Ray-AABB (Axis Aligned Bounding Box) intersection.

intersectAABB.glsl

// adapted from intersectCube in https://github.com/evanw/webgl-path-tracing/blob/master/webgl-path-tracing.js

// compute the near and far intersections of the cube (stored in the x and y components) using the slab method
// no intersection means vec.x > vec.y (really tNear > tFar)
vec2 intersectAABB(vec3 rayOrigin, vec3 rayDir, vec3 boxMin, vec3 boxMax) {
    vec3 tMin = (boxMin - rayOrigin) / rayDir;
    vec3 tMax = (boxMax - rayOrigin) / rayDir;
    vec3 t1 = min(tMin, tMax);
    vec3 t2 = max(tMin, tMax);
    float tNear = max(max(t1.x, t1.y), t1.z);
    float tFar = min(min(t2.x, t2.y), t2.z);
    return vec2(tNear, tFar);
};

Leaving this here if anyone needed this for C# with 3 dimensional vectors.
SIMD optimized implementation that returns true if an intersection was found.

public bool Intersects(Ray ray)
{
    Vector4 tMin = (MinPosition - ray.Origin) / ray.Direction;
    Vector4 tMax = (MaxPosition - ray.Origin) / ray.Direction;
    Vector128<float> t1 = Vector4.Min(tMin, tMax).AsVector128();
    Vector128<float> t2 = Vector4.Max(tMin, tMax).AsVector128();

    Vector128<float> tNear = Sse.Max(Sse.Max(Sse.MoveHighToLow(t1, t1),
                                             Sse.Shuffle(t1, t1, 0b00_00_11_01)),
                                     t1);

    Vector128<float> tFar = Sse.Min(Sse.Min(Sse.MoveHighToLow(t2, t2),
                                            Sse.Shuffle(t2, t2, 0b00_00_11_01)),
                                    t2);

    return tNear.GetElement(0) <= tFar.GetElement(0) && tFar.GetElement(0) >= 0;
}

Obviously if you need an optimal solution for a single ray to check many boxes then get rid of the division operators.

public bool Intersects(RayAxisAlignBoxOptimizedIntersection optimizedRay)
{
    Vector4 tMin = (MinPosition - optimizedRay.Start) * optimizedRay.InverseDirection;
    Vector4 tMax = (MaxPosition - optimizedRay.Start) * optimizedRay.InverseDirection;

That type name is absurdly long.