How to prevent Halide 'select' from evaluating both branches

The select expression in Halide is similar to if or switch statement in C. That is to say, the expression select(cond, a, b) will evaluate to a if cond is true, otherwise it evaluates to b.

So, I thought select evaluates only one branch between a and b, just like if does. But it turns out not true, I found select nearly always evaluates both branches. This is annoying especially when a or b is very costly. Maybe I'm not using Halide in the right way?

This problem was asked a long time ago in here, however the answer in that post cannot solve my problem. Here is some actual code:

Var x("x"), y("y");  Func f("f"), g("g");  f(x, y) = x >= 10000;           // a1. f is calculated  // f(x, y) = cast<bool>(0);     // a2. f is constant  Expr power = pow(pow(x, x), y);  g(x, y) = select(f(x, y), power, 0.f);  f.compute_root();               // b1. compute f in advance  // f.compute_inline();          // b2. compute f inline    Buffer<float> result(1000, 1000);  g.realize(result);    double time = Tools::benchmark(3, 100, [&] {      g.realize(result);  });  cout << "time: " << time * 1e3 << " ms" << endl;  

In this example, select evaluates to power(a costly function) or 0.f depending on f(x, y). And f(x, y) always evaluates to false, which means select should always evaluates to 0.f. There are some different test choices:

• a1: The value of f is calculated, and the result will always be false
• a2: f is set to the constant value false
• b1: Compute f in advance
• b2: Compute f inline

The test result (time cost) is:

• a1+b1: 4.22 ms
• a1+b2: 4.15 ms
• a2+b1: 4.15 ms
• a2+b2: 0.0788 ms

It seems clearly that power is evaluated in the first three cases, and it should not happen.

An other example is my FAST corner detector. If the threshold is set to 256 (which can never be reached), is_corner will always be false. However, select in L50 always evaluates score_val (very time-consuming) unless vectorize in L64 is disabled.

https://stackoverflow.com/questions/66792921/how-to-prevent-halide-select-from-evaluating-both-branches March 25, 2021 at 12:05PM