Re: Optimize flag breaks code on many versions of gcc (not all)

[tbp <tbptbp at gmail dot com>]

On 6/19/06, Richard Guenther <richard.guenther@gmail.com> wrote:
> Using -mfpmath=sse -msse2 is a workaround if you have a processor that supports
> SSE2 instructions.  As opposed to -ffloat-store, it works reliably and
> with no performance
> impact.
Such slab test can be turned into a branchless sequence of SSE
min/max, even for filtering infinities around dir ~= 0; it's much
simpler and efficient to intersect 4 rays against one box at once
though.
Without intrinsics a NaN oblivious version would be like:

static float minf(const float a, const float b) { return (a < b) ? a : b; }
static float maxf(const float a, const float b) { return (a > b) ? a : b; }

bool_t intersect_ray_box(const aabb_t &box, const rt::mono::ray_t
&ray, float &lmin, float &lmax)
{
	float
		l1	= (box.min.x - ray.pos.x) * ray.inv_dir.x,
		l2	= (box.max.x - ray.pos.x) * ray.inv_dir.x;
	lmin	= minf(l1,l2);
	lmax	= maxf(l1,l2);

	l1	= (box.min.y - ray.pos.y) * ray.inv_dir.y;
	l2	= (box.max.y - ray.pos.y) * ray.inv_dir.y;
	lmin	= maxf(minf(l1,l2), lmin);
	lmax	= minf(maxf(l1,l2), lmax);
	
	l1	= (box.min.z - ray.pos.z) * ray.inv_dir.z;
	l2	= (box.max.z - ray.pos.z) * ray.inv_dir.z;
	lmin	= maxf(minf(l1,l2), lmin);
	lmax	= minf(maxf(l1,l2), lmax);

	return (lmax >= lmin) & (lmax >= 0.f);
}