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

[Jason Ekstrand]

On Mon, Dec 3, 2018 at 3:50 PM Dylan Baker <dylan at pnwbakers.com> wrote:

> 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"
>

I just saw this one in v2.  Agreed; it should be "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`).
>

While it may not be pythonic, I think I prefer == 0 because that makes the
reader explicitly aware that it's an integer.


> > +         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?
>

I could go either way.  I didn't find it too hard but I agree it's kind-of
mashed together.  For that matter,

if self.is_search:
   unify_bit_size(...)
else:
   unify_bit_size(...)

would be even better.

--Jason
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