[Mesa-dev] [PATCH 01/12] nir: Add an "exact" bit to nir_alu_instr

[Matt Turner]

On Thu, Mar 17, 2016 at 5:51 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> ---
>  src/compiler/nir/nir.h       | 11 +++++++++++
>  src/compiler/nir/nir_clone.c |  1 +
>  src/compiler/nir/nir_print.c |  2 ++
>  3 files changed, 14 insertions(+)
>
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index 34f31eb..94b981b 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -671,6 +671,17 @@ extern const nir_op_info nir_op_infos[nir_num_opcodes];
>  typedef struct nir_alu_instr {
>     nir_instr instr;
>     nir_op op;
> +
> +   /** Indicates that this ALU instruction generates an exact value
> +    *
> +    * This is kind-of a mixture of GLSL "precise" and "invariant" and not

"kind of" isn't hyphenated.

> +    * really equivalent to either.  This indicates that the value generated by
> +    * this operation is high-precision and any code transformations that touch
> +    * it must ensure that the resulting value is bit-for-bit identical to the
> +    * original.

I think this is a lot of the problem -- we don't seem to have a good
idea of what these keywords mean, concretely.

Precise is more clear to me: don't optimize things in such a way as to
change the result.

Invariant is much less clear to me though. I've read the GLSL spec of
course, but can someone give me an example?