luisa-python-lang (WIP)

A new Python DSL frontend for LuisaCompute. Will be integrated into LuisaCompute python package once it's ready.

Content

• Introduction
• Basics
  • Difference from Python
  • Types
  • Value & Reference Semantics
  • Functions
  • User-defined Structs
  • Control Flow
• Advanced Usage
  • Generics
  • Metaprogramming
  • User-defined Builtins
  • Standalone Compilation

Introduction

import luisa_lang as lc

Basic Syntax

Difference from Python

There are some notable differences between luisa_lang and Python:

• Variables have value semantics by default. Use byref to indicate that an argument that is passed by reference.
• Generic functions and structs are implemented via monomorphization (a.k.a instantiation) at compile time rather than via type erasure.
• Overloading subscript operator and attribute access is different from Python. Only __getitem__ and __getattr__ are needed, which returns a local reference.

Types

Functions

Functions are defined using the @lc.func decorator. The function body can contain any valid LuisaCompute code. You can also include normal Python code that will be executed at DSL comile time using lc.comptime(). (See Metaprogramming for more details)

@lc.func
def add(a: lc.float, b: lc.float) -> lc.float:
    with lc.comptime():
        print('compiliing add function')
    return a + b

Value & Reference Semantics

Variables have value semantics by default. This means that when you assign a variable to another, a copy is made.

a = lc.float3(1.0, 2.0, 3.0)
b = a
a.x = 2.0
lc.print(f'{a.x} {b.x}') # prints 2.0 1.0

You can use byref to indicate that a variable is passed as a local reference. Assigning to an byref variable will update the original variable.

@luisa.func(a=byref, b=byref)
def swap(a: int, b: int):
    a, b = b, a

a = lc.float3(1.0, 2.0, 3.0)
b = lc.float3(4.0, 5.0, 6.0)
swap(a.x, b.x)
lc.print(f'{a.x} {b.x}') # prints 4.0 1.0

When overloading subscript operator or attribute access, you actually return a local reference to the object.

Local References

Local references are like pointers in C++. However, they cannot escape the expression boundary. This means that you cannot store a local reference in a variable and use it later. While you can return a local reference from a function, it must be returned from a uniform path. That is you cannot return different local references based on a condition.

@lc.struct
class InfiniteArray:
    def __getitem__(self, index: int) -> int:
        return self.data[index] # returns a local reference

    # this method will be ignored by the compiler. but you can still put it here for linting
    def __setitem__(self, index: int, value: int):
        pass

    # Not allowed, non-uniform return
    def __getitem__(self, index: int) -> int:
        if index == 0:
            return self.data[0]
        else:
            return self.data[1]

User-defined Structs

@lc.struct
class Sphere:
    center: lc.float3
    radius: lc.float

Generics

T = TypeVar('T', bound=Any)
@lc.func
def add(a: T, b: T) -> T:
    return a + b

Metaprogramming

luisa_lang provides a metaprogramming feature similar to C++ that allows users to generate code at compile time.

# Compile time reflection
@lc.func
def get_x_or_zero(x: Any):
    t = lc.comptime(type(x))
    if lc.comptime(hasattr(t, 'x')):
        return x.x
    else:
        return 0.0

# Lambdas/Function objects
F = TypeVar('F', bound=Callable) # TypeVar bounds facilliatate type inference in Mypy etc. but are not strictly necessary
T = TypeVar('T', bound=Any)

@lc.func
def apply_func(f: F, x: T):
    return f(x)

# Generate code at compile time
@lc.func
def call_n_times(f: F):
    with lc.comptime():
        n = input('how many times to call?')
        for i in range(n):
            # lc.embed_code(expr) will generate add expr to the DSL code
            lc.embed_code(apply_func(f, i))
            # or 
            lc.embed_code('apply_func(f, i)')

# Hint a parameter is constexpr
@lc.func(n=lc.comptime) # without this, n will be treated as a runtime variable and result in an error
def pow(x: lc.float, n: int) -> lc.float:
    p = 1.0
    with lc.comptime():
        for _ in range(n):
            lc.embed_code('p *= x')
    return p

Limitation & Caveats

• Lambda and nested function do not support updating nonlocal variables.
• All DSL types have value semantics, which means they are copied when passed around.
• lc.embed_code cannot be used to generate a new variable that is referenced in later code. Declare the variable with Any type prior to using lc.embed_code.
• lc.struct can only be used at the top level of a module.

Standalone Compiler

It is possible to retarget luisa_lang to serve as another DSL. For example, one may want to use it as a customized shader language different from what LuisaCompute provides, or use it to generate customized C++/CUDA source. In this case, one can use the standalone compiler to compile the luisa_lang code into a standalone shader file.