From 63b0692eac5cbbce0669b0a8c256e10a50eb7da5 Mon Sep 17 00:00:00 2001
From: Rhys Perry <pendingchaos02@gmail.com>
Date: Thu, 10 Oct 2024 14:17:34 +0100
Subject: [PATCH] aco: don't use uniform continues if exec might be empty
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Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31143>
---
 .../compiler/aco_instruction_selection.cpp    | 25 ++++++++----
 src/amd/compiler/tests/test_isel.cpp          | 38 +++++++++++++++++++
 2 files changed, 56 insertions(+), 7 deletions(-)

diff --git a/src/amd/compiler/aco_instruction_selection.cpp b/src/amd/compiler/aco_instruction_selection.cpp
index 2b1e8609ffc..1d18ae9ca09 100644
--- a/src/amd/compiler/aco_instruction_selection.cpp
+++ b/src/amd/compiler/aco_instruction_selection.cpp
@@ -10249,6 +10249,14 @@ emit_loop_jump(isel_context* ctx, bool is_break)
    append_logical_end(ctx->block);
    unsigned idx = ctx->block->index;
 
+   /* If exec is empty inside uniform control flow in a loop, we can assume that all invocations
+    * of the loop are inactive. Breaking from the loop is the right thing to do in that case.
+    * We shouldn't perform a uniform continue, or else we might never reach a break.
+    */
+   bool potentially_empty_exec = ctx->cf_info.exec.potentially_empty_discard ||
+                                 ctx->cf_info.exec.potentially_empty_break ||
+                                 ctx->cf_info.exec.potentially_empty_continue;
+
    if (is_break) {
       logical_target = ctx->cf_info.parent_loop.exit;
       add_logical_edge(idx, logical_target);
@@ -10274,7 +10282,7 @@ emit_loop_jump(isel_context* ctx, bool is_break)
       add_logical_edge(idx, logical_target);
       ctx->block->kind |= block_kind_continue;
 
-      if (!ctx->cf_info.parent_if.is_divergent) {
+      if (!ctx->cf_info.parent_if.is_divergent && !potentially_empty_exec) {
          /* uniform continue - directly jump to the loop header */
          ctx->block->kind |= block_kind_uniform;
          ctx->cf_info.has_branch = true;
@@ -10283,14 +10291,17 @@ emit_loop_jump(isel_context* ctx, bool is_break)
          return;
       }
 
-      /* for potential uniform breaks after this continue,
-         we must ensure that they are handled correctly */
-      ctx->cf_info.parent_loop.has_divergent_continue = true;
       ctx->cf_info.parent_loop.has_divergent_branch = true;
 
-      if (!ctx->cf_info.exec.potentially_empty_continue) {
-         ctx->cf_info.exec.potentially_empty_continue = true;
-         ctx->cf_info.exec.potentially_empty_continue_depth = ctx->block->loop_nest_depth;
+      if (ctx->cf_info.parent_if.is_divergent) {
+         /* for potential uniform breaks after this continue,
+            we must ensure that they are handled correctly */
+         ctx->cf_info.parent_loop.has_divergent_continue = true;
+
+         if (!ctx->cf_info.exec.potentially_empty_continue) {
+            ctx->cf_info.exec.potentially_empty_continue = true;
+            ctx->cf_info.exec.potentially_empty_continue_depth = ctx->block->loop_nest_depth;
+         }
       }
    }
 
diff --git a/src/amd/compiler/tests/test_isel.cpp b/src/amd/compiler/tests/test_isel.cpp
index 78bdeb47743..58317bc0d1b 100644
--- a/src/amd/compiler/tests/test_isel.cpp
+++ b/src/amd/compiler/tests/test_isel.cpp
@@ -1741,3 +1741,41 @@ BEGIN_TEST(isel.cf.empty_exec.repair_ssa)
 
    finish_isel_test();
 END_TEST
+
+/*
+ * loop {
+ *   if (uniform) {
+ *     terminate_if
+ *     //potentially empty
+ *     continue
+ *   }
+ *   //potentially empty
+ *   break
+ * }
+ */
+BEGIN_TEST(isel.cf.empty_exec.loop_uniform_continue)
+   if (!setup_nir_cs(GFX11))
+      return;
+
+   nir_push_loop(nb);
+   {
+      //>> BB1
+      //! /* logical preds: BB0, BB2, / linear preds: BB0, BB3, / kind: uniform, loop-header, */
+
+      nir_push_if(nb, nir_unit_test_uniform_amd(nb, 1, 1, .base = 0));
+      {
+         //>> BB2
+         //! /* logical preds: BB1, / linear preds: BB1, / kind: continue, discard, */
+         nir_terminate_if(nb, nir_unit_test_uniform_amd(nb, 1, 1, .base = 1));
+         nir_jump(nb, nir_jump_continue);
+      }
+      nir_pop_if(nb, NULL);
+
+      //>> BB6
+      //! /* logical preds: BB5, / linear preds: BB4, BB5, / kind: uniform, break, */
+      nir_jump(nb, nir_jump_break);
+   }
+   nir_pop_loop(nb, NULL);
+
+   finish_isel_test();
+END_TEST