diff --git a/libs/vkd3d-shader/hlsl.c b/libs/vkd3d-shader/hlsl.c
index 01be78571..dc9886b53 100644
--- a/libs/vkd3d-shader/hlsl.c
+++ b/libs/vkd3d-shader/hlsl.c
@@ -1976,14 +1976,15 @@ static struct hlsl_ir_node *hlsl_new_error_expr(struct hlsl_ctx *ctx)
     return hlsl_new_expr(ctx, HLSL_OP0_ERROR, operands, ctx->builtin_types.error, &loc);
 }
 
-struct hlsl_ir_node *hlsl_new_if(struct hlsl_ctx *ctx, struct hlsl_ir_node *condition,
-        struct hlsl_block *then_block, struct hlsl_block *else_block, const struct vkd3d_shader_location *loc)
+struct hlsl_ir_node *hlsl_new_if(struct hlsl_ctx *ctx, struct hlsl_ir_node *condition, struct hlsl_block *then_block,
+        struct hlsl_block *else_block, enum hlsl_if_flatten_type flatten_type, const struct vkd3d_shader_location *loc)
 {
     struct hlsl_ir_if *iff;
 
     if (!(iff = hlsl_alloc(ctx, sizeof(*iff))))
         return NULL;
     init_node(&iff->node, HLSL_IR_IF, NULL, loc);
+    iff->flatten_type = flatten_type;
     hlsl_src_from_node(&iff->condition, condition);
     hlsl_block_init(&iff->then_block);
     hlsl_block_add_block(&iff->then_block, then_block);
@@ -1993,10 +1994,11 @@ struct hlsl_ir_node *hlsl_new_if(struct hlsl_ctx *ctx, struct hlsl_ir_node *cond
     return &iff->node;
 }
 
-void hlsl_block_add_if(struct hlsl_ctx *ctx, struct hlsl_block *block, struct hlsl_ir_node *condition,
-        struct hlsl_block *then_block, struct hlsl_block *else_block, const struct vkd3d_shader_location *loc)
+void hlsl_block_add_if(struct hlsl_ctx *ctx, struct hlsl_block *block,
+        struct hlsl_ir_node *condition, struct hlsl_block *then_block, struct hlsl_block *else_block,
+        enum hlsl_if_flatten_type flatten_type, const struct vkd3d_shader_location *loc)
 {
-    struct hlsl_ir_node *instr = hlsl_new_if(ctx, condition, then_block, else_block, loc);
+    struct hlsl_ir_node *instr = hlsl_new_if(ctx, condition, then_block, else_block, flatten_type, loc);
 
     if (instr)
     {
@@ -2674,7 +2676,8 @@ static struct hlsl_ir_node *clone_if(struct hlsl_ctx *ctx, struct clone_instr_ma
         return NULL;
     }
 
-    if (!(dst = hlsl_new_if(ctx, map_instr(map, src->condition.node), &then_block, &else_block, &src->node.loc)))
+    if (!(dst = hlsl_new_if(ctx, map_instr(map, src->condition.node),
+            &then_block, &else_block, src->flatten_type, &src->node.loc)))
     {
         hlsl_block_cleanup(&then_block);
         hlsl_block_cleanup(&else_block);
diff --git a/libs/vkd3d-shader/hlsl.h b/libs/vkd3d-shader/hlsl.h
index 4f79f2845..e52cb7e6b 100644
--- a/libs/vkd3d-shader/hlsl.h
+++ b/libs/vkd3d-shader/hlsl.h
@@ -661,12 +661,20 @@ struct hlsl_ir_call
     struct hlsl_ir_function_decl *decl;
 };
 
+enum hlsl_if_flatten_type
+{
+    HLSL_IF_FLATTEN_DEFAULT,
+    HLSL_IF_FORCE_FLATTEN,
+    HLSL_IF_FORCE_BRANCH
+};
+
 struct hlsl_ir_if
 {
     struct hlsl_ir_node node;
     struct hlsl_src condition;
     struct hlsl_block then_block;
     struct hlsl_block else_block;
+    enum hlsl_if_flatten_type flatten_type;
 };
 
