[channel_args] Size optimizations (#33901)

- Make `Value` a simple wrapper around `Pointer` and use some blessed
vtables to distinguish strings vs ints vs actual pointers - this saves 8
bytes per value stored
- introduce `RcString` as a lightweight container around an immutable
string - this saves some bytes vs the shared_ptr<std::string> approach
we previously had, and importantly opens up the technique (via
`RcStringValue`) to channel node keys also, which should increase
sharing and consequently also decrease total memory usage

---------

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

build_autogenerated.yaml

@@ -7076,9 +7076,7 @@ targets:
   - src/core/lib/event_engine/channel_args_endpoint_config.h
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/dual_ref_counted.h
-  - src/core/lib/gprpp/match.h
   - src/core/lib/gprpp/orphanable.h
-  - src/core/lib/gprpp/overload.h
   - src/core/lib/gprpp/ref_counted.h
   - src/core/lib/gprpp/ref_counted_ptr.h
   - src/core/lib/gprpp/time.h

src/core/BUILD

@@ -2624,7 +2624,6 @@ grpc_cc_library(
         "absl/strings",
         "absl/strings:str_format",
         "absl/types:optional",
-        "absl/types:variant",
     ],
     language = "c++",
     visibility = [
@@ -2634,7 +2633,6 @@ grpc_cc_library(
         "avl",
         "channel_stack_type",
         "dual_ref_counted",
-        "match",
         "ref_counted",
         "time",
         "useful",

src/core/lib/channel/channel_args.cc

@@ -27,6 +27,9 @@
 #include <algorithm>
 #include <initializer_list>
 #include <map>
+#include <memory>
+#include <new>
+#include <string>
 #include <vector>
 
 #include "absl/strings/match.h"
@@ -40,11 +43,47 @@
 #include <grpc/support/string_util.h>
 
 #include "src/core/lib/gpr/useful.h"
-#include "src/core/lib/gprpp/crash.h"
-#include "src/core/lib/gprpp/match.h"
 
 namespace grpc_core {
 
+RefCountedPtr<RcString> RcString::Make(absl::string_view src) {
+  void* p = gpr_malloc(sizeof(Header) + src.length() + 1);
+  return RefCountedPtr<RcString>(new (p) RcString(src));
+}
+
+RcString::RcString(absl::string_view src) : header_{{}, src.length()} {
+  memcpy(payload_, src.data(), header_.length);
+  // Null terminate because we frequently need to convert to char* still to go
+  // back and forth to the old c-style api.
+  payload_[header_.length] = 0;
+}
+
+void RcString::Destroy() { gpr_free(this); }
+
+const grpc_arg_pointer_vtable ChannelArgs::Value::int_vtable_{
+    // copy
+    [](void* p) { return p; },
+    // destroy
+    [](void*) {},
+    // cmp
+    [](void* p1, void* p2) -> int {
+      return QsortCompare(reinterpret_cast<intptr_t>(p1),
+                          reinterpret_cast<intptr_t>(p2));
+    },
+};
+
+const grpc_arg_pointer_vtable ChannelArgs::Value::string_vtable_{
+    // copy
+    [](void* p) -> void* { return static_cast<RcString*>(p)->Ref().release(); },
+    // destroy
+    [](void* p) { static_cast<RcString*>(p)->Unref(); },
+    // cmp
+    [](void* p1, void* p2) -> int {
+      return QsortCompare(static_cast<RcString*>(p1)->as_string_view(),
+                          static_cast<RcString*>(p2)->as_string_view());
+    },
+};
+
 ChannelArgs::Pointer::Pointer(void* p, const grpc_arg_pointer_vtable* vtable)
     : p_(p), vtable_(vtable == nullptr ? EmptyVTable() : vtable) {}
 
@@ -100,7 +139,7 @@ bool ChannelArgs::WantMinimalStack() const {
   return GetBool(GRPC_ARG_MINIMAL_STACK).value_or(false);
 }
 
-ChannelArgs::ChannelArgs(AVL<std::string, Value> args)
+ChannelArgs::ChannelArgs(AVL<RcStringValue, Value> args)
     : args_(std::move(args)) {}
 
