Added a codegen parameter for whether fasttables are generated or not.

Example:

$ CC=clang bazel build -c opt --copt=-g benchmarks:benchmark --//:fasttable_enabled=false
INFO: Build option --//:fasttable_enabled has changed, discarding analysis cache.
INFO: Analyzed target //benchmarks:benchmark (0 packages loaded, 913 targets configured).
INFO: Found 1 target...
Target //benchmarks:benchmark up-to-date:
  bazel-bin/benchmarks/benchmark
INFO: Elapsed time: 0.760s, Critical Path: 0.58s
INFO: 7 processes: 1 internal, 6 linux-sandbox.
INFO: Build completed successfully, 7 total actions
$ bazel-bin/benchmarks/benchmark --benchmark_filter=BM_Parse_Upb
------------------------------------------------------------------------------
Benchmark                                       Time           CPU Iterations
------------------------------------------------------------------------------
BM_Parse_Upb_FileDesc_WithArena             10985 ns      10984 ns      63567   651.857MB/s
BM_Parse_Upb_FileDesc_WithInitialBlock      10556 ns      10554 ns      66138   678.458MB/s
$ CC=clang bazel build -c opt --copt=-g benchmarks:benchmark --//:fasttable_enabled=true
INFO: Build option --//:fasttable_enabled has changed, discarding analysis cache.
INFO: Analyzed target //benchmarks:benchmark (0 packages loaded, 913 targets configured).
INFO: Found 1 target...
Target //benchmarks:benchmark up-to-date:
  bazel-bin/benchmarks/benchmark
INFO: Elapsed time: 0.744s, Critical Path: 0.58s
INFO: 7 processes: 1 internal, 6 linux-sandbox.
INFO: Build completed successfully, 7 total actions
$ bazel-bin/benchmarks/benchmark --benchmark_filter=BM_Parse_Upb
------------------------------------------------------------------------------
Benchmark                                       Time           CPU Iterations
------------------------------------------------------------------------------
BM_Parse_Upb_FileDesc_WithArena              3284 ns       3284 ns     213495    2.1293GB/s
BM_Parse_Upb_FileDesc_WithInitialBlock       2882 ns       2882 ns     243069    2.4262GB/s

Biggest unknown is whether this parameter should default to true or false.

BUILD

@@ -5,6 +5,7 @@ load(
 )
 load(
     "//bazel:upb_proto_library.bzl",
+    "upb_fasttable_enabled",
     "upb_proto_library",
     "upb_proto_reflection_library",
 )
@@ -35,6 +36,12 @@ config_setting(
     constraint_values = ["@bazel_tools//platforms:windows"],
 )
 
+upb_fasttable_enabled(
+    name = "fasttable_enabled",
+    build_setting_default = True,
+    visibility = ["//visibility:public"],
+)
+
 # Public C/C++ libraries #######################################################
 
 cc_library(

bazel/upb_proto_library.bzl

@@ -106,6 +106,28 @@ def _cc_library_func(ctx, name, hdrs, srcs, dep_ccinfos):
         linking_context = linking_context,
     )
 
+# Build setting for whether fasttable code generation is enabled ###############
+
+_FastTableEnabled = provider(
+    fields = {
+        "enabled": "whether fasttable is enabled",
+    },
+)
+
+def fasttable_enabled_impl(ctx):
+    raw_setting = ctx.build_setting_value
+
+    if raw_setting:
+      # TODO(haberman): check that the target CPU supports fasttable.
+      pass
+
+    return _FastTableEnabled(enabled = raw_setting)
+
+upb_fasttable_enabled = rule(
+    implementation = fasttable_enabled_impl,
+    build_setting = config.bool(flag = True)
+)
+
 # upb_proto_library / upb_proto_reflection_library shared code #################
 
