# Hybrid Frontend Developer Guide¶

If you are a developer:

1. who is trying writing some preliminary patterns that have not been supported by TVM yet, maybe Hybrid Frontend Language Reference is a better place for you.

1. who wants to know the implementation details of this module, you are right here!

## Features¶

### Software Emulation¶

In software emulation, the most interesting thing is the decorator tvm.hybrid.script. This decorator helps 2 things:

1. Importing runtime variables

2. Overloading the function according to the arguments passed

Correct me if I am wrong: I believe that how 1. is implemented is dangerous, but I have no choice. What I did is to add those names into python dict func.__global__ and after the call to func is done, those names will be cleaned up.

Overload is simple: the decorator checks the arguments’ types and determines which function should be actually called.

### Backend Compilation¶

Compilation is a large module, you can see python/tvm/hybrid/var_decl.py and python/tvm/hybrid/parser.py for more details. The first stage determines the usage, or more accurately the declaration of each variable and the second stage does the actual IR generation.

### Attributes¶

So far, ONLY tensors’ shape attribute is supported. You can see visit_Subscript in python/tvm/hybrid/parser.py for more details. This is a hacky solution, I just check the attributes when subscript.

### Loops¶

In HalideIR, loops have in total 4 types: serial, unrolled, parallel, and vectorized.

Note

Unlike what that is in HalideIR, in loop_type(a, b), a is the starting point and b is the trip count of iterations. Here loop_type(a, b) indicates [a, b). Thus, when lowering it to HalideIR, we need to do start, extent = a, b - a

Note

In HalideIR those are enums, they are in passive form. Here we use active form to annotate loops, because they are ready to run.

### Variables¶

Because there is no variables in HalideIR, all the mutable variables will be lowered to an array with size 1. It takes the first store of a variable as its declaration.

### Math Intrinsics¶

So far, these math intrinsics, log, exp, sigmoid, tanh, power, and popcount, are supported. Math intrinsics will be imported by the decorator. Most of the intrinsics are borrowed by library implementation except popcount and sigmoid. I implemented them manually.