 ChannelArgs ChannelArgs::Set(grpc_arg arg) const {
@@ -130,52 +169,22 @@ ChannelArgs ChannelArgs::FromC(const grpc_channel_args* args) {
 
 grpc_arg ChannelArgs::Value::MakeCArg(const char* name) const {
   char* c_name = const_cast<char*>(name);
-  return Match(
-      rep_,
-      [c_name](int i) { return grpc_channel_arg_integer_create(c_name, i); },
-      [c_name](const std::shared_ptr<const std::string>& s) {
-        return grpc_channel_arg_string_create(c_name,
-                                              const_cast<char*>(s->c_str()));
-      },
-      [c_name](const Pointer& p) {
-        return grpc_channel_arg_pointer_create(c_name, p.c_pointer(),
-                                               p.c_vtable());
-      });
+  if (rep_.c_vtable() == &int_vtable_) {
+    return grpc_channel_arg_integer_create(
+        c_name, reinterpret_cast<intptr_t>(rep_.c_pointer()));
   }
-
-bool ChannelArgs::Value::operator<(const Value& rhs) const {
-  if (rhs.rep_.index() != rep_.index()) return rep_.index() < rhs.rep_.index();
-  switch (rep_.index()) {
-    case 0:
-      return absl::get<int>(rep_) < absl::get<int>(rhs.rep_);
-    case 1:
-      return *absl::get<std::shared_ptr<const std::string>>(rep_) <
-             *absl::get<std::shared_ptr<const std::string>>(rhs.rep_);
-    case 2:
-      return absl::get<Pointer>(rep_) < absl::get<Pointer>(rhs.rep_);
-    default:
-      Crash("unreachable");
-  }
-}
-
-bool ChannelArgs::Value::operator==(const Value& rhs) const {
-  if (rhs.rep_.index() != rep_.index()) return false;
-  switch (rep_.index()) {
-    case 0:
-      return absl::get<int>(rep_) == absl::get<int>(rhs.rep_);
-    case 1:
-      return *absl::get<std::shared_ptr<const std::string>>(rep_) ==
-             *absl::get<std::shared_ptr<const std::string>>(rhs.rep_);
-    case 2:
-      return absl::get<Pointer>(rep_) == absl::get<Pointer>(rhs.rep_);
-    default:
-      Crash("unreachable");
+  if (rep_.c_vtable() == &string_vtable_) {
+    return grpc_channel_arg_string_create(
+        c_name,
+        const_cast<char*>(static_cast<RcString*>(rep_.c_pointer())->c_str()));
   }
+  return grpc_channel_arg_pointer_create(c_name, rep_.c_pointer(),
+                                         rep_.c_vtable());
 }
 
 ChannelArgs::CPtr ChannelArgs::ToC() const {
   std::vector<grpc_arg> c_args;
-  args_.ForEach([&c_args](const std::string& key, const Value& value) {
+  args_.ForEach([&c_args](const RcStringValue& key, const Value& value) {
     c_args.push_back(value.MakeCArg(key.c_str()));
   });
   return CPtr(static_cast<const grpc_channel_args*>(
@@ -194,7 +203,7 @@ ChannelArgs ChannelArgs::Set(absl::string_view name, Value value) const {
   if (const auto* p = args_.Lookup(name)) {
     if (*p == value) return *this;  // already have this value for this key
   }
-  return ChannelArgs(args_.Add(std::string(name), std::move(value)));
+  return ChannelArgs(args_.Add(RcStringValue(name), std::move(value)));
 }
 
 ChannelArgs ChannelArgs::Set(absl::string_view name,
@@ -218,8 +227,8 @@ ChannelArgs ChannelArgs::Remove(absl::string_view name) const {
 ChannelArgs ChannelArgs::RemoveAllKeysWithPrefix(
     absl::string_view prefix) const {
   auto args = args_;
-  args_.ForEach([&args, prefix](const std::string& key, const Value&) {
-    if (absl::StartsWith(key, prefix)) args = args.Remove(key);
+  args_.ForEach([&](const RcStringValue& key, const Value&) {
+    if (absl::StartsWith(key.as_string_view(), prefix)) args = args.Remove(key);
   });
   return ChannelArgs(std::move(args));
 }
@@ -227,9 +236,7 @@ ChannelArgs ChannelArgs::RemoveAllKeysWithPrefix(
 absl::optional<int> ChannelArgs::GetInt(absl::string_view name) const {
   auto* v = Get(name);
   if (v == nullptr) return absl::nullopt;
-  const auto* i = v->GetIfInt();
-  if (i == nullptr) return absl::nullopt;
-  return *i;
+  return v->GetIfInt();
 }
 
