Improve shared_vector::push_back

push_back now allocates additional space
in powers of 2 up to 1k elements, then
in blocks of 1k elements.

pvDataApp/misc/sharedVector.h

@@ -6,6 +6,8 @@
 #include <stdexcept>
 #include <iterator>
 
+#include <cassert>
+
 #include "pv/sharedPtr.h"
 
 namespace epics { namespace pvData {
@@ -423,9 +425,34 @@ public:
 
     // Modifications
 
+private:
+    void _push_resize() {
+        if(this->m_count==this->m_total || !this->unique()) {
+            size_t next;
+            if(this->m_total<1024) {
+                // round m_total+1 up to the next power of 2
+                next = this->m_total;
+                next |= next >> 1;
+                next |= next >> 2;
+                next |= next >> 4;
+                next |= next >> 8;
+                next++;
+            } else {
+                // pad m_total up to the next multiple of 1024
+                next = this->m_total+1024;
+                next &= ~0x3ff;
+            }
+            assert(next > this->m_total);
+            reserve(next);
+        }
+        resize(this->size()+1);
+    }
+
+public:
+
     void push_back(param_type v)
     {
-        resize(this->size()+1);
+        _push_resize();
         back() = v;
     }
 

testApp/misc/testSharedVector.cpp

@@ -329,6 +329,31 @@ static void testCapacity()
     testOk1(vect[1]==124);
 }
 
+static void testPush()
+{
+    epics::pvData::shared_vector<int> vect;
+
+    testDiag("Test push_back optimizations");
+
+    size_t nallocs = 0;
+    size_t cap = vect.capacity();
+
+    for(size_t s=0; s<16*1024; s++) {
+        vect.push_back(s);
+
+        if(cap!=vect.capacity()) {
+            nallocs++;
+            cap = vect.capacity();
+        }
+    }
+
+    testDiag("push_back %ld times caused %ld re-allocations",
+             (unsigned long)vect.size(),
+             (unsigned long)nallocs);
+
+    testOk1(nallocs==26);
+}
+
 static void testVoid()
 {
     testDiag("Test vecter cast to/from void");
@@ -409,7 +434,7 @@ static void testWeak()
 
 MAIN(testSharedVector)
 {
-    testPlan(121);
+    testPlan(122);
     testDiag("Tests for shared_vector");
 
     testDiag("sizeof(shared_vector<int>)=%lu",
@@ -422,6 +447,7 @@ MAIN(testSharedVector)
     testShare();
     testConst();
     testSlice();
+    testPush();
     testVoid();
     testNonPOD();
     testWeak();