Merge bitcoin#9505: Prevector Quick Destruct

45a5aaf Only call clear on prevector if it isn't trivially destructible and don't loop in clear (Jeremy Rubin)
aaa02e7 Add prevector destructor benchmark (Jeremy Rubin)

Tree-SHA512: 52bc8163b65b71310252f2d578349d0ddc364a6c23795c5e06e101f5449f04c96cbdca41c0cffb1974b984b8e33006471137d92b8dd4a81a98e922610a94132a

Author: laanwj

src/Makefile.bench.include

@@ -29,6 +29,7 @@ bench_bench_dash_SOURCES = \
   bench/lockedpool.cpp \
   bench/perf.cpp \
   bench/perf.h \
+  bench/prevector_destructor.cpp \ 
   bench/string_cast.cpp
 
 nodist_bench_bench_dash_SOURCES = $(GENERATED_TEST_FILES)

src/bench/prevector_destructor.cpp

@@ -0,0 +1,36 @@
+// Copyright (c) 2015-2017 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.h"
+#include "prevector.h"
+
+static void PrevectorDestructor(benchmark::State& state)
+{
+    while (state.KeepRunning()) {
+        for (auto x = 0; x < 1000; ++x) {
+            prevector<28, unsigned char> t0;
+            prevector<28, unsigned char> t1;
+            t0.resize(28);
+            t1.resize(29);
+        }
+    }
+}
+
+static void PrevectorClear(benchmark::State& state)
+{
+
+    while (state.KeepRunning()) {
+        for (auto x = 0; x < 1000; ++x) {
+            prevector<28, unsigned char> t0;
+            prevector<28, unsigned char> t1;
+            t0.resize(28);
+            t0.clear();
+            t1.resize(29);
+            t0.clear();
+        }
+    }
+}
+
+BENCHMARK(PrevectorDestructor);
+BENCHMARK(PrevectorClear);

src/prevector.h

@@ -11,6 +11,7 @@
 #include <string.h>
 
 #include <iterator>
+#include <type_traits>
 
 #pragma pack(push, 1)
 /** Implements a drop-in replacement for std::vector<T> which stores up to N
@@ -388,10 +389,14 @@ class prevector {
     iterator erase(iterator first, iterator last) {
         iterator p = first;
         char* endp = (char*)&(*end());
-        while (p != last) {
-            (*p).~T();
-            _size--;
-            ++p;
+        if (!std::is_trivially_destructible<T>::value) {
+            while (p != last) {
+                (*p).~T();
+                _size--;
+                ++p;
+            }
+        } else {
+            _size -= last - p;
         }
         memmove(&(*first), &(*last), endp - ((char*)(&(*last))));
         return first;
@@ -432,7 +437,9 @@ class prevector {
     }
 
     ~prevector() {
-        clear();
+        if (!std::is_trivially_destructible<T>::value) {
+            clear();
+        }
         if (!is_direct()) {
             free(_union.indirect);
             _union.indirect = NULL;