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Cycles: Avoid branching in SSE version of intersection pre-calculation 

Similar to the previous commit, avoid negative effect of bad branch prediction.

Gives measurable performance up to ~2% in tests here.

Once again, thanks to Maxym Dmytrychenko!
This commit is contained in:
Sergey Sharybin 2016-10-25 14:18:32 +02:00
parent af411d918e
commit 81c9e0d295
1 changed files with 14 additions and 2 deletions
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16
intern/cycles/kernel/geom/geom_triangle_intersect.h
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@ -59,21 +59,33 @@ void triangle_intersect_precalc(float3 dir,
IsectPrecalc *isect_precalc)
{
/* Calculate dimension where the ray direction is maximal. */
#ifndef __KERNEL_SSE__
int kz = util_max_axis(make_float3(fabsf(dir.x),
fabsf(dir.y),
fabsf(dir.z)));
int kx = kz + 1; if(kx == 3) kx = 0;
int ky = kx + 1; if(ky == 3) ky = 0;
#else
int kx, ky, kz;
/* Avoiding mispredicted branch on direction. */
kz = util_max_axis(fabs(dir));
static const char inc_xaxis[] = {1, 2, 0, 55};
static const char inc_yaxis[] = {2, 0, 1, 55};
kx = inc_xaxis[kz];
ky = inc_yaxis[kz];
#endif
float dir_kz = IDX(dir, kz);
/* Swap kx and ky dimensions to preserve winding direction of triangles. */
if(IDX(dir, kz) < 0.0f) {
if(dir_kz < 0.0f) {
int tmp = kx;
kx = ky;
ky = tmp;
}
/* Calculate the shear constants. */
float inv_dir_z = 1.0f / IDX(dir, kz);
float inv_dir_z = 1.0f / dir_kz;
isect_precalc->Sx = IDX(dir, kx) * inv_dir_z;
isect_precalc->Sy = IDX(dir, ky) * inv_dir_z;
isect_precalc->Sz = inv_dir_z;