nir/gcm: be more conservative about moving instructions from loops

Here we only pull instructions further up control flow if they are
constant or texture instructions. See the code comment for more
information.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4636>

diff --git a/src/compiler/nir/nir_opt_gcm.c b/src/compiler/nir/nir_opt_gcm.c
index 0d4c23ab63136045583ef23bb73671069e3dfea3..9a337c9a97f3fd6ad264e02872cba0ca0d6af939 100644 (file)
--- a/src/compiler/nir/nir_opt_gcm.c
+++ b/src/compiler/nir/nir_opt_gcm.c
@@ -277,8 +277,22 @@ gcm_choose_block_for_instr(nir_instr *instr, nir_block *early_block,
 
    nir_block *best = late_block;
    for (nir_block *block = late_block; block != NULL; block = block->imm_dom) {
+      /* Being too aggressive with how we pull instructions out of loops can
+       * result in extra register pressure and spilling. For example its fairly
+       * common for loops in compute shaders to calculate SSBO offsets using
+       * the workgroup id, subgroup id and subgroup invocation, pulling all
+       * these calculations outside the loop causes register pressure.
+       *
+       * To work around these issues for now we only allow constant and texture
+       * instructions to be moved outside their original loops.
+       *
+       * TODO: figure out some heuristics to allow more to be moved out of loops.
+       */
       if (state->blocks[block->index].loop_depth <
-          state->blocks[best->index].loop_depth)
+          state->blocks[best->index].loop_depth &&
+          (nir_block_dominates(instr->block, block) ||
+           instr->type == nir_instr_type_load_const ||
+           instr->type == nir_instr_type_tex))
          best = block;
       else if (block == instr->block)
          best = block;