StrCat: do not use intermediate buffer when result fits in SSO.

PiperOrigin-RevId: 557811632
Change-Id: I370fa17d2fb82a1f1ca86f84529bae31b34b18e4

absl/strings/str_cat.h

@@ -89,16 +89,21 @@
 
 #include <algorithm>
 #include <array>
+#include <cassert>
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
+#include <limits>
 #include <string>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
 #include "absl/base/attributes.h"
 #include "absl/base/port.h"
+#include "absl/meta/type_traits.h"
 #include "absl/strings/internal/has_absl_stringify.h"
+#include "absl/strings/internal/resize_uninitialized.h"
 #include "absl/strings/internal/stringify_sink.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/string_view.h"
@@ -444,10 +449,69 @@ std::string CatPieces(std::initializer_list<absl::string_view> pieces);
 void AppendPieces(std::string* dest,
                   std::initializer_list<absl::string_view> pieces);
 
+template <typename Integer>
+std::string IntegerToString(Integer i) {
+  // Any integer (signed/unsigned) up to 64 bits can be formatted into a buffer
+  // with 22 bytes (including NULL at the end).
+  constexpr size_t kMaxDigits10 = 22;
+  std::string result;
+  strings_internal::STLStringResizeUninitialized(&result, kMaxDigits10);
+  char* start = &result[0];
+  // note: this can be optimized to not write last zero.
+  char* end = numbers_internal::FastIntToBuffer(i, start);
+  auto size = static_cast<size_t>(end - start);
+  assert((size < result.size()) &&
+         "StrCat(Integer) does not fit into kMaxDigits10");
+  result.erase(size);
+  return result;
+}
+template <typename Float>
+std::string FloatToString(Float f) {
+  std::string result;
+  strings_internal::STLStringResizeUninitialized(
+      &result, numbers_internal::kSixDigitsToBufferSize);
+  char* start = &result[0];
+  result.erase(numbers_internal::SixDigitsToBuffer(f, start));
+  return result;
+}
+
+// `SingleArgStrCat` overloads take built-in `int`, `long` and `long long` types
+// (signed / unsigned) to avoid ambiguity on the call side. If we used int32_t
+// and int64_t, then at least one of the three (`int` / `long` / `long long`)
+// would have been ambiguous when passed to `SingleArgStrCat`.
+inline std::string SingleArgStrCat(int x) { return IntegerToString(x); }
+inline std::string SingleArgStrCat(unsigned int x) {
+  return IntegerToString(x);
+}
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(long x) { return IntegerToString(x); }
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(unsigned long x) {
+  return IntegerToString(x);
+}
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(long long x) { return IntegerToString(x); }
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(unsigned long long x) {
+  return IntegerToString(x);
+}
+inline std::string SingleArgStrCat(float x) { return FloatToString(x); }
+inline std::string SingleArgStrCat(double x) { return FloatToString(x); }
+
+
+template <typename T, typename = std::enable_if_t<std::is_arithmetic<T>{} &&
+                                                  !std::is_same<T, char>{}>>
+using EnableIfFastCase = T;
+
 }  // namespace strings_internal
 
 ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }
 
+template <typename T>
+ABSL_MUST_USE_RESULT inline std::string StrCat(
+    strings_internal::EnableIfFastCase<T> a) {
+  return strings_internal::SingleArgStrCat(a);
+}
 ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a) {
   return std::string(a.data(), a.size());
 }

absl/strings/str_cat_benchmark.cc

@@ -188,4 +188,15 @@ void StrAppendConfig(B* benchmark) {
 
 BENCHMARK(BM_StrAppend)->Apply(StrAppendConfig);
 
+void BM_StrCat_int(benchmark::State& state) {
+  int i = 0;
+  for (auto s : state) {
+    std::string result = absl::StrCat(i);
+    benchmark::DoNotOptimize(result);
+    i = IncrementAlternatingSign(i);
+  }
+}
+
+BENCHMARK(BM_StrCat_int);
+
 }  // namespace

absl/strings/str_cat_test.cc

@@ -665,4 +665,20 @@ TEST(StrCat, AbslStringifyWithEnum) {
   EXPECT_EQ(absl::StrCat(e), "Choices");
 }
 
+template <typename Integer>
+void CheckSingleArgumentIntegerLimits() {
+  Integer max = std::numeric_limits<Integer>::max();
+  Integer min = std::numeric_limits<Integer>::min();
+
+  EXPECT_EQ(absl::StrCat(max), std::to_string(max));
+  EXPECT_EQ(absl::StrCat(min), std::to_string(min));
+}
+
+TEST(StrCat, SingleArgumentLimits) {
+  CheckSingleArgumentIntegerLimits<int32_t>();
+  CheckSingleArgumentIntegerLimits<uint32_t>();
+  CheckSingleArgumentIntegerLimits<int64_t>();
+  CheckSingleArgumentIntegerLimits<uint64_t>();
+}
+
 }  // namespace