<bit>: Improve has_single_bit() codegen

benchmarks/CMakeLists.txt

@@ -114,6 +114,7 @@ add_benchmark(efficient_nonlocking_print src/efficient_nonlocking_print.cpp)
 add_benchmark(filesystem src/filesystem.cpp)
 add_benchmark(find_and_count src/find_and_count.cpp)
 add_benchmark(find_first_of src/find_first_of.cpp)
+add_benchmark(has_single_bit src/has_single_bit.cpp)
 add_benchmark(iota src/iota.cpp)
 add_benchmark(locale_classic src/locale_classic.cpp)
 add_benchmark(minmax_element src/minmax_element.cpp)

benchmarks/src/has_single_bit.cpp

@@ -0,0 +1,52 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <benchmark/benchmark.h>
+#include <bit>
+#include <cstdint>
+
+#include <utility.hpp>
+
+using namespace std;
+
+template <typename T>
+void bm_has_single_bit_if(benchmark::State& state) {
+    const auto random_v = random_vector<T>(8);
+    for (auto _ : state) {
+        benchmark::DoNotOptimize(random_v);
+        unsigned int count_true  = 0;
+        unsigned int count_false = 0;
+        for (const auto& x : random_v) {
+            if (has_single_bit(x)) {
+                benchmark::DoNotOptimize(++count_true);
+            } else {
+                benchmark::DoNotOptimize(++count_false);
+            }
+        }
+    }
+}
+
+template <typename T>
+void bm_has_single_bit(benchmark::State& state) {
+    const auto random_v = random_vector<T>(8);
+    for (auto _ : state) {
+        benchmark::DoNotOptimize(random_v);
+        unsigned int r = 0;
+        for (const auto& x : random_v) {
+            r += has_single_bit(x);
+        }
+        benchmark::DoNotOptimize(r);
+    }
+}
+
+BENCHMARK(bm_has_single_bit_if<uint8_t>);
+BENCHMARK(bm_has_single_bit_if<uint16_t>);
+BENCHMARK(bm_has_single_bit_if<uint32_t>);
+BENCHMARK(bm_has_single_bit_if<uint64_t>);
+
+BENCHMARK(bm_has_single_bit<uint8_t>);
+BENCHMARK(bm_has_single_bit<uint16_t>);
+BENCHMARK(bm_has_single_bit<uint32_t>);
+BENCHMARK(bm_has_single_bit<uint64_t>);
+
+BENCHMARK_MAIN();

stl/inc/bit

@@ -84,7 +84,7 @@ _NODISCARD constexpr int countl_zero(_Ty _Val) noexcept;
 
 _EXPORT_STD template <_Standard_unsigned_integral _Ty>
 _NODISCARD constexpr bool has_single_bit(const _Ty _Val) noexcept {
-    return _Val != 0 && (_Val & (_Val - 1)) == 0;
+    return (_Val ^ (_Val - 1)) > _Val - 1;
 }
 
 inline void _Precondition_violation_in_bit_ceil() noexcept {}