nak: Fix divergence test for redux availability

nak's divergence differs slightly from nir's divergence. Fix the test to
match what the backend will use, since we need to allocate a ureg.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/13964
Fixes: 295373f29f ("nak: Implement nir_intrinsic_reduce with REDUX")
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37585>

src/nouveau/compiler/nak_nir_lower_cf.c

@@ -498,3 +498,36 @@ nak_nir_lower_cf(nir_shader *nir)
 
    return progress;
 }
+
+bool
+nak_block_is_divergent(const nir_block *block)
+{
+   const nir_cf_node* node = &block->cf_node;
+
+   while (node) {
+      switch (node->type) {
+         case nir_cf_node_if: {
+            nir_if *nif = nir_cf_node_as_if(node);
+            if (nir_src_is_divergent(&nif->condition))
+               return true;
+            break;
+         }
+         case nir_cf_node_loop: {
+            nir_loop *loop = nir_cf_node_as_loop(node);
+            if (nir_loop_is_divergent(loop))
+               return true;
+            break;
+         }
+         case nir_cf_node_block: {
+            nir_block *block = nir_cf_node_as_block(node);
+            if (block->divergent)
+               return true;
+            break;
+         }
+         case nir_cf_node_function:
+            break;
+      }
+      node = node->parent;
+   }
+   return false;
+}

src/nouveau/compiler/nak_nir_lower_scan_reduce.c

@@ -229,7 +229,7 @@ nak_nir_lower_scan_reduce_intrin(nir_builder *b,
               nir_op_infos[red_op].output_type != nir_type_float &&
               intrin->src[0].ssa->bit_size == 32 &&
               cluster_size == 32 &&
-              !intrin->instr.block->divergent) {
+              !nak_block_is_divergent(intrin->instr.block)) {
       /* TODO: We could probably also use REDUX for the non-uniform case if we
        *       were allowed to write uregs from non-uniform control flow.
        */

src/nouveau/compiler/nak_private.h

@@ -343,6 +343,16 @@ bool nak_nir_add_barriers(nir_shader *nir, const struct nak_compiler *nak);
 bool nak_nir_lower_cf(nir_shader *nir);
 bool nak_nir_lower_cmat(nir_shader *shader, const struct nak_compiler *nak);
 
+/**
+ * Check if, for nak's purposes, a block is divergent
+ *
+ * Note that this differs from block->divergent because in nir's terms, the
+ * start block of a loop with divergent continues can be non-divergent but nak
+ * will always consider this divergent. This matters because nak does not allow
+ * writing uregs from these blocks.
+ */
+bool nak_block_is_divergent(const nir_block *block);
+
 void nak_optimize_nir(nir_shader *nir, const struct nak_compiler *nak);
 
 #ifdef __cplusplus