What does Intel PIN instruction count depend on and why it varies between executions? - intel-pin

Question

I would like to get instruction count for bubblesort algorithm using Intel PIN. I thought that maybe I could analyse how instruction count increases, when there is more data to sort. But the result varies between executions for the same set of data.
For example few outputs: 1662097, 1811453, 1832990, 1745621
It varies quite a lot. Why isn't it the same? Is there a way to achieve it? How am I supposed to get any useful information from that? There is no way of telling how increase of data set influenced number of instructions, when it isn't even stable for the same execution.
Pintool code (copy-pasted from Pin User Guide):
#include <iostream>
#include <fstream>
#include "pin.H"
ofstream OutFile;
// The running count of instructions is kept here
// make it static to help the compiler optimize docount
static UINT64 icount = 0;
// This function is called before every instruction is executed
VOID docount() { icount++; }
// Pin calls this function every time a new instruction is encountered
VOID Instruction(INS ins, VOID *v)
{
// Insert a call to docount before every instruction, no arguments are passed
INS_InsertCall(ins, IPOINT_BEFORE, (AFUNPTR)docount, IARG_END);
}
KNOB<string> KnobOutputFile(KNOB_MODE_WRITEONCE, "pintool",
"o", "inscount.out", "specify output file name");
// This function is called when the application exits
VOID Fini(INT32 code, VOID *v)
{
// Write to a file since cout and cerr maybe closed by the application
OutFile.setf(ios::showbase);
OutFile << "Count " << icount << endl;
OutFile.close();
}
/* ===================================================================== */
/* Print Help Message */
/* ===================================================================== */
INT32 Usage()
{
cerr << "This tool counts the number of dynamic instructions executed" << endl;
cerr << endl << KNOB_BASE::StringKnobSummary() << endl;
return -1;
}
/* ===================================================================== */
/* Main */
/* ===================================================================== */
/* argc, argv are the entire command line: pin -t <toolname> -- ... */
/* ===================================================================== */
int main(int argc, char * argv[])
{
// Initialize pin
if (PIN_Init(argc, argv)) return Usage();
OutFile.open(KnobOutputFile.Value().c_str());
// Register Instruction to be called to instrument instructions
INS_AddInstrumentFunction(Instruction, 0);
// Register Fini to be called when the application exits
PIN_AddFiniFunction(Fini, 0);
// Start the program, never returns
PIN_StartProgram();
return 0;
}
Bubblesort implementation (nothing extraordinary, intentionally O(n^2)):
void bubblesort(int table[], int size) {
int i, j, temp;
for (i = 0; i < size - 1; i++)
{
for (j = 0; j < size - 1 - i; j++)
{
if (table[j] > table[j + 1])
{
temp = table[j + 1];
table[j + 1] = table[j];
table[j] = temp;
}
}
}
}