Merge bitcoin/bitcoin#30326: optimization: Reduce cache lookups in CCoinsViewCache::FetchCoin

204ca67 Reduce cache lookups in CCoinsViewCache::FetchCoin (Lőrinc)

Pull request description:

  Enhanced efficiency and readability of `CCoinsViewCache::FetchCoin` by replacing separate `find()` and `emplace()` calls with a single `try_emplace()`, reducing map lookups and potential insertions.

  `AssembleBlock` shows `FetchCoin` as one of its bottlenecks:
  <img src="https://github.com/bitcoin/bitcoin/assets/1841944/79c7f480-aac2-46da-9ac9-526a02a8eafa">

  These changes result in a modest performance improvement:

  >  ./src/bench/bench_bitcoin --filter='AssembleBlock' --min-time=10000

  before:
  |               ns/op |                op/s |    err% |     total | benchmark
  |--------------------:|--------------------:|--------:|----------:|:----------
  |          156,160.70 |            6,403.66 |    0.6% |     10.91 | `AssembleBlock`

  after:
  |               ns/op |                op/s |    err% |     total | benchmark
  |--------------------:|--------------------:|--------:|----------:|:----------
  |          152,971.97 |            6,537.15 |    0.2% |     10.95 | `AssembleBlock`

  Further benchmarks: bitcoin/bitcoin#30326 (comment)

ACKs for top commit:
  sipa:
    utACK 204ca67
  achow101:
    ACK 204ca67
  andrewtoth:
    re-ACK 204ca67

Tree-SHA512: 65743a5d4edd442672a59d7b3de38fe197c61270a5c8df65712413904559f360fc58b512234558c7e5169ffb4dda3b2d2f7ded92ad5b04ca999828986b251066

Author: achow101

src/coins.cpp

@@ -43,19 +43,18 @@ size_t CCoinsViewCache::DynamicMemoryUsage() const {
 }
 
 CCoinsMap::iterator CCoinsViewCache::FetchCoin(const COutPoint &outpoint) const {
-    CCoinsMap::iterator it = cacheCoins.find(outpoint);
-    if (it != cacheCoins.end())
-        return it;
-    Coin tmp;
-    if (!base->GetCoin(outpoint, tmp))
-        return cacheCoins.end();
-    CCoinsMap::iterator ret = cacheCoins.emplace(std::piecewise_construct, std::forward_as_tuple(outpoint), std::forward_as_tuple(std::move(tmp))).first;
-    if (ret->second.coin.IsSpent()) {
-        // The parent only has an empty entry for this outpoint; we can consider our
-        // version as fresh.
-        ret->second.AddFlags(CCoinsCacheEntry::FRESH, *ret, m_sentinel);
+    const auto [ret, inserted] = cacheCoins.try_emplace(outpoint);
+    if (inserted) {
+        if (!base->GetCoin(outpoint, ret->second.coin)) {
+            cacheCoins.erase(ret);
+            return cacheCoins.end();
+        }
+        if (ret->second.coin.IsSpent()) {
+            // The parent only has an empty entry for this outpoint; we can consider our version as fresh.
+            ret->second.AddFlags(CCoinsCacheEntry::FRESH, *ret, m_sentinel);
+        }
+        cachedCoinsUsage += ret->second.coin.DynamicMemoryUsage();
     }
-    cachedCoinsUsage += ret->second.coin.DynamicMemoryUsage();
     return ret;
 }