 enum hlsl_loop_unroll_type
@@ -1586,8 +1594,9 @@ struct hlsl_ir_node *hlsl_block_add_expr(struct hlsl_ctx *ctx, struct hlsl_block
         struct hlsl_type *data_type, const struct vkd3d_shader_location *loc);
 struct hlsl_ir_node *hlsl_block_add_float_constant(struct hlsl_ctx *ctx, struct hlsl_block *block,
         float f, const struct vkd3d_shader_location *loc);
-void hlsl_block_add_if(struct hlsl_ctx *ctx, struct hlsl_block *block, struct hlsl_ir_node *condition,
-        struct hlsl_block *then_block, struct hlsl_block *else_block, const struct vkd3d_shader_location *loc);
+void hlsl_block_add_if(struct hlsl_ctx *ctx, struct hlsl_block *block,
+        struct hlsl_ir_node *condition, struct hlsl_block *then_block, struct hlsl_block *else_block,
+        enum hlsl_if_flatten_type flatten_type, const struct vkd3d_shader_location *loc);
 struct hlsl_ir_node *hlsl_block_add_index(struct hlsl_ctx *ctx, struct hlsl_block *block,
         struct hlsl_ir_node *val, struct hlsl_ir_node *idx, const struct vkd3d_shader_location *loc);
 struct hlsl_ir_node *hlsl_block_add_int_constant(struct hlsl_ctx *ctx, struct hlsl_block *block,
@@ -1704,8 +1713,8 @@ struct hlsl_ir_node *hlsl_new_copy(struct hlsl_ctx *ctx, struct hlsl_ir_node *no
 struct hlsl_ir_function_decl *hlsl_new_func_decl(struct hlsl_ctx *ctx,
         struct hlsl_type *return_type, const struct hlsl_func_parameters *parameters,
         const struct hlsl_semantic *semantic, const struct vkd3d_shader_location *loc);
-struct hlsl_ir_node *hlsl_new_if(struct hlsl_ctx *ctx, struct hlsl_ir_node *condition,
-        struct hlsl_block *then_block, struct hlsl_block *else_block, const struct vkd3d_shader_location *loc);
+struct hlsl_ir_node *hlsl_new_if(struct hlsl_ctx *ctx, struct hlsl_ir_node *condition, struct hlsl_block *then_block,
+        struct hlsl_block *else_block, enum hlsl_if_flatten_type flatten_type, const struct vkd3d_shader_location *loc);
 struct hlsl_type *hlsl_new_stream_output_type(struct hlsl_ctx *ctx,
         enum hlsl_so_object_type so_type, struct hlsl_type *type);
 struct hlsl_ir_node *hlsl_new_ternary_expr(struct hlsl_ctx *ctx, enum hlsl_ir_expr_op op,
diff --git a/libs/vkd3d-shader/hlsl.y b/libs/vkd3d-shader/hlsl.y
index 8ec963c86..92bfd7040 100644
--- a/libs/vkd3d-shader/hlsl.y
+++ b/libs/vkd3d-shader/hlsl.y
@@ -471,7 +471,7 @@ static void append_conditional_break(struct hlsl_ctx *ctx, struct hlsl_block *co
 
     hlsl_block_init(&then_block);
     hlsl_block_add_jump(ctx, &then_block, HLSL_IR_JUMP_BREAK, NULL, &condition->loc);
-    hlsl_block_add_if(ctx, cond_block, not, &then_block, NULL, &condition->loc);
+    hlsl_block_add_if(ctx, cond_block, not, &then_block, NULL, HLSL_IF_FLATTEN_DEFAULT, &condition->loc);
 }
 
 static void check_attribute_list_for_duplicates(struct hlsl_ctx *ctx, const struct parse_attribute_list *attrs)
@@ -9139,6 +9139,7 @@ selection_statement:
         {
             struct hlsl_ir_node *condition = node_from_block($4);
             const struct parse_attribute_list *attributes = &$1;
+            enum hlsl_if_flatten_type flatten_type = HLSL_IF_FLATTEN_DEFAULT;
             unsigned int i;
 
             check_attribute_list_for_duplicates(ctx, attributes);
@@ -9147,10 +9148,19 @@ selection_statement:
             {
                 const struct hlsl_attribute *attr = attributes->attrs[i];
 
