prevector: store P2WSH/P2TR/P2PK scripts inline

The current `prevector` size of 28 bytes (chosen to fill the `sizeof(CScript)` aligned size) was introduced in 2015 (bitcoin/bitcoin#6914) before SegWit and TapRoot.
However, the increasingly common `P2WSH` and `P2TR` scripts are both 34 bytes, and are forced to use heap (re)allocation rather than efficient inline storage.

The core trade-off of this change is to eliminate heap allocations for common 34-36 byte scripts at the cost of increasing the base memory footprint of all `CScript` objects by 8 bytes (while still respecting peak memory usage defined by `-dbcache`).

Increasing the `prevector` size allows these scripts to be stored inline, avoiding extra heap allocations, reducing potential memory fragmentation, and improving performance during cache flushes. Massif analysis confirms a lower stable memory usage after flushing, suggesting the elimination of heap allocations outweighs the larger base size for common workloads.

Due to memory alignment, increasing the `prevector` size to 36 bytes doesn't change the overall `sizeof(CScript)` compared to an increase to 34 bytes, allowing us to include `P2PK` scripts as well at no additional memory cost.

Performance benchmarks for AssumeUTXO load and flush show:
* Small dbcache (450MB): ~1-3% performance improvement (despite more frequent flushes)
* Large dbcache (4500MB): ~6-8% performance improvement due to fewer heap allocations (and basically the number of flushes)
* Very large dbcache (4500MB): ~5-6% performance improvement due to fewer heap allocations (and memory limit not being reached, so there's no memory penalty)

Full IBD and reindex-chainstate with larger `dbcache` values also show an overall ~3-4% speedup.

Co-authored-by: Ava Chow <[email protected]>
Co-authored-by: Andrew Toth <[email protected]>
Co-authored-by: maflcko <[email protected]>

Author: 4 people

src/script/script.h

@@ -403,10 +403,8 @@ class CScriptNum
 /**
  * We use a prevector for the script to reduce the considerable memory overhead
  *  of vectors in cases where they normally contain a small number of small elements.
- * Tests in October 2015 showed use of this reduced dbcache memory usage by 23%
- *  and made an initial sync 13% faster.
  */
-using CScriptBase = prevector<28, uint8_t>;
+using CScriptBase = prevector<36, uint8_t>;
 
 bool GetScriptOp(CScriptBase::const_iterator& pc, CScriptBase::const_iterator end, opcodetype& opcodeRet, std::vector<unsigned char>* pvchRet);
 

src/test/script_tests.cpp

@@ -1175,10 +1175,10 @@ static TxoutType GetTxoutType(const CScript& output_script)
 BOOST_AUTO_TEST_CASE(script_size_and_capacity_test)
 {
     BOOST_CHECK_EQUAL(sizeof(CompressedScript), 40);
-    BOOST_CHECK_EQUAL(sizeof(CScriptBase), 32);
+    BOOST_CHECK_EQUAL(sizeof(CScriptBase), 40);
     BOOST_CHECK_NE(sizeof(CScriptBase), sizeof(prevector<CScriptBase::STATIC_SIZE + 1, uint8_t>)); // CScriptBase size should be set to avoid wasting space in padding
-    BOOST_CHECK_EQUAL(sizeof(CScript), 32);
-    BOOST_CHECK_EQUAL(sizeof(CTxOut), 40);
+    BOOST_CHECK_EQUAL(sizeof(CScript), 40);
+    BOOST_CHECK_EQUAL(sizeof(CTxOut), 48);
 
     CKey dummy_key;
     dummy_key.MakeNewKey(/*fCompressed=*/true);
@@ -1212,25 +1212,25 @@ BOOST_AUTO_TEST_CASE(script_size_and_capacity_test)
         CHECK_SCRIPT_STATIC_SIZE(script, 25);
     }
 
-    // P2WSH needs extra allocation
+    // P2WSH has direct allocation
     {
         const auto script{GetScriptForDestination(WitnessV0ScriptHash{CScript{} << OP_TRUE})};
         BOOST_CHECK(script.IsPayToWitnessScriptHash());
-        CHECK_SCRIPT_DYNAMIC_SIZE(script, 34, 34);
+        CHECK_SCRIPT_STATIC_SIZE(script, 34);
     }
 
-    // P2TR needs extra allocation
+    // P2TR has direct allocation
     {
         const auto script{GetScriptForDestination(WitnessV1Taproot{XOnlyPubKey{dummy_pubkey}})};
         BOOST_CHECK_EQUAL(GetTxoutType(script), TxoutType::WITNESS_V1_TAPROOT);
-        CHECK_SCRIPT_DYNAMIC_SIZE(script, 34, 34);
+        CHECK_SCRIPT_STATIC_SIZE(script, 34);
     }
 
-    // Compressed P2PK needs extra allocation
+    // Compressed P2PK has direct allocation
     {
         const auto script{GetScriptForRawPubKey(dummy_pubkey)};
         BOOST_CHECK_EQUAL(GetTxoutType(script), TxoutType::PUBKEY);
-        CHECK_SCRIPT_DYNAMIC_SIZE(script, 35, 35);
+        CHECK_SCRIPT_STATIC_SIZE(script, 35);
     }
 
     // Uncompressed P2PK needs extra allocation

src/test/validation_flush_tests.cpp

@@ -26,9 +26,8 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
     LOCK(::cs_main);
     auto& view = chainstate.CoinsTip();
 
-    // The number of bytes consumed by coin's heap data, i.e. CScript
-    // (prevector<28, unsigned char>) when assigned 56 bytes of data per above.
-    //
+    // The number of bytes consumed by coin's heap data, i.e.
+    // CScript (prevector<36, unsigned char>) when assigned 56 bytes of data per above.
     // See also: Coin::DynamicMemoryUsage().
     constexpr unsigned int COIN_SIZE = is_64_bit ? 80 : 64;