_TEST_REQUIRE(upper != std::string_view::npos);
+            BOOST_TEST_REQUIRE(lower != std::string_view::npos);
+            BOOST_TEST_REQUIRE(i == upper*16 + lower);
+        }
+    }
 }
 
 BOOST_AUTO_TEST_CASE(span_write_bytes)