[Mesa-dev] [PATCH 27/29] nir/lower_to_source_mod: Skip unsafe operations

[Connor Abbott]

This seems a little ugly to me. Since these ops are just moving bits
around, maybe we should just convert the float64 src/dest to uint64?
We do this for other opcodes that just shuffle bits around.

On Mon, Mar 21, 2016 at 8:06 AM, Samuel Iglesias Gonsálvez
<siglesias at igalia.com> wrote:
> From: Iago Toral Quiroga <itoral at igalia.com>
>
> Some special alu operations cannot absorve src modifiers safely. For example,
> double unpack operations split a double into separate 32-bit chunks that
> are to be manipulated at bit level. If we allow these operations to absorve
> the srcmod we run into two issues:
>
> 1. We would be applying the source modifiers to both 32-bit chunks, which is
>    not correct.
> 2. If the 32-bit chunks are handled as unsigned data by the driver, the
>    the effect of the source modifier could be lost.
>
> So just skip srcmod lowering for these operations.
> ---
>  src/compiler/nir/nir_lower_to_source_mods.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/src/compiler/nir/nir_lower_to_source_mods.c b/src/compiler/nir/nir_lower_to_source_mods.c
> index 1e8c3c2..8964dc0 100644
> --- a/src/compiler/nir/nir_lower_to_source_mods.c
> +++ b/src/compiler/nir/nir_lower_to_source_mods.c
> @@ -33,6 +33,26 @@
>   * easier to not have them when we're doing optimizations.
>   */
>
> +/*
> + * Some special alu operations cannot absorve src modifiers safely. For example,
> + * double unpack operations split a double into separate 32-bit chunks that
> + * are to be manipulated at bit level. If we allow these operations to absorve
> + * the srcmod we run into two issues:
> + *
> + * 1. We would be applying the source modifiers to both 32-bit chunks, which is
> + *    not correct.
> + * 2. If the 32-bit chunks are handled as unsigned data by the driver, the
> + *    the effect of the source modifier could be lost.
> + *
> + * So just skip srcmod lowering for these operations.
> + */
> +static bool
> +nir_alu_op_unsafe_for_srcmods(unsigned opcode)
> +{
> +   return opcode == nir_op_unpack_double_2x32_split_x ||
> +          opcode == nir_op_unpack_double_2x32_split_y;
> +}
> +
>  static bool
>  nir_lower_to_source_mods_block(nir_block *block, void *state)
>  {
> @@ -41,6 +61,8 @@ nir_lower_to_source_mods_block(nir_block *block, void *state)
>           continue;
>
>        nir_alu_instr *alu = nir_instr_as_alu(instr);
> +      if (nir_alu_op_unsafe_for_srcmods(alu->op))
> +         continue;
>
>        for (unsigned i = 0; i < nir_op_infos[alu->op].num_inputs; i++) {
>           if (!alu->src[i].src.is_ssa)
> --
> 2.5.0
>
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