 absl::optional<Duration> ChannelArgs::GetDurationFromIntMillis(
@@ -245,9 +252,9 @@ absl::optional<absl::string_view> ChannelArgs::GetString(
     absl::string_view name) const {
   auto* v = Get(name);
   if (v == nullptr) return absl::nullopt;
-  const auto* s = v->GetIfString();
+  const auto s = v->GetIfString();
   if (s == nullptr) return absl::nullopt;
-  return *s;
+  return s->as_string_view();
 }
 
 absl::optional<std::string> ChannelArgs::GetOwnedString(
@@ -268,8 +275,8 @@ void* ChannelArgs::GetVoidPointer(absl::string_view name) const {
 absl::optional<bool> ChannelArgs::GetBool(absl::string_view name) const {
   auto* v = Get(name);
   if (v == nullptr) return absl::nullopt;
-  auto* i = v->GetIfInt();
-  if (i == nullptr) {
+  auto i = v->GetIfInt();
+  if (!i.has_value()) {
     gpr_log(GPR_ERROR, "%s ignored: it must be an integer",
             std::string(name).c_str());
     return absl::nullopt;
@@ -286,18 +293,22 @@ absl::optional<bool> ChannelArgs::GetBool(absl::string_view name) const {
   }
 }
 
+std::string ChannelArgs::Value::ToString() const {
+  if (rep_.c_vtable() == &int_vtable_) {
+    return std::to_string(reinterpret_cast<intptr_t>(rep_.c_pointer()));
+  }
+  if (rep_.c_vtable() == &string_vtable_) {
+    return std::string(
+        static_cast<RcString*>(rep_.c_pointer())->as_string_view());
+  }
+  return absl::StrFormat("%p", rep_.c_pointer());
+}
+
 std::string ChannelArgs::ToString() const {
   std::vector<std::string> arg_strings;
-  args_.ForEach([&arg_strings](const std::string& key, const Value& value) {
-    std::string value_str;
-    if (auto* i = value.GetIfInt()) {
-      value_str = std::to_string(*i);
-    } else if (auto* s = value.GetIfString()) {
-      value_str = *s;
-    } else if (auto* p = value.GetIfPointer()) {
-      value_str = absl::StrFormat("%p", p->c_pointer());
-    }
-    arg_strings.push_back(absl::StrCat(key, "=", value_str));
+  args_.ForEach([&arg_strings](const RcStringValue& key, const Value& value) {
+    arg_strings.push_back(
+        absl::StrCat(key.as_string_view(), "=", value.ToString()));
   });
   return absl::StrCat("{", absl::StrJoin(arg_strings, ", "), "}");
 }
@@ -306,13 +317,14 @@ ChannelArgs ChannelArgs::UnionWith(ChannelArgs other) const {
   if (args_.Empty()) return other;
   if (other.args_.Empty()) return *this;
   if (args_.Height() <= other.args_.Height()) {
-    args_.ForEach([&other](const std::string& key, const Value& value) {
+    args_.ForEach([&other](const RcStringValue& key, const Value& value) {
       other.args_ = other.args_.Add(key, value);
     });
     return other;
   } else {
     auto result = *this;
-    other.args_.ForEach([&result](const std::string& key, const Value& value) {
+    other.args_.ForEach(
+        [&result](const RcStringValue& key, const Value& value) {
           if (result.args_.Lookup(key) == nullptr) {
             result.args_ = result.args_.Add(key, value);
           }
@@ -323,7 +335,7 @@ ChannelArgs ChannelArgs::UnionWith(ChannelArgs other) const {
 
