Sorting float array in fragment shader without dynamic branching in OpenGL ES (iPad) - opengl-es-2.0

Question

We have a somewhat complicated fragment shader, which we cannot split into smaller shaders.
One of the thing the shader does is calculate the distance of the fragment/pixel to 7 other points, and find the two nearest ones.
On the iPad, we are having huge performance problems with this part of the code, and Instruments tells us the bottleneck is the shader code which leads to dynamic branches in the GPU.
We sacrificed readability and tried several code changes to avoid these dynamic branches:
Changed the loop so that it's not really a loop, but a repeating series of instructions.
Simplified the code so that it contains only very simple if-like instructions.
This is the 'simplest' version of the code we came up with:
bool isLowest;
float currentDistance, lowestDistance, newLowest;
lowestDistance = distances[1];
int indexOfLowest = 1, newIndexOfLowest;
// 'loop' for index 2
currentDistance = distances[2];
isLowest = currentDistance < lowestDistance;
newLowest = isLowest ? currentDistance : lowestDistance;
lowestDistance = newLowest;
newIndexOfLowest = isLowest ? 2 : indexOfLowest; // BAD LINE
indexOfLowest = newIndexOfLowest;
// 'loop' for index 3
currentDistance = distances[3];
isLowest = currentDistance < lowestDistance;
newLowest = isLowest ? currentDistance : lowestDistance;
lowestDistance = newLowest;
newIndexOfLowest = isLowest ? 3 : indexOfLowest; // BAD LINE
indexOfLowest = newIndexOfLowest;
// etc. until index 8
As you can see, the code finds the index of the lowest distance. In our opinion, this code can be executed without dynamic branches. The last four lines of one 'loop' are just simple calculations, they are not real branches.
The problem is the second-last row in a 'loop, where indexOfLowest gets a new value:
newIndexOfLowest = isLowest ? 2 : indexOfLowest;
If we comment out that line, everything works fine on 60 FPS and Instruments doesn't report dynamic branches. But with this line, it's not more than 8 FPS.
How can we rewrite this code such that it does not cause dynamic branches in the GPU?
EDIT: Just updated the code to an even simpler version, which has the same problems. The old code had a for loop with fixed limits. But the problem is still there.
EDIT 2: We just analyzed the code using PowerVR's PVRShaderEditor (previously PVRUniSCoEditor), where it shows the expected GPU cycles per shader source code line. The line in question (BAD LINE in the code above) is shown with only 1-2 GPU cycles. The fact that this line causes dynamic branches which lead to huge performance problems is not mentioned. Any other ideas how we could debug this code, so we can understand why it's causing these branches?

Answer 1

We did it. Casting the bool to float (1.0 for true, 0.0 for false) was helpful here:
bool isLowest;
float currentDistance, lowestDistance, newLowest;
lowestDistance = distances[1];
int indexOfLowest = 1, newIndexOfLowest;
// 'loop' for index 2
currentDistance = distances[2];
isLowest = currentDistance < lowestDistance;
indexOfLowest = float(isLowest) * float(2 - indexOfLowest);
lowestDistance = isLowest ? currentDistance : lowestDistance;
// 'loop' for index 3
currentDistance = distances[3];
isLowest = currentDistance < lowestDistance;
indexOfLowest = float(isLowest) * float(3 - indexOfLowest);
lowestDistance = isLowest ? currentDistance : lowestDistance;
// etc. until index 8
This calculates the index of the lowest distance, without using dynamic branches.