r600/sfn: simplify if clauses with empty then branch

nir_opt_if doesn't catch all the possible cases of empty then branches,
so resolve this on the fly when creating the backend IR.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20141>

src/gallium/drivers/r600/sfn/sfn_shader.cpp

@@ -693,36 +693,12 @@ Shader::process_if(nir_if *if_stmt)
 {
    SFN_TRACE_FUNC(SfnLog::flow, "IF");
 
-   if (!emit_if_start(if_stmt))
-      return false;
-
-   foreach_list_typed(nir_cf_node, n, node, &if_stmt->then_list)
-   {
-      SFN_TRACE_FUNC(SfnLog::flow, "IF-then");
-      if (!process_cf_node(n))
-         return false;
-   }
-
-   if (!child_block_empty(if_stmt->else_list)) {
-      if (!emit_control_flow(ControlFlowInstr::cf_else))
-         return false;
-      foreach_list_typed(nir_cf_node,
-                         n,
-                         node,
-                         &if_stmt->else_list) if (!process_cf_node(n)) return false;
-   }
-
-   if (!emit_control_flow(ControlFlowInstr::cf_endif))
-      return false;
-
-   return true;
-}
-
-bool
-Shader::emit_if_start(nir_if *if_stmt)
-{
    auto value = value_factory().src(if_stmt->condition, 0);
-   AluInstr *pred = new AluInstr(op2_pred_setne_int,
+
+   EAluOp op = child_block_empty(if_stmt->then_list) ? op2_prede_int :
+                                                       op2_pred_setne_int;
+
+   AluInstr *pred = new AluInstr(op,
                                  value_factory().temp_register(),
                                  value,
                                  value_factory().zero(),
@@ -734,6 +710,35 @@ Shader::emit_if_start(nir_if *if_stmt)
    IfInstr *ir = new IfInstr(pred);
    emit_instruction(ir);
    start_new_block(1);
+
+   if (!child_block_empty(if_stmt->then_list)) {
+      foreach_list_typed(nir_cf_node, n, node, &if_stmt->then_list)
+      {
+         SFN_TRACE_FUNC(SfnLog::flow, "IF-then");
+         if (!process_cf_node(n))
+            return false;
+      }
+      if (!child_block_empty(if_stmt->else_list)) {
+         if (!emit_control_flow(ControlFlowInstr::cf_else))
+            return false;
+         foreach_list_typed(nir_cf_node,
+                            n,
+                            node,
+                            &if_stmt->else_list)
+               if (!process_cf_node(n)) return false;
+      }
+   } else {
+      assert(!child_block_empty(if_stmt->else_list));
+      foreach_list_typed(nir_cf_node,
+                         n,
+                         node,
+                         &if_stmt->else_list)
+            if (!process_cf_node(n)) return false;
+   }
+
+   if (!emit_control_flow(ControlFlowInstr::cf_endif))
+      return false;
+
    return true;
 }
 

src/gallium/drivers/r600/sfn/sfn_shader.h

@@ -303,7 +303,6 @@ private:
    bool read_input(std::istream& is);
    virtual bool read_prop(std::istream& is) = 0;
 
-   bool emit_if_start(nir_if *if_stmt);
    bool emit_control_flow(ControlFlowInstr::CFType type);
    bool emit_store_scratch(nir_intrinsic_instr *intr);
    bool emit_load_scratch(nir_intrinsic_instr *intr);