JS Optimization

After profiling, I see that roughly 25-30% of my program's time is spent inside this function.

Unfortunately, this isn't my algorithm, and I don't think I can improve it. The paper it came from is here: http://www.dgp.toronto.edu/people/stam/reality/Research/pdf/GDC03.pdf

My only option seems to be "performance tricks", but I don't see any openings. The only thing I can say for sure that I haven't tried is ``loop unrolling'', but I don't have much experience doing that myself, and I don't see how I could do it here while staying in bounds. If it helps, height, width, and iterations are all set by the user.

So: any ideas on how to improve the performance of this code?

function lin_solve(b, x, x0, a, c) {
        if (a === 0 && c === 1) {
            for (var j = 1; j <= height; j++) {
                var currentRow = j * rowSize;
                ++currentRow;
        var i = width - 1; do {
                    x[currentRow] = x0[currentRow];
                    ++currentRow;
                } while (i--);
            }
            set_bnd(b, x);
        } else {
            var invC = 1 / c;
            for (var k = 0; k < iterations; k++) {
                for (var j = 1; j <= height; j++) {
                    var lastRow = (j - 1) * rowSize;
                    var currentRow = j * rowSize;
                    var nextRow = (j + 1) * rowSize;
                    var lastX = x[currentRow];
                    ++currentRow;
                   
            var i = width; do {
                        lastX = x[currentRow] = (x0[currentRow] + a*(lastX+x[++currentRow]+x[++lastRow]+x[++nextRow])) * invC;
            } while (i-- > 1)
                }
                set_bnd(b, x);
            }
        }
    }

Can you run it through Google Closure for extra juice?