diff options
| -rw-r--r-- | absl/strings/ascii.cc | 22 | ||||
| -rw-r--r-- | absl/strings/ascii_benchmark.cc | 18 |
2 files changed, 29 insertions, 11 deletions
diff --git a/absl/strings/ascii.cc b/absl/strings/ascii.cc index 8c6b1e05..1e6566e9 100644 --- a/absl/strings/ascii.cc +++ b/absl/strings/ascii.cc @@ -159,6 +159,20 @@ ABSL_DLL const char kToUpper[256] = { }; // clang-format on +// Returns whether `c` is in the a-z/A-Z range (w.r.t. `ToUpper`). +// Implemented by: +// 1. Pushing the a-z/A-Z range to [SCHAR_MIN, SCHAR_MIN + 26). +// 2. Comparing to SCHAR_MIN + 26. +template <bool ToUpper> +constexpr bool AsciiInAZRange(unsigned char c) { + constexpr unsigned char sub = (ToUpper ? 'a' : 'A') - SCHAR_MIN; + constexpr signed char threshold = SCHAR_MIN + 26; // 26 = alphabet size. + // Using unsigned arithmetic as overflows/underflows are well defined. + unsigned char u = c - sub; + // Using signed cmp, as SIMD unsigned cmp isn't available in many platforms. + return static_cast<signed char>(u) < threshold; +} + template <bool ToUpper> constexpr void AsciiStrCaseFold(char* p, char* end) { // The upper- and lowercase versions of ASCII characters differ by only 1 bit. @@ -168,15 +182,9 @@ constexpr void AsciiStrCaseFold(char* p, char* end) { // have the same single bit difference. constexpr unsigned char kAsciiCaseBitFlip = 'a' ^ 'A'; - constexpr char ch_a = ToUpper ? 'a' : 'A'; - constexpr char ch_z = ToUpper ? 'z' : 'Z'; for (; p < end; ++p) { unsigned char v = static_cast<unsigned char>(*p); - // We use & instead of && to ensure this always stays branchless - // We use static_cast<int> to suppress -Wbitwise-instead-of-logical - bool is_in_range = static_cast<bool>(static_cast<int>(ch_a <= v) & - static_cast<int>(v <= ch_z)); - v ^= is_in_range ? kAsciiCaseBitFlip : 0; + v ^= AsciiInAZRange<ToUpper>(v) ? kAsciiCaseBitFlip : 0; *p = static_cast<char>(v); } } diff --git a/absl/strings/ascii_benchmark.cc b/absl/strings/ascii_benchmark.cc index b04b28ca..3aecdb8b 100644 --- a/absl/strings/ascii_benchmark.cc +++ b/absl/strings/ascii_benchmark.cc @@ -105,18 +105,28 @@ static void BM_StrToLower(benchmark::State& state) { const int size = state.range(0); std::string s(size, 'X'); for (auto _ : state) { - benchmark::DoNotOptimize(absl::AsciiStrToLower(s)); + benchmark::DoNotOptimize(s); + std::string res = absl::AsciiStrToLower(s); + benchmark::DoNotOptimize(res); } } -BENCHMARK(BM_StrToLower)->Range(1, 1 << 20); +BENCHMARK(BM_StrToLower) + ->DenseRange(0, 32) + ->RangeMultiplier(2) + ->Range(64, 1 << 20); static void BM_StrToUpper(benchmark::State& state) { const int size = state.range(0); std::string s(size, 'x'); for (auto _ : state) { - benchmark::DoNotOptimize(absl::AsciiStrToUpper(s)); + benchmark::DoNotOptimize(s); + std::string res = absl::AsciiStrToUpper(s); + benchmark::DoNotOptimize(res); } } -BENCHMARK(BM_StrToUpper)->Range(1, 1 << 20); +BENCHMARK(BM_StrToUpper) + ->DenseRange(0, 32) + ->RangeMultiplier(2) + ->Range(64, 1 << 20); } // namespace |