The possible reason for getting r1=-Inf in the expression log(exp(f(x))), is due to the limited precision of floating point numbers. This can cause numeric underflow.
In this case the value of the term exp(-(x.^2)/2) will become very small as the magnitude of x grows large. Eventually the value will become so small that it will get rounded down to zero. This will cause the entire expression to evaluate to -Inf as log of 0 is negative infinity.
A workaround that you could try is using symbolic integration instead of numerical integration. This would allow you to simplify the expression log(exp(f(x))) and evaluate the integral analytically.
syms x;
f = log(exp(-(x.^2)/2));
r = simplify(int(f, x, -100, 100))