 GeneratedSrcsInfo = provider(
@@ -127,6 +149,8 @@ def _compile_upb_protos(ctx, proto_info, proto_sources, ext):
     srcs = [_generate_output_file(ctx, name, ext + ".c") for name in proto_sources]
     hdrs = [_generate_output_file(ctx, name, ext + ".h") for name in proto_sources]
     transitive_sets = proto_info.transitive_descriptor_sets.to_list()
+    fasttable_enabled = ctx.attr._fasttable_enabled[_FastTableEnabled].enabled
+    codegen_params = "fasttable:" if fasttable_enabled else ""
     ctx.actions.run(
         inputs = depset(
             direct = [proto_info.direct_descriptor_set],
@@ -136,7 +160,7 @@ def _compile_upb_protos(ctx, proto_info, proto_sources, ext):
         outputs = srcs + hdrs,
         executable = ctx.executable._protoc,
         arguments = [
-                        "--upb_out=" + _get_real_root(srcs[0]),
+                        "--upb_out=" + codegen_params + _get_real_root(srcs[0]),
                         "--plugin=protoc-gen-upb=" + ctx.executable._upbc.path,
                         "--descriptor_set_in=" + ctx.configuration.host_path_separator.join([f.path for f in transitive_sets]),
                     ] +
@@ -240,6 +264,7 @@ _upb_proto_library_aspect = aspect(
             "//:upb",
         ]),
         "_ext": attr.string(default = ".upb"),
+        "_fasttable_enabled": attr.label(default = "//:fasttable_enabled")
     }),
     implementation = _upb_proto_library_aspect_impl,
     provides = [
@@ -295,6 +320,7 @@ _upb_proto_reflection_library_aspect = aspect(
             ],
         ),
         "_ext": attr.string(default = ".upbdefs"),
+        "_fasttable_enabled": attr.label(default = "//:fasttable_enabled")
     }),
     implementation = _upb_proto_reflection_library_aspect_impl,
     provides = [

cmake/make_cmakelists.py

@@ -141,6 +141,9 @@ class BuildFileFunctions(object):
   def config_setting(self, **kwargs):
     pass
 
+  def upb_fasttable_enabled(self, **kwargs):
+    pass
+
   def select(self, arg_dict):
     return []
 

upbc/generator.cc

@@ -883,7 +883,8 @@ std::vector<TableEntry> FastDecodeTable(const protobuf::Descriptor* message,
   return table;
 }
 
-void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
+void WriteSource(const protobuf::FileDescriptor* file, Output& output,
+                 bool fasttable_enabled) {
   EmitFileWarning(file, output);
 
   output(
@@ -975,7 +976,13 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
       output("};\n\n");
     }
 
-    std::vector<TableEntry> table = FastDecodeTable(message, layout);
+    std::vector<TableEntry> table;
+    uint8_t table_mask = -1;
+
+    if (fasttable_enabled) {
+      table = FastDecodeTable(message, layout);
+      table_mask = (table.size() - 1) << 3;
+    }
 
     output("const upb_msglayout $0 = {\n", MessageInit(message));
     output("  $0,\n", submsgs_array_ref);
@@ -983,7 +990,7 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
     output("  $0, $1, $2, $3,\n", GetSizeInit(layout.message_size()),
            field_number_order.size(),
            "false",  // TODO: extendable
-           (table.size() - 1) << 3
+           table_mask
     );
     output("  {\n");
     for (const auto& ent : table) {
@@ -1120,14 +1127,15 @@ void WriteDefSource(const protobuf::FileDescriptor* file, Output& output) {
 }
 
 bool Generator::Generate(const protobuf::FileDescriptor* file,
-                         const std::string& /* parameter */,
+                         const std::string& parameter,
                          protoc::GeneratorContext* context,
                          std::string* /* error */) const {
+  bool fasttable_enabled = parameter == "fasttable";
   Output h_output(context->Open(HeaderFilename(file->name())));
   WriteHeader(file, h_output);
 
   Output c_output(context->Open(SourceFilename(file->name())));
-  WriteSource(file, c_output);
+  WriteSource(file, c_output, fasttable_enabled);
 
   Output h_def_output(context->Open(DefHeaderFilename(file->name())));
   WriteDefHeader(file, h_def_output);