float Helper_ShuffleDown(float x, uint index, uint mask, out bool valid)
{
uint clamp = mask & 0x1fu;
uint segMask = (mask >> 8) & 0x1fu;
uint minThreadId = $SUBGROUP_INVOCATION$ & segMask;
uint maxThreadId = minThreadId | (clamp & ~segMask);
uint srcThreadId = $SUBGROUP_INVOCATION$ + index;
valid = srcThreadId <= maxThreadId;
float v = $SUBGROUP_BROADCAST$(x, srcThreadId);
return valid ? v : x;
}