 ChannelArgs ChannelArgs::FuzzingReferenceUnionWith(ChannelArgs other) const {
   // DO NOT OPTIMIZE THIS!!
-  args_.ForEach([&other](const std::string& key, const Value& value) {
+  args_.ForEach([&other](const RcStringValue& key, const Value& value) {
     other.args_ = other.args_.Add(key, value);
   });
   return other;

src/core/lib/channel/channel_args.h

@@ -22,6 +22,7 @@
 #include <grpc/support/port_platform.h>
 
 #include <stddef.h>
+#include <stdint.h>
 
 #include <algorithm>  // IWYU pragma: keep
 #include <iosfwd>
@@ -33,7 +34,6 @@
 #include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
-#include "absl/types/variant.h"
 
 #include <grpc/event_engine/event_engine.h>
 #include <grpc/grpc.h>
@@ -220,6 +220,91 @@ struct GetObjectImpl<T, absl::enable_if_t<!WrapInSharedPtr<T>::value, void>> {
   };
 };
 
+// Immutable reference counted string
+class RcString {
+ public:
+  static RefCountedPtr<RcString> Make(absl::string_view src);
+
+  RefCountedPtr<RcString> Ref() {
+    IncrementRefCount();
+    return RefCountedPtr<RcString>(this);
+  }
+  void IncrementRefCount() { header_.rc.Ref(); }
+  void Unref() {
+    if (header_.rc.Unref()) Destroy();
+  }
+
+  absl::string_view as_string_view() const {
+    return absl::string_view(payload_, header_.length);
+  }
+
+  char* c_str() { return payload_; }
+
+ private:
+  explicit RcString(absl::string_view src);
+  void Destroy();
+
+  struct Header {
+    RefCount rc;
+    size_t length;
+  };
+  Header header_;
+  char payload_[];
+};
+
+// Wrapper around RefCountedPtr<RcString> to give value semantics, especially to
+// overloaded operators.
+class RcStringValue {
+ public:
+  RcStringValue() : str_{} {}
+  explicit RcStringValue(absl::string_view str) : str_(RcString::Make(str)) {}
+
+  absl::string_view as_string_view() const {
+    return str_ == nullptr ? absl::string_view() : str_->as_string_view();
+  }
+
+  const char* c_str() const { return str_ == nullptr ? "" : str_->c_str(); }
+
+ private:
+  RefCountedPtr<RcString> str_;
+};
+
+inline bool operator==(const RcStringValue& lhs, absl::string_view rhs) {
+  return lhs.as_string_view() == rhs;
+}
+
+inline bool operator==(absl::string_view lhs, const RcStringValue& rhs) {
+  return lhs == rhs.as_string_view();
+}
+
+inline bool operator==(const RcStringValue& lhs, const RcStringValue& rhs) {
+  return lhs.as_string_view() == rhs.as_string_view();
+}
+
+inline bool operator<(const RcStringValue& lhs, absl::string_view rhs) {
+  return lhs.as_string_view() < rhs;
+}
+
+inline bool operator<(absl::string_view lhs, const RcStringValue& rhs) {
+  return lhs < rhs.as_string_view();
+}
+
+inline bool operator<(const RcStringValue& lhs, const RcStringValue& rhs) {
+  return lhs.as_string_view() < rhs.as_string_view();
+}
+
+inline bool operator>(const RcStringValue& lhs, absl::string_view rhs) {
+  return lhs.as_string_view() > rhs;
+}
+
+inline bool operator>(absl::string_view lhs, const RcStringValue& rhs) {
+  return lhs > rhs.as_string_view();
+}
+
+inline bool operator>(const RcStringValue& lhs, const RcStringValue& rhs) {
+  return lhs.as_string_view() > rhs.as_string_view();
+}
+
 // Provide the canonical name for a type's channel arg key
 template <typename T>
 struct ChannelArgNameTraits {
@@ -283,32 +368,43 @@ class ChannelArgs {
 
   class Value {
    public:
-    explicit Value(int n) : rep_(n) {}
+    explicit Value(int n) : rep_(reinterpret_cast<void*>(n), &int_vtable_) {}
     explicit Value(std::string s)
-        : rep_(std::make_shared<const std::string>(std::move(s))) {}
+        : rep_(RcString::Make(s).release(), &string_vtable_) {}
     explicit Value(Pointer p) : rep_(std::move(p)) {}
 
