Bump targeted C++ standard to C++20

.github/workflows/ci.yml

@@ -242,7 +242,7 @@ jobs:
     - run: cmake .
     - run: make libstim -j 2
     - run: echo -e '#include "stim.h"\nint main(int argc,const char **argv) {return !stim::find_bool_argument("test", argc, argv);}' > test.cc
-    - run: g++ -std=c++17 test.cc out/libstim.a -I src
+    - run: g++ -std=c++20 test.cc out/libstim.a -I src
     - run: ./a.out test
   build_lib_install:
     runs-on: ubuntu-latest
@@ -253,7 +253,7 @@ jobs:
     - run: make -j 2
     - run: make install
     - run: echo -e '#include "stim.h"\nint main(int argc,const char **argv) {return !stim::find_bool_argument("test", argc, argv);}' > test.cc
-    - run: g++ -std=c++17 test.cc install_dir/lib/libstim.a -I install_dir/include
+    - run: g++ -std=c++20 test.cc install_dir/lib/libstim.a -I install_dir/include
     - run: ./a.out test
     - run: echo -e "H 0 \n CNOT 0 1 \n M 0 1" | install_dir/bin/stim --sample
   benchmark_windows:

BUILD

@@ -44,7 +44,7 @@ cc_library(
     name = "stim_lib",
     srcs = SOURCE_FILES_NO_MAIN,
     copts = [
-        "-std=c++17",
+        "-std=c++20",
     ],
     includes = ["src/"],
 )
@@ -53,7 +53,7 @@ cc_binary(
     name = "stim",
     srcs = SOURCE_FILES_NO_MAIN + glob(["src/**/main.cc"]),
     copts = [
-        "-std=c++17",
+        "-std=c++20",
         "-march=native",
         "-O3",
     ],
@@ -64,7 +64,7 @@ cc_binary(
     name = "stim_benchmark",
     srcs = SOURCE_FILES_NO_MAIN + PERF_FILES,
     copts = [
-        "-std=c++17",
+        "-std=c++20",
         "-march=native",
         "-O3",
     ],
@@ -75,7 +75,7 @@ cc_test(
     name = "stim_test",
     srcs = SOURCE_FILES_NO_MAIN + TEST_FILES,
     copts = [
-        "-std=c++17",
+        "-std=c++20",
         "-march=native",
     ],
     data = glob(["testdata/**"]),
@@ -91,7 +91,7 @@ cc_binary(
     srcs = SOURCE_FILES_NO_MAIN + PYBIND_FILES,
     copts = [
         "-O3",
-        "-std=c++17",
+        "-std=c++20",
         "-fvisibility=hidden",
         "-march=native",
         "-DSTIM_PYBIND11_MODULE_NAME=stim",

CMakeLists.txt

@@ -15,7 +15,7 @@
 cmake_minimum_required(VERSION 3.13)
 project(stim)
 include_directories(src)
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY out)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY out)
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY out)

doc/developer_documentation.md

@@ -166,7 +166,7 @@ bazel run stim
 find src \
     | grep "\\.cc$" \
     | grep -v "\\.\(test\|perf\|pybind\)\\.cc$" \
-    | xargs g++ -I src -pthread -std=c++17 -O3 -march=native
+    | xargs g++ -I src -pthread -std=c++20 -O3 -march=native
 
