aco: improve check for unreachable loop continue blocks

The old code would have previously caught:
loop {
   ...
   break
}
when it was meant to just catch:
loop {
   if (...)
      break
   else
      break
}

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3658>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/amd/compiler/aco_instruction_selection.cpp b/src/amd/compiler/aco_instruction_selection.cpp
index 59668fb4bc1..a8b318c63c5 100644
--- a/src/amd/compiler/aco_instruction_selection.cpp
+++ b/src/amd/compiler/aco_instruction_selection.cpp
@@ -90,7 +90,7 @@ struct if_context {
    Block BB_endif;
 };
 
-static void visit_cf_list(struct isel_context *ctx,
+static bool visit_cf_list(struct isel_context *ctx,
                           struct exec_list *list);
 
 static void add_logical_edge(unsigned pred_idx, Block *succ)
@@ -8566,7 +8566,7 @@ static void visit_loop(isel_context *ctx, nir_loop *loop)
    unsigned loop_header_idx = loop_header->index;
    loop_info_RAII loop_raii(ctx, loop_header_idx, &loop_exit);
    append_logical_start(ctx->block);
-   visit_cf_list(ctx, &loop->body);
+   bool unreachable = visit_cf_list(ctx, &loop->body);
 
    //TODO: what if a loop ends with a unconditional or uniformly branched continue and this branch is never taken?
    if (!ctx->cf_info.has_branch) {
@@ -8611,8 +8611,11 @@ static void visit_loop(isel_context *ctx, nir_loop *loop)
       bld.branch(aco_opcode::p_branch);
    }
 
-   /* fixup phis in loop header from unreachable blocks */
-   if (ctx->cf_info.has_branch || ctx->cf_info.parent_loop.has_divergent_branch) {
+   /* Fixup phis in loop header from unreachable blocks.
+    * has_branch/has_divergent_branch also indicates if the loop ends with a
+    * break/continue instruction, but we don't emit those if unreachable=true */
+   if (unreachable) {
+      assert(ctx->cf_info.has_branch || ctx->cf_info.parent_loop.has_divergent_branch);
       bool linear = ctx->cf_info.has_branch;
       bool logical = ctx->cf_info.has_branch || ctx->cf_info.parent_loop.has_divergent_branch;
       for (aco_ptr<Instruction>& instr : ctx->program->blocks[loop_header_idx].instructions) {
@@ -8956,7 +8959,7 @@ static bool visit_if(isel_context *ctx, nir_if *if_stmt)
    }
 }
 
-static void visit_cf_list(isel_context *ctx,
+static bool visit_cf_list(isel_context *ctx,
                           struct exec_list *list)
 {
    foreach_list_typed(nir_cf_node, node, node, list) {
@@ -8966,7 +8969,7 @@ static void visit_cf_list(isel_context *ctx,
          break;
       case nir_cf_node_if:
          if (!visit_if(ctx, nir_cf_node_as_if(node)))
-            return;
+            return true;
          break;
       case nir_cf_node_loop:
          visit_loop(ctx, nir_cf_node_as_loop(node));
@@ -8975,6 +8978,7 @@ static void visit_cf_list(isel_context *ctx,
          unreachable("unimplemented cf list type");
       }
    }
+   return false;
 }
 
 static void export_vs_varying(isel_context *ctx, int slot, bool is_pos, int *next_pos)