aco: skip continue_or_break LCSSA phis when not needed

Fixes:
//exec is empty here
loop {
   %1:s[16-17] = ...
   if () {
      break
   }
   %2:s[16-17] = ...
   continue_or_break
}
%3 = phi %1, undef
//because of the undef, %2 can use s[16-17] and overwrite the address
load(%3:s[16-17])

No fossil-db changes.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Fixes: bbe4652430 ("aco: create lcssa phis for continue_or_break loops when necessary")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11333
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29838>
(cherry picked from commit e3ffc244f5)

.pick_status.json

@@ -84,7 +84,7 @@
         "description": "aco: skip continue_or_break LCSSA phis when not needed",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "bbe46524307e5e834d5b3d593763b72120d6a3ee",
         "notes": null

src/amd/compiler/aco_instruction_selection.cpp

@@ -10446,6 +10446,17 @@ lcssa_workaround(isel_context* ctx, nir_loop* loop)
 
    std::map<unsigned, unsigned> renames;
    nir_foreach_block_in_cf_node (block, &loop->cf_node) {
+      /* These values are reachable from the loop exit even when continue_or_break is used. We
+       * shouldn't create phis with undef operands in case the contents are important even if exec
+       * is zero (for example, memory access addresses). */
+      if (nir_block_dominates(block, nir_loop_last_block(loop)))
+         continue;
+
+      /* Definitions in this block are not reachable from the loop exit, and so all uses are inside
+       * the loop. */
+      if (!nir_block_dominates(block, block_after_loop))
+         continue;
+
       nir_foreach_instr (instr, block) {
          nir_def* def = nir_instr_def(instr);
          if (!def)

src/amd/compiler/aco_instruction_selection_setup.cpp

@@ -288,7 +288,7 @@ init_context(isel_context* ctx, nir_shader* shader)
    nir_metadata_preserve(impl, nir_metadata_none);
 
    /* we'll need these for isel */
-   nir_metadata_require(impl, nir_metadata_block_index);
+   nir_metadata_require(impl, nir_metadata_block_index | nir_metadata_dominance);
 
    if (ctx->options->dump_preoptir) {
       fprintf(stderr, "NIR shader before instruction selection:\n");