[Mesa-dev] [PATCH] nir: Make vec-to-movs handle src/dest aliasing.

[Connor Abbott]

What happens if you have something like foo = vec3(foo.z, bar.x,
foo.x)? I don't think emitting vector mov's for only the contiguous
components is enough.

On Thu, Jan 22, 2015 at 4:51 PM, Eric Anholt <eric at anholt.net> wrote:
> It now emits vector MOVs instead of a series of individual MOVs, which
> should be useful to any vector backends.  This pushes the problem of
> src/dest aliasing of channels on a scalar chip to the backend, but if
> there are any vector operations in your shader then you needed to be
> handling this already.
>
> Fixes fs-swap-problem with my scalarizing patches.
> ---
>  src/glsl/nir/nir_lower_vec_to_movs.c | 74 +++++++++++++++++++++++++++++++-----
>  1 file changed, 64 insertions(+), 10 deletions(-)
>
> diff --git a/src/glsl/nir/nir_lower_vec_to_movs.c b/src/glsl/nir/nir_lower_vec_to_movs.c
> index 022889e..489901e 100644
> --- a/src/glsl/nir/nir_lower_vec_to_movs.c
> +++ b/src/glsl/nir/nir_lower_vec_to_movs.c
> @@ -33,6 +33,49 @@
>   */
>
>  static bool
> +src_matches_dest_reg(nir_dest *dest, nir_src *src)
> +{
> +   if (dest->is_ssa || src->is_ssa)
> +      return false;
> +
> +   return (dest->reg.reg == src->reg.reg &&
> +           dest->reg.base_offset == src->reg.base_offset &&
> +           !dest->reg.indirect &&
> +           !src->reg.indirect);
> +}
> +
> +static unsigned
> +insert_movs(nir_alu_instr *vec, unsigned start_channel,
> +            unsigned start_src_idx, void *mem_ctx)
> +{
> +   unsigned src_idx = start_src_idx;
> +   assert(src_idx < nir_op_infos[vec->op].num_inputs);
> +
> +   nir_alu_instr *mov = nir_alu_instr_create(mem_ctx, nir_op_imov);
> +   nir_alu_src_copy(&mov->src[0], &vec->src[src_idx], mem_ctx);
> +   nir_alu_dest_copy(&mov->dest, &vec->dest, mem_ctx);
> +
> +   mov->dest.write_mask = (1u << start_channel);
> +   mov->src[0].swizzle[start_channel] = vec->src[src_idx].swizzle[0];
> +   src_idx++;
> +
> +   for (unsigned i = start_channel + 1; i < 4; i++) {
> +      if (!(vec->dest.write_mask & (1 << i)))
> +         continue;
> +
> +      if (nir_srcs_equal(vec->src[src_idx].src, vec->src[start_src_idx].src)) {
> +         mov->dest.write_mask |= (1 << i);
> +         mov->src[0].swizzle[i] = vec->src[src_idx].swizzle[0];
> +      }
> +      src_idx++;
> +   }
> +
> +   nir_instr_insert_before(&vec->instr, &mov->instr);
> +
> +   return mov->dest.write_mask;
> +}
> +
> +static bool
>  lower_vec_to_movs_block(nir_block *block, void *mem_ctx)
>  {
>     nir_foreach_instr_safe(block, instr) {
> @@ -50,22 +93,33 @@ lower_vec_to_movs_block(nir_block *block, void *mem_ctx)
>           continue; /* The loop */
>        }
>
> +      /* Since we insert multiple MOVs, we have to be non-SSA. */
> +      assert(!vec->dest.dest.is_ssa);
> +
> +      unsigned finished_write_mask = 0;
> +
> +      /* First, emit a MOV for all the src channels that are in the
> +       * destination reg, in case other values we're populating in the dest
> +       * might overwrite them.
> +       */
>        for (unsigned i = 0, src_idx = 0; i < 4; i++) {
>           if (!(vec->dest.write_mask & (1 << i)))
>              continue;
>
> -         assert(src_idx < nir_op_infos[vec->op].num_inputs);
> -
> -         nir_alu_instr *mov = nir_alu_instr_create(mem_ctx, nir_op_imov);
> -         nir_alu_src_copy(&mov->src[0], &vec->src[src_idx], mem_ctx);
> -
> -         /* We only care about the one swizzle */
> -         mov->src[0].swizzle[i] = vec->src[src_idx].swizzle[0];
> +         if (src_matches_dest_reg(&vec->dest.dest, &vec->src[src_idx].src)) {
> +            finished_write_mask |= insert_movs(vec, i, src_idx, mem_ctx);
> +            break;
> +         }
> +         src_idx++;
> +      }
>
> -         nir_alu_dest_copy(&mov->dest, &vec->dest, mem_ctx);
> -         mov->dest.write_mask = (1u << i);
> +      /* Now, emit MOVs for all the other src channels. */
> +      for (unsigned i = 0, src_idx = 0; i < 4; i++) {
> +         if (!(vec->dest.write_mask & (1 << i)))
> +            continue;
>
> -         nir_instr_insert_before(&vec->instr, &mov->instr);
> +         if (!(finished_write_mask & (1 << i)))
> +            finished_write_mask |= insert_movs(vec, i, src_idx, mem_ctx);
>
>           src_idx++;
>        }
> --
> 2.1.3
>
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