[Mesa-dev] [PATCH 1/2] nir/algebraic: Rewrite bit-size inference

[Dylan Baker]

Quoting Connor Abbott (2018-11-29 10:32:02)
> Before this commit, there were two copies of the algorithm: one in C,
> that we would use to figure out what bit-size to give the replacement
> expression, and one in Python, that emulated the C one and tried to
> prove that the C algorithm would never fail to correctly assign
> bit-sizes. That seemed pretty fragile, and likely to fall over if we
> make any changes. Furthermore, the C code was really just recomputing
> more-or-less the same thing as the Python code every time. Instead, we
> can just store the results of the Python algorithm in the C
> datastructure, and consult it to compute the bitsize of each value,
> moving the "brains" entirely into Python. Since the Python algorithm no
> longer has to match C, it's also a lot easier to change it to something
> more closely approximating an actual type-inference algorithm. The
> algorithm used is based on Hindley-Milner, although deliberately
> weakened a little. It's a few more lines than the old one, judging by
> the diffstat, but I think it's easier to verify that it's correct while
> being as general as possible.
> 
> We could split this up into two changes, first making the C code use the
> results of the Python code and then rewriting the Python algorithm, but
> since the old algorithm never tracked which variable each equivalence
> class, it would mean we'd have to add some non-trivial code which would
> then get thrown away. I think it's better to see the final state all at
> once, although I could also try splitting it up.
> ---
>  src/compiler/nir/nir_algebraic.py | 518 ++++++++++++++++--------------
>  src/compiler/nir/nir_search.c     | 146 +--------
>  src/compiler/nir/nir_search.h     |   2 +-
>  3 files changed, 295 insertions(+), 371 deletions(-)
> 
> diff --git a/src/compiler/nir/nir_algebraic.py b/src/compiler/nir/nir_algebraic.py
> index 728196136ab..48390dbde38 100644
> --- a/src/compiler/nir/nir_algebraic.py
> +++ b/src/compiler/nir/nir_algebraic.py
> @@ -88,7 +88,7 @@ class Value(object):
>  
>     __template = mako.template.Template("""
>  static const ${val.c_type} ${val.name} = {
> -   { ${val.type_enum}, ${val.bit_size} },
> +   { ${val.type_enum}, ${val.c_bit_size} },
>  % if isinstance(val, Constant):
>     ${val.type()}, { ${val.hex()} /* ${val.value} */ },
>  % elif isinstance(val, Variable):
> @@ -112,6 +112,40 @@ static const ${val.c_type} ${val.name} = {
>     def __str__(self):
>        return self.in_val
>  
> +   def get_bit_size(self):
> +      """Get the physical bit-size that has been chosen for this value, or if
> +      there is none, the canonical value which currently represents this
> +      bit-size class. Variables will be preferred, i.e. if there are any
> +      variables in the equivalence class, the canonical value will be a
> +      variable. We do this since we'll need to know which variable each value
> +      is equivalent to when constructing the replacement expression. This is
> +      the "find" part of the union-find algorithm.
> +      """
> +      bit_size = self
> +
> +      while isinstance(bit_size, Value):
> +         if bit_size._bit_size == None:

Use "is" and "is not" instead of "==" and "!=" when comparing singletons like
None, True, False; the former are the identity operators, they'll be faster and
avoid any surprises.

> +            break
> +         bit_size = bit_size._bit_size
> +
> +      if bit_size != self:

Is this a comparison of identity or equality? If it's identity you should use
"is not"

> +         self._bit_size = bit_size
> +      return bit_size
> +

[snip]

>           else:
> -            if val.common_class != 0:
> -               assert val.bit_size == 0 or val.bit_size == val.common_class
> -            else:
> -               val.common_class = val.bit_size
> -            return val.common_class
> +            self.unify_bit_size(src, src_type_bits,
> +               lambda src_bit_size, unused:
> +                  '{} must have {} bits, but as a source of nir_op_{} '\
> +                  'it must have {} bits'.format(src, src_bit_size, nir_op.name, src_type_bits)
> +                  if self.is_search else
> +                  '{} has the bit size of {}, but as a source of nir_op_{} '\
> +                  'it must have {} bits, which may not be the same'.format(
> +                     src, src_bit_size, nir_op.name, src_type_bits))
> +
> +      if dst_type_bits == 0:

The common idiom is `if not dst_type_bits`, which will also be faster (for
ints', `if val` is equivalent to `if val != 0`, and `if not val` is equivalent
to `if val == 0`).

> +         for src_type, src in zip(nir_op.input_types, val.sources):
> +            src_type_bits = type_bits(src_type)
> +            if src_type_bits == 0:
> +               self.unify_bit_size(val, src,
> +                  lambda val_bit_size, src_bit_size:
> +                     '{} must have the bit size of {}, while its source {} must '
> +                     'have incompatible bit size {}'.format(
> +                        val, val_bit_size, src, src_bit_size)
> +                     if self.is_search else
> +                     '{} must have {} bits, but its source {} ' \
> +                     '(bit size of {}) may not have that bit size ' \
> +                     'when building the replacement.'.format(
> +                        val, val_bit_size, src, src_bit_size))

This is a case where a ternary hurts readability, how about just using a local
function?

Dylan
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