[Mesa-dev] [PATCH 4/9] nir: Move the compare-with-zero optimizations to the late section

[Matt Turner]

On Mon, Mar 23, 2015 at 8:13 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> total instructions in shared programs: 4422307 -> 4422363 (0.00%)
> instructions in affected programs:     4230 -> 4286 (1.32%)
> helped:                                0
> HURT:                                  12
>
> While this does hurt some things, the losses are minor and it prevents the
> compare-with-zero optimization from fighting with ffma which is much more
> important.

Is it actually "fighting" (i.e., undoing the other pass' work) or just
preventing some ffmas from being generated?

If we did have something that would be recognized by both these and
the ffma pattern, it'd look like

fge(fadd(a, fmul(b, c)), 0.0)

which we could turn into

fge(ffma(a, b, c), 0.0) if ffma runs first; or
fge(a, fneg(fmul(b, c)) otherwise

I guess the first one is better for i965, since we can do that in one
instruction. In fact, maybe we don't want to do these optimizations at
all? I'm kind of surprised that it hurts.

> ---
>  src/glsl/nir/nir_opt_algebraic.py | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/src/glsl/nir/nir_opt_algebraic.py b/src/glsl/nir/nir_opt_algebraic.py
> index 84d4c72..cfb1429 100644
> --- a/src/glsl/nir/nir_opt_algebraic.py
> +++ b/src/glsl/nir/nir_opt_algebraic.py
> @@ -82,10 +82,6 @@ optimizations = [
>     (('inot', ('fge', a, b)), ('flt', a, b)),
>     (('inot', ('ilt', a, b)), ('ige', a, b)),
>     (('inot', ('ige', a, b)), ('ilt', a, b)),
> -   (('flt', ('fadd', a, b), 0.0), ('flt', a, ('fneg', b))),
> -   (('fge', ('fadd', a, b), 0.0), ('fge', a, ('fneg', b))),
> -   (('feq', ('fadd', a, b), 0.0), ('feq', a, ('fneg', b))),
> -   (('fne', ('fadd', a, b), 0.0), ('fne', a, ('fneg', b))),
>     (('fge', ('fneg', ('fabs', a)), 0.0), ('feq', a, 0.0)),
>     (('bcsel', ('flt', a, b), a, b), ('fmin', a, b)),
>     (('bcsel', ('flt', a, b), b, a), ('fmax', a, b)),
> @@ -163,9 +159,6 @@ optimizations = [
>     (('iadd', a, ('isub', 0, b)), ('isub', a, b)),
>     (('fabs', ('fsub', 0.0, a)), ('fabs', a)),
>     (('iabs', ('isub', 0, a)), ('iabs', a)),
> -
> -# This one may not be exact
> -   (('feq', ('fadd', a, b), 0.0), ('feq', a, ('fneg', b))),

Separate commit?

>  ]
>
>  # Add optimizations to handle the case where the result of a ternary is
> @@ -194,6 +187,10 @@ for op in ['flt', 'fge', 'feq', 'fne',
>  # they help code generation but do not necessarily produce code that is
>  # more easily optimizable.
>  late_optimizations = [
> +   (('flt', ('fadd', a, b), 0.0), ('flt', a, ('fneg', b))),
> +   (('fge', ('fadd', a, b), 0.0), ('fge', a, ('fneg', b))),
> +   (('feq', ('fadd', a, b), 0.0), ('feq', a, ('fneg', b))),
> +   (('fne', ('fadd', a, b), 0.0), ('fne', a, ('fneg', b))),
>  ]
>
>  print nir_algebraic.AlgebraicPass("nir_opt_algebraic", optimizations).render()
> --
> 2.3.3