VS optimizer bug sample

repro.cpp

#include <iostream>
#include <cmath>

// Sample of an optimizer bug in Visual Studio 2003 - 2019 (or more)
//
// Compiling this program in Release (/O1, /O2 or /Ox and LTCG) in both x86 and x64
// configurations produces a different output from unoptimized (Debug) configurations.
//
// Expected output: 709
// Produced output: 31
//
// GCC, ICC, clang-cl, as well as ideone.com output the correct value.
//
// By Adrian Zdanowicz and Rafael Rivera

int zero = 0; // Can't seem to make this a local, delete it or make const.

int main()
{
	int	iterations = 0, maxvis = 0, i2 = 0, qx0 = 0, ix = 0, iy = 0, q1 = 0, qx2 = 0, qy2 = 0;
	
	qx0 = std::atoi("0");// This cannot be reduced.
	// qx0 = static_cast<int>(std::floor(0)); // Or this - both fail the test.
		
	if (zero) // Can't remove this.
		i2 = 12;
	else
		i2 = 15;
	if (i2 < 2) i2 = 2; // Nor this.

	q1 = -15;
	qx2 = qx0 + 15; // Can't make this a constant = 15.
	qy2 = 15; // Can't remove this in favour of using qx2.
	maxvis = i2;

	for (iy = q1; iy <= qy2; iy++) // qy2 can be replaced with a constant 15, but not with qx2.
	{
		for (ix = q1; ix <= qx2; ix++)
		{
			i2 = iy - qx0; // Nor this.
			// std::cout << i2 << std::endl; // Uncommenting this "fixes" the code.
			if ((ix * ix + i2 * i2) > (maxvis * maxvis)) continue; // Can't touch this either.
			
			iterations++;

			if (iterations >= 961) break; // Can't remove this.
		}
		if (iterations >= 961) break; // Nor this.
	}

	std::cout << iterations;
}