[Mesa-dev] [PATCH 09/11] nir/lower_vec_to_movs: Get rid of start_idx and swizzle compacting

[Eduardo Lima Mitev]

On 09/10/2015 02:50 AM, Jason Ekstrand wrote:
> Previously, we did this thing with keeping track of a separate start_idx
> which was different from the iteration variable.  I think this was a relic
> of the way that GLSL IR implements writemasks.  In NIR, if a given bit in
> the writemask is unset then that channel is just "unused", not missing.  In
> particular, a vec4 operation with a writemask of 0xd will use sources 0, 2,
> and 3 and leave source 1 alone.  We can simplify things a good deal (and
> make them correct) by removing this "compacting" step.
> 

Indeed, much clearer now.

Reviewed-by: Eduardo Lima Mitev <elima at igalia.com>

> Cc: Eric Anholt <eric at anholt.net>
> ---
>  src/glsl/nir/nir_lower_vec_to_movs.c | 33 +++++++++++++--------------------
>  1 file changed, 13 insertions(+), 20 deletions(-)
> 
> diff --git a/src/glsl/nir/nir_lower_vec_to_movs.c b/src/glsl/nir/nir_lower_vec_to_movs.c
> index 7d31e36..ed8ec9b 100644
> --- a/src/glsl/nir/nir_lower_vec_to_movs.c
> +++ b/src/glsl/nir/nir_lower_vec_to_movs.c
> @@ -58,29 +58,25 @@ src_matches_dest_reg(nir_dest *dest, nir_src *src)
>   * which ones have been processed.
>   */
>  static unsigned
> -insert_mov(nir_alu_instr *vec, unsigned start_channel,
> -            unsigned start_src_idx, nir_shader *shader)
> +insert_mov(nir_alu_instr *vec, unsigned start_idx, nir_shader *shader)
>  {
> -   unsigned src_idx = start_src_idx;
> -   assert(src_idx < nir_op_infos[vec->op].num_inputs);
> +   assert(start_idx < nir_op_infos[vec->op].num_inputs);
>  
>     nir_alu_instr *mov = nir_alu_instr_create(shader, nir_op_imov);
> -   nir_alu_src_copy(&mov->src[0], &vec->src[src_idx], mov);
> +   nir_alu_src_copy(&mov->src[0], &vec->src[start_idx], mov);
>     nir_alu_dest_copy(&mov->dest, &vec->dest, mov);
>  
> -   mov->dest.write_mask = (1u << start_channel);
> -   mov->src[0].swizzle[start_channel] = vec->src[src_idx].swizzle[0];
> -   src_idx++;
> +   mov->dest.write_mask = (1u << start_idx);
> +   mov->src[0].swizzle[start_idx] = vec->src[start_idx].swizzle[0];
>  
> -   for (unsigned i = start_channel + 1; i < 4; i++) {
> +   for (unsigned i = start_idx + 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)) {
> +      if (nir_srcs_equal(vec->src[i].src, vec->src[start_idx].src)) {
>           mov->dest.write_mask |= (1 << i);
> -         mov->src[0].swizzle[i] = vec->src[src_idx].swizzle[0];
> +         mov->src[0].swizzle[i] = vec->src[i].swizzle[0];
>        }
> -      src_idx++;
>     }
>  
>     nir_instr_insert_before(&vec->instr, &mov->instr);
> @@ -129,26 +125,23 @@ lower_vec_to_movs_block(nir_block *block, void *void_state)
>         * 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++) {
> +      for (unsigned i = 0; i < 4; i++) {
>           if (!(vec->dest.write_mask & (1 << i)))
>              continue;
>  
> -         if (src_matches_dest_reg(&vec->dest.dest, &vec->src[src_idx].src)) {
> -            finished_write_mask |= insert_mov(vec, i, src_idx, state->shader);
> +         if (src_matches_dest_reg(&vec->dest.dest, &vec->src[i].src)) {
> +            finished_write_mask |= insert_mov(vec, i, state->shader);
>              break;
>           }
> -         src_idx++;
>        }
>  
>        /* Now, emit MOVs for all the other src channels. */
> -      for (unsigned i = 0, src_idx = 0; i < 4; i++) {
> +      for (unsigned i = 0; i < 4; i++) {
>           if (!(vec->dest.write_mask & (1 << i)))
>              continue;
>  
>           if (!(finished_write_mask & (1 << i)))
> -            finished_write_mask |= insert_mov(vec, i, src_idx, state->shader);
> -
> -         src_idx++;
> +            finished_write_mask |= insert_mov(vec, i, state->shader);
>        }
>  
>        nir_instr_remove(&vec->instr);
>