-                if (!strcmp(attr->name, "branch")
-                        || !strcmp(attr->name, "flatten"))
+                if (!strcmp(attr->name, "branch"))
                 {
-                    hlsl_warning(ctx, &@1, VKD3D_SHADER_WARNING_HLSL_IGNORED_ATTRIBUTE, "Unhandled attribute '%s'.", attr->name);
+                    if (flatten_type == HLSL_IF_FORCE_FLATTEN)
+                        hlsl_error(ctx, &@1, VKD3D_SHADER_ERROR_HLSL_INVALID_SYNTAX,
+                                "The 'branch' and 'flatten' attributes are mutually exclusive.");
+                    flatten_type = HLSL_IF_FORCE_BRANCH;
+                }
+                else if (!strcmp(attr->name, "flatten"))
+                {
+                    if (flatten_type == HLSL_IF_FORCE_BRANCH)
+                        hlsl_error(ctx, &@1, VKD3D_SHADER_ERROR_HLSL_INVALID_SYNTAX,
+                                "The 'branch' and 'flatten' attributes are mutually exclusive.");
+                    flatten_type = HLSL_IF_FORCE_FLATTEN;
                 }
                 else
                 {
@@ -9158,10 +9168,16 @@ selection_statement:
                 }
             }
 
+            if (flatten_type == HLSL_IF_FORCE_BRANCH && hlsl_version_lt(ctx, 2, 1))
+            {
+                hlsl_error(ctx, &@1, VKD3D_SHADER_ERROR_HLSL_INCOMPATIBLE_PROFILE,
+                        "The 'branch' attribute requires shader model 2.1 or higher.");
+            }
+
             check_condition_type(ctx, condition);
 
             condition = add_cast(ctx, $4, condition, hlsl_get_scalar_type(ctx, HLSL_TYPE_BOOL), &@4);
-            hlsl_block_add_if(ctx, $4, condition, $6.then_block, $6.else_block, &@2);
+            hlsl_block_add_if(ctx, $4, condition, $6.then_block, $6.else_block, flatten_type, &@2);
 
             destroy_block($6.then_block);
             destroy_block($6.else_block);
diff --git a/libs/vkd3d-shader/hlsl_codegen.c b/libs/vkd3d-shader/hlsl_codegen.c
index 51dd9f80f..ef0a8a802 100644
--- a/libs/vkd3d-shader/hlsl_codegen.c
+++ b/libs/vkd3d-shader/hlsl_codegen.c
@@ -1060,7 +1060,7 @@ static void insert_early_return_break(struct hlsl_ctx *ctx,
 
     hlsl_block_add_jump(ctx, &then_block, HLSL_IR_JUMP_BREAK, NULL, &cf_instr->loc);
 
-    if (!(iff = hlsl_new_if(ctx, &load->node, &then_block, NULL, &cf_instr->loc)))
+    if (!(iff = hlsl_new_if(ctx, &load->node, &then_block, NULL, HLSL_IF_FLATTEN_DEFAULT, &cf_instr->loc)))
         return;
     list_add_after(&load->node.entry, &iff->entry);
 }
@@ -1245,7 +1245,7 @@ static bool lower_return(struct hlsl_ctx *ctx, struct hlsl_ir_function_decl *fun
 
         load = hlsl_block_add_simple_load(ctx, block, func->early_return_var, &cf_instr->loc);
         not = hlsl_block_add_unary_expr(ctx, block, HLSL_OP1_LOGIC_NOT, load, &cf_instr->loc);
-        hlsl_block_add_if(ctx, block, not, &then_block, NULL, &cf_instr->loc);
+        hlsl_block_add_if(ctx, block, not, &then_block, NULL, HLSL_IF_FLATTEN_DEFAULT, &cf_instr->loc);
     }
 
     return has_early_return;
@@ -13866,7 +13866,7 @@ static struct hlsl_ir_if *loop_unrolling_generate_var_check(struct hlsl_ctx *ctx
     load = hlsl_block_add_simple_load(ctx, dst, var, loc);
     cond = hlsl_block_add_unary_expr(ctx, dst, HLSL_OP1_LOGIC_NOT, load, loc);
 
-    if (!(iff = hlsl_new_if(ctx, cond, &then_block, NULL, loc)))
+    if (!(iff = hlsl_new_if(ctx, cond, &then_block, NULL, HLSL_IF_FLATTEN_DEFAULT, loc)))
         return NULL;
     hlsl_block_add_instr(dst, iff);
 
diff --git a/tests/hlsl/conditional.shader_test b/tests/hlsl/conditional.shader_test
index 607fcb065..1f9d5b5db 100644
--- a/tests/hlsl/conditional.shader_test
+++ b/tests/hlsl/conditional.shader_test
@@ -58,7 +58,7 @@ float main() : sv_target
         return float4(0.9, 0.8, 0.7, 0.6);
 }
 
-[pixel shader fail todo]
+[pixel shader fail]
 float4 u;
 
 float main() : sv_target