 # output binary ends up at:
 # ./a.out
@@ -387,7 +387,7 @@ Ending a filter with a `*` turns it into a prefix filter `--only=sim_*`.
 find src \
     | grep "\\.cc" \
     | grep -v "\\.\(test\|perf\|pybind\)\\.cc" \
-    | xargs g++ -I src -pthread -std=c++17 -O3 -march=native -g -fno-omit-frame-pointer
+    | xargs g++ -I src -pthread -std=c++20 -O3 -march=native -g -fno-omit-frame-pointer
 sudo perf record -g ./a.out  # [ADD STIM FLAGS FOR THE CASE YOU WANT TO PROFILE]
 sudo perf report
 ```

glue/javascript/build_wasm.sh

@@ -29,7 +29,7 @@ echo '</script>' >> out/all_stim_tests.html
 
 # Build web assembly module using emscripten.
 emcc \
-    -std=c++17 \
+    -std=c++20 \
     -s NO_DISABLE_EXCEPTION_CATCHING \
     -s EXPORT_NAME="load_stim_module"  \
     -s MODULARIZE=1  \

setup.py

@@ -29,7 +29,7 @@
 
 if sys.platform.startswith('win'):
     common_compile_args = [
-        '/std:c++17',
+        '/std:c++20',
         '/O2',
         f'/DVERSION_INFO={__version__}',
     ]
@@ -38,7 +38,7 @@
     arch_basic = []
 else:
     common_compile_args = [
-        '-std=c++17',
+        '-std=c++20',
         '-fno-strict-aliasing',
         '-O3',
         '-g0',

src/stim/mem/bitword_128_sse.h

@@ -20,6 +20,7 @@
 
 #include <algorithm>
 #include <array>
+#include <bit>
 #include <immintrin.h>
 #include <sstream>
 #include <stdexcept>
@@ -140,7 +141,7 @@ struct bitword<128> {
     }
 
     inline uint16_t popcount() const {
-        return popcnt64(u64[0]) + popcnt64(u64[1]);
+        return std::popcount(u64[0]) + std::popcount(u64[1]);
     }
 
     inline bitword<128> shifted(int offset) const {

src/stim/mem/bitword_256_avx.h

@@ -19,6 +19,7 @@
 #if __AVX2__
 
 #include <array>
+#include <bit>
 #include <immintrin.h>
 #include <iostream>
 #include <sstream>
@@ -139,8 +140,8 @@ struct bitword<256> {
     }
 
     inline uint16_t popcount() const {
-        return stim::popcnt64(u64[0]) + stim::popcnt64(u64[1]) + stim::popcnt64(u64[2]) +
-               (uint16_t)stim::popcnt64(u64[3]);
+        return std::popcount(u64[0]) + std::popcount(u64[1]) + std::popcount(u64[2]) +
+               (uint16_t)std::popcount(u64[3]);
     }
 
     inline bitword<256> shifted(int offset) const {

src/stim/mem/bitword_64.h

@@ -18,6 +18,7 @@
 #define _STIM_MEM_SIMD_WORD_64_STD_H
 
 #include <array>
+#include <bit>
 #include <sstream>
 #include <stdlib.h>
 
@@ -109,7 +110,7 @@ struct bitword<64> {
     }
 
     inline uint16_t popcount() const {
-        return popcnt64(u64[0]);
+        return std::popcount(u64[0]);
     }
 
     inline std::string str() const {

src/stim/mem/simd_bits_range_ref.inl

@@ -216,7 +216,7 @@ size_t simd_bits_range_ref<W>::popcnt() const {
     auto end = u64 + num_u64_padded();
     size_t result = 0;
     for (const uint64_t *p = u64; p != end; p++) {
-        result += popcnt64(*p);
+        result += std::popcount(*p);
     }
     return result;
 }

src/stim/mem/simd_util.h

@@ -37,16 +37,6 @@ inline uint64_t spread_bytes_32_to_64(uint32_t v) {
 
 void inplace_transpose_64x64(uint64_t *data, size_t stride);
 
-inline uint8_t popcnt64(uint64_t val) {
-    val -= (val >> 1) & 0x5555555555555555ULL;
-    val = (val & 0x3333333333333333ULL) + ((val >> 2) & 0x3333333333333333ULL);
-    val += val >> 4;
-    val &= 0xF0F0F0F0F0F0F0FULL;
-    val *= 0x101010101010101ULL;
-    val >>= 56;
-    return (uint8_t)val;
-}
-
 }  // namespace stim
 
 #endif

src/stim/mem/simd_util.test.cc

