(Prototype) Generate the frontend AST when traversing the Python one

[k-ye]

Concisely describe the proposed feature

Right now Taichi generates its C++ AST in the following steps:

1. Traverse the Python AST, substitutes many parts into function calls to build the C++ AST
2. Compile this transformed Python AST back to a Python function
3. Invoke the Python function, so that the C++ AST is built

Describe the solution you'd like (if any)

Build the C++ AST directly while traversing the Python AST.

AST Node Implementation Progress

Based on https://docs.python.org/3.9/library/ast.html#abstract-grammar

mod

• Module
• Interactive
• Expression
• FunctionType

stmt

• FunctionDef

  • return value
  • kernel arguments
    • ti.template
    • ti.sparse_matrix_builder
    • ti.any_arr
    • scalar
  • function arguments

• AsyncFunctionDef

• ClassDef

• Return

• Delete

• Assign

  • static assign
  • normal assign

• AugAssign

• AnnAssign

• For

  • static range/ndrange for
  • static grouped for
  • range for
  • ndrange for
  • grouped ndrange for
  • struct for
  • grouped struct for

• AsyncFor

• While

• If

  • static if
  • normal if

• With

• AsyncWith

• Raise

• Try

• Assert

• Import

• ImportFrom

• Global

• Nonlocal

• Expr

• Pass

• Break

• Continue

expr

• BoolOp

• NamedExpr

• BinOp

• UnaryOp

• Lambda

• IfExp

• Dict

• Set

• ListComp

• SetComp

• DictComp

• GeneratorExp

• Await

• Yield

• YieldFrom

• Compare

  • Chain comparison (left op[0] comparators[0]) and (comparators[0] op[1] comparators[1]) and ...

• Call

  • basic function call
  • built-in to ti functions

• FormattedValue

• JoinedStr

• Constant

• Attribute

• Subscript

• Starred

• Name

• List

• Tuple

• Slice

other

• real function support