tools/codegen/api/translate.py

from typing import Dict, Sequence, List, NoReturn, Union
from tools.codegen.api.types import *

# This file implements a small program synthesis engine that implements
# conversions between one API to another.
#
# The key data type in this file in CType, short for C++ semantic type.  A CType
# represents a C++ type, plus semantic information about what it represents.
# For example, consider the argument "bool pin_memory"; its normal C++ type is
# "bool", but its C++ semantic type also keeps track that this represents a
# "pin_memory"; you can't just use a random other boolean in a context where you
# need a "pin_memory"!
#
# The translator takes a list of needed CTypes, and then figures out how
# to construct expressions with these CTypes from the given bindings.  Many
# of these expressions are trivial (I need a Tensor other; there's a Tensor
# other scope); others are more nontrivial and may require packing/unpacking.
# Some examples of non-trivial action:
#
#   - Need the "dtype" binding?  Well, maybe "dtype" isn't available
#     in the context, instead, "options" is, and you need to extract
#     it from there.  (Gather)
#
#   - Need the "context" binding?  Well, maybe "context" isn't available
#     in the context, and you need to construct it from "dtype", "device",
#     etc.  (Scatter)
#
#   - Need the "memory_format" binding?  Well, actually, it's available
#     from both "memory_format" and "options", so you had better make sure
#     they are consistent.  (Join)

options_ctype = ConstRefCType(BaseCType("TensorOptions", "options"))

class UnsatError(RuntimeError):
    pass

# Given a set of in-scope bindings and a set of target bindings, synthesize
# a list of expressions that uses only the in-scope bindings (bindings) that
# have all of the types of goals.  You may want to use this function if
# you're generating code for a function like:
#
#   void f({args}) {
#     g({exprs}); // g is a different API
#   }
#
# and you need to generate "exprs".
#
# Typically, a list of Bindings is convenient to get (you usually call something
# like arguments() to get them); but technically you only need less information:
# for 'bindings' an (un-ordered) list of Exprs is sufficient; similarly, for
# 'goals', an (ordered) list of CType goals is sufficient.  If you are doing
# something more complicated, e.g., tracking the set of bindings in a context,
# you may find using these smaller types more convenient.
def translate(
    bindings: Sequence[Union[Expr, Binding]],
    goals: Sequence[Union[CType, Binding]],
    *, method: bool = False
) -> List[Expr]:

    binding_exprs: List[Expr] = []
    for b in bindings:
        if isinstance(b, Binding):
            binding_exprs.append(Expr(
                expr=b.name,
                type=b.ctype,
            ))
        else:
            binding_exprs.append(b)

    goal_ctypes: List[CType] = []
    for g in goals:
        if isinstance(g, Binding):
            goal_ctypes.append(g.ctype)
        else:
            goal_ctypes.append(g)

    # Add all the bindings to the context
    ctx: Dict[CType, str] = {}
    for b in binding_exprs:
        ctx[b.type] = b.expr

        # While we're at it, do some simple forward inference, looking through
        # constructors.
        # TODO: My kingdom for a pattern matcher
        # https://www.python.org/dev/peps/pep-0634/
        # TODO: This could get us in recomputation trouble if b.expr is nontrivial
        t = b.type
        if isinstance(t, ConstRefCType) and isinstance(t.elem, OptionalCType) and \
                isinstance(t.elem.elem, BaseCType) and t.elem.elem.type == 'Tensor':
            ctx[ConstRefCType(BaseCType("Tensor", t.elem.elem.name))] = \
                f'({b.expr}.has_value() ? *{b.expr} : at::Tensor())'

    # Add implicit bindings if the generated code is inside a Tensor method
    if method:
        ctx[MutRefCType(BaseCType("Tensor", "self"))] = "const_cast<Tensor&>(*this)"
        ctx[ConstRefCType(BaseCType("Tensor", "self"))] = "const_cast<Tensor&>(*this)"
        # This is better!  Byte-for-byte compat
        # ctx[ConstRefCType(BaseCType("Tensor", "self"))] = "*this"

    def unsat(goal: CType) -> NoReturn:
        ctx_desc = '\n'.join(f"  {t.cpp_type()} {t.name}; // {e}" for t, e in ctx.items())
        raise UnsatError(f'''
Failed to synthesize the expression "{goal.cpp_type()} {goal.name}".
When I failed, the following bindings were available in the context:

{ctx_desc}

This probably means there is a missing rule in the rules of tools.codegen.api.translate.
Check this module for more information.
''')

    # A shitty backtracking search implementation.  It's shitty because it
    # doesn't actually do backtracing or search. In particular, if
    # direct=True, we won't try to do any fancy synthesis, just trivial
    # conversions (e.g., "T a" is OK for "const T& a").  So all of the
    # existing rules in this function simply try to solve immediately,
    # and bail if things don't work out.
    def solve(goal: CType, *, direct: bool) -> str:
        def direct_solve(goal: CType) -> str:
            return solve(goal, direct=True)

        if goal in ctx:
            # Trivial
            return ctx[goal]

        # const & is satisfied with mutable &
        if isinstance(goal, ConstRefCType):
            try:
                # WARNING: not strictly decreasing; be careful not
                # to add a direct conversion that goes satisfies
                # mutable& with const&
                return solve(MutRefCType(goal.elem), direct=direct)
            except UnsatError:
                pass

        # mutable & is satisfied with value
        if isinstance(goal, MutRefCType):
            try:
                return solve(goal.elem, direct=direct)
            except UnsatError:
                pass

        if direct:
            unsat(goal)

        # For now, all of these rules are mutually exclusive.
        if goal == OptionalCType(BaseCType("MemoryFormat", "memory_format")):
            memory_format = direct_solve(
                OptionalCType(BaseCType("MemoryFormat", SpecialArgName.possibly_redundant_memory_format))
            )
            # No need to join "memory_format" and "options" if the target API takes "options" directly.
            # Otherwise it will cause the redundant memory_format error.
            if options_ctype in goal_ctypes:
                return memory_format
            try:
                options = direct_solve(options_ctype)
                return f"c10::impl::check_tensor_options_and_extract_memory_format({options}, {memory_format})"
            except UnsatError:
                return memory_format

        elif goal == BaseCType("TensorOptions", "options"):
            dtype = direct_solve(OptionalCType(BaseCType("ScalarType", "dtype")))
            pin_memory = direct_solve(OptionalCType(BaseCType("bool", "pin_memory")))
            device = direct_solve(OptionalCType(BaseCType("Device", "device")))
            layout = direct_solve(OptionalCType(BaseCType("Layout", "layout")))
            return f'TensorOptions().dtype({dtype}).layout({layout}).device({device}).pinned_memory({pin_memory})'

        elif goal == OptionalCType(BaseCType("ScalarType", "dtype")):
            options = direct_solve(options_ctype)
            return f'optTypeMetaToScalarType({options}.dtype_opt())'

        elif goal == OptionalCType(BaseCType("Layout", "layout")):
            options = direct_solve(options_ctype)
            return f'{options}.layout_opt()'

        elif goal == OptionalCType(BaseCType("Device", "device")):
            options = direct_solve(options_ctype)
            return f'{options}.device_opt()'

        elif goal == OptionalCType(BaseCType("bool", "pin_memory")):
            options = direct_solve(options_ctype)
            return f'{options}.pinned_memory_opt()'

        unsat(goal)

    return [Expr(solve(g, direct=False), g) for g in goal_ctypes]