From 83a425d25af033086744c1c8c892015014ed46bd Mon Sep 17 00:00:00 2001
From: William Casarin <jb55@jb55.com>
Date: Fri, 1 May 2020 17:31:38 -0700
Subject: compressor: use a prevector in compressed script serialization

Use a prevector for stack allocation instead of heap allocation during
script compression and decompression. These functions were doing
millions of unnecessary heap allocations during IBD.

We introduce a CompressedScript type alias for this prevector. It is
size 33 as that is the maximum size of a compressed script.

Fix the DecompressScript header to match the variable name from
compressor.cpp

Signed-off-by: William Casarin <jb55@jb55.com>
---
 src/compressor.h | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

(limited to 'src/compressor.h')

diff --git a/src/compressor.h b/src/compressor.h
index 478bfff0b6..40b2496f06 100644
--- a/src/compressor.h
+++ b/src/compressor.h
@@ -6,14 +6,26 @@
 #ifndef BITCOIN_COMPRESSOR_H
 #define BITCOIN_COMPRESSOR_H
 
+#include <prevector.h>
 #include <primitives/transaction.h>
 #include <script/script.h>
 #include <serialize.h>
 #include <span.h>
 
-bool CompressScript(const CScript& script, std::vector<unsigned char> &out);
+/**
+ * This saves us from making many heap allocations when serializing
+ * and deserializing compressed scripts.
+ *
+ * This prevector size is determined by the largest .resize() in the
+ * CompressScript function. The largest compressed script format is a
+ * compressed public key, which is 33 bytes.
+ */
+using CompressedScript = prevector<33, unsigned char>;
+
+
+bool CompressScript(const CScript& script, CompressedScript& out);
 unsigned int GetSpecialScriptSize(unsigned int nSize);
-bool DecompressScript(CScript& script, unsigned int nSize, const std::vector<unsigned char> &out);
+bool DecompressScript(CScript& script, unsigned int nSize, const CompressedScript& in);
 
 /**
  * Compress amount.
@@ -51,7 +63,7 @@ struct ScriptCompression
 
     template<typename Stream>
     void Ser(Stream &s, const CScript& script) {
-        std::vector<unsigned char> compr;
+        CompressedScript compr;
         if (CompressScript(script, compr)) {
             s << MakeSpan(compr);
             return;
@@ -66,7 +78,7 @@ struct ScriptCompression
         unsigned int nSize = 0;
         s >> VARINT(nSize);
         if (nSize < nSpecialScripts) {
-            std::vector<unsigned char> vch(GetSpecialScriptSize(nSize), 0x00);
+            CompressedScript vch(GetSpecialScriptSize(nSize), 0x00);
             s >> MakeSpan(vch);
             DecompressScript(script, nSize, vch);
             return;
-- 
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