@@ -426,20 +426,3 @@ TEST(simd_util, interleave_mask) {
     ASSERT_EQ(interleave_mask(16), 0x0000FFFF0000FFFFULL);
     ASSERT_EQ(interleave_mask(32), 0x00000000FFFFFFFFULL);
 }
-
-TEST(simd_util, popcnt64) {
-    for (size_t expected = 0; expected <= 64; expected++) {
-        std::vector<uint64_t> bits{};
-        for (size_t i = 0; i < 64; i++) {
-            bits.push_back(i < expected);
-        }
-        for (size_t reps = 0; reps < 100; reps++) {
-            std::shuffle(bits.begin(), bits.end(), INDEPENDENT_TEST_RNG());
-            uint64_t v = 0;
-            for (size_t i = 0; i < 64; i++) {
-                v |= bits[i] << i;
-            }
-            ASSERT_EQ(popcnt64(v), expected);
-        }
-    }
-}

src/stim/probability_util.test.cc

@@ -58,7 +58,7 @@ TEST_EACH_WORD_SIZE_W(probability_util, biased_random, {
         biased_randomize_bits(p, data.u64, data.u64 + data.num_u64_padded(), rng);
         size_t t = 0;
         for (size_t k = 0; k < data.num_u64_padded(); k++) {
-            t += popcnt64(data.u64[k]);
+            t += std::popcount(data.u64[k]);
         }
         float dev = sqrtf(p * (1 - p) * n);
         float min_expected = n * p - dev * 5;

src/stim/simulators/error_analyzer.cc

@@ -1160,7 +1160,7 @@ void ErrorAnalyzer::decompose_helper_add_error_combinations(
     // Count number of detectors affected by each error.
     std::array<uint8_t, 1 << s> detector_counts{};
     for (size_t k = 1; k < 1 << s; k++) {
-        detector_counts[k] = popcnt64(detector_masks[k]);
+        detector_counts[k] = std::popcount(detector_masks[k]);
     }
 
     // Find single-detector errors (and empty errors).

src/stim/simulators/vector_simulator.h

@@ -151,7 +151,7 @@ struct VectorSimulator {
         float mag2 = 0;
         for (size_t i = 0; i < state.size(); i++) {
             bool reject = observable.sign;
-            reject ^= (popcnt64(i & mask) & 1) != 0;
+            reject ^= (std::popcount(i & mask) & 1) != 0;
             if (reject) {
                 state[i] = 0;
             } else {

src/stim/stabilizers/pauli_string.pybind.cc

@@ -65,7 +65,7 @@ pybind11::object PyPauliString::to_unitary_matrix(const std::string &endian) con
         }
     }
     uint8_t start_phase = 0;
-    start_phase += popcnt64(x & z);
+    start_phase += std::popcount(x & z);
     if (imag) {
         start_phase += 1;
     }
@@ -75,7 +75,7 @@ pybind11::object PyPauliString::to_unitary_matrix(const std::string &endian) con
     for (size_t col = 0; col < n; col++) {
         size_t row = col ^ x;
         uint8_t phase = start_phase;
-        if (popcnt64(col & z) & 1) {
+        if (std::popcount(col & z) & 1) {
             phase += 2;
         }
         std::complex<float> v{1, 0};
@@ -433,7 +433,7 @@ PyPauliString PyPauliString::from_unitary_matrix(
     }
     for (size_t k = 0; k < n; k++) {
         uint8_t expected_phase = phases[0];
-        if (popcnt64(k & z) & 1) {
+        if (std::popcount(k & z) & 1) {
             expected_phase += 2;
         }
         if ((expected_phase & 3) != phases[k]) {
@@ -442,7 +442,7 @@ PyPauliString PyPauliString::from_unitary_matrix(
         }
     }
 
-    uint8_t leftover_phase = phases[0] + popcnt64(x & z);
+    uint8_t leftover_phase = phases[0] + std::popcount(x & z);
     PyPauliString result(PauliString<MAX_BITWORD_WIDTH>(q), (leftover_phase & 1) != 0);
     result.value.sign = (leftover_phase & 2) != 0;
     auto &rx = result.value.xs.u64[0];