-    const int* GetIfInt() const { return absl::get_if<int>(&rep_); }
-    const std::string* GetIfString() const {
-      auto* p = absl::get_if<std::shared_ptr<const std::string>>(&rep_);
-      if (p == nullptr) return nullptr;
-      return p->get();
+    absl::optional<int> GetIfInt() const {
+      if (rep_.c_vtable() != &int_vtable_) return absl::nullopt;
+      return reinterpret_cast<intptr_t>(rep_.c_pointer());
+    }
+    RefCountedPtr<RcString> GetIfString() const {
+      if (rep_.c_vtable() != &string_vtable_) return nullptr;
+      return static_cast<RcString*>(rep_.c_pointer())->Ref();
+    }
+    const Pointer* GetIfPointer() const {
+      if (rep_.c_vtable() == &int_vtable_) return nullptr;
+      if (rep_.c_vtable() == &string_vtable_) return nullptr;
+      return &rep_;
     }
-    const Pointer* GetIfPointer() const { return absl::get_if<Pointer>(&rep_); }
+
+    std::string ToString() const;
 
     grpc_arg MakeCArg(const char* name) const;
 
-    bool operator<(const Value& rhs) const;
-    bool operator==(const Value& rhs) const;
+    bool operator<(const Value& rhs) const { return rep_ < rhs.rep_; }
+    bool operator==(const Value& rhs) const { return rep_ == rhs.rep_; }
     bool operator!=(const Value& rhs) const { return !this->operator==(rhs); }
     bool operator==(absl::string_view rhs) const {
-      auto* p = absl::get_if<std::shared_ptr<const std::string>>(&rep_);
-      if (p == nullptr) return false;
-      return **p == rhs;
+      auto str = GetIfString();
+      if (str == nullptr) return false;
+      return str->as_string_view() == rhs;
     }
 
    private:
-    absl::variant<int, std::shared_ptr<const std::string>, Pointer> rep_;
+    static const grpc_arg_pointer_vtable int_vtable_;
+    static const grpc_arg_pointer_vtable string_vtable_;
+
+    Pointer rep_;
   };
 
   struct ChannelArgsDeleter {
@@ -462,12 +558,12 @@ class ChannelArgs {
   std::string ToString() const;
 
  private:
-  explicit ChannelArgs(AVL<std::string, Value> args);
+  explicit ChannelArgs(AVL<RcStringValue, Value> args);
 
   GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name,
                                        Value value) const;
 
-  AVL<std::string, Value> args_;
+  AVL<RcStringValue, Value> args_;
 };
 
 std::ostream& operator<<(std::ostream& out, const ChannelArgs& args);

src/core/lib/compression/compression_internal.cc

@@ -32,6 +32,7 @@
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/debug/trace.h"
 #include "src/core/lib/gprpp/crash.h"
+#include "src/core/lib/gprpp/ref_counted_ptr.h"
 #include "src/core/lib/surface/api_trace.h"
 
 namespace grpc_core {
@@ -227,11 +228,13 @@ absl::optional<grpc_compression_algorithm>
 DefaultCompressionAlgorithmFromChannelArgs(const ChannelArgs& args) {
   auto* value = args.Get(GRPC_COMPRESSION_CHANNEL_DEFAULT_ALGORITHM);
   if (value == nullptr) return absl::nullopt;
-  if (auto* p = value->GetIfInt()) {
-    return static_cast<grpc_compression_algorithm>(*p);
+  auto ival = value->GetIfInt();
+  if (ival.has_value()) {
+    return static_cast<grpc_compression_algorithm>(*ival);
   }
-  if (auto* p = value->GetIfString()) {
-    return ParseCompressionAlgorithm(*p);
+  auto sval = value->GetIfString();
+  if (sval != nullptr) {
+    return ParseCompressionAlgorithm(sval->as_string_view());
   }
   return absl::nullopt;
 }

src/core/lib/gprpp/ref_counted.h

@@ -45,12 +45,14 @@ class RefCount {
  public:
   using Value = intptr_t;
 
+  RefCount() : RefCount(1) {}
+
   // `init` is the initial refcount stored in this object.
   //
   // `trace` is a string to be logged with trace events; if null, no
   // trace logging will be done.  Tracing is a no-op in non-debug builds.
   explicit RefCount(
-      Value init = 1,
+      Value init,
       const char*
 #ifndef NDEBUG
           // Leave unnamed if NDEBUG to avoid unused parameter warning