[libc++] Optimize the std::mismatch tail (#83440)

This adds vectorization to the last 0-3 vectors and, if the range is
large enough, the remaining elements that don't fill a vector
completely.
```
-----------------------------------------------------------------------
Benchmark                           old    full vectors  partial vector
-----------------------------------------------------------------------
bm_mismatch<char>/1             1.40 ns         1.62 ns         2.09 ns
bm_mismatch<char>/2             1.88 ns         2.10 ns         2.33 ns
bm_mismatch<char>/3             2.67 ns         2.56 ns         2.72 ns
bm_mismatch<char>/4             3.01 ns         3.20 ns         3.70 ns
bm_mismatch<char>/5             3.51 ns         3.73 ns         3.64 ns
bm_mismatch<char>/6             4.71 ns         4.85 ns         4.37 ns
bm_mismatch<char>/7             5.12 ns         5.33 ns         4.37 ns
bm_mismatch<char>/8             5.79 ns         6.02 ns         4.75 ns
bm_mismatch<char>/15            9.20 ns         10.5 ns         7.23 ns
bm_mismatch<char>/16            10.2 ns         10.1 ns         7.46 ns
bm_mismatch<char>/17            10.2 ns         10.8 ns         7.57 ns
bm_mismatch<char>/31            17.6 ns         17.1 ns         10.8 ns
bm_mismatch<char>/32            17.4 ns         1.64 ns         1.64 ns
bm_mismatch<char>/33            23.3 ns         2.10 ns         2.33 ns
bm_mismatch<char>/63            31.8 ns         16.9 ns         2.33 ns
bm_mismatch<char>/64            32.6 ns         2.10 ns         2.10 ns
bm_mismatch<char>/65            33.6 ns         2.57 ns         2.80 ns
bm_mismatch<char>/127           67.3 ns         18.1 ns         3.27 ns
bm_mismatch<char>/128           2.17 ns         2.14 ns         2.57 ns
bm_mismatch<char>/129           2.36 ns         2.80 ns         3.27 ns
bm_mismatch<char>/255           67.5 ns         19.6 ns         4.68 ns
bm_mismatch<char>/256           3.76 ns         3.71 ns         3.97 ns
bm_mismatch<char>/257           3.77 ns         4.04 ns         4.43 ns
bm_mismatch<char>/511           70.8 ns         22.1 ns         7.47 ns
bm_mismatch<char>/512           7.27 ns         7.30 ns         6.95 ns
bm_mismatch<char>/513           7.11 ns         7.05 ns         6.96 ns
bm_mismatch<char>/1023          75.9 ns         27.4 ns         13.3 ns
bm_mismatch<char>/1024          13.9 ns         13.8 ns         12.4 ns
bm_mismatch<char>/1025          13.6 ns         13.6 ns         12.8 ns
bm_mismatch<char>/2047          87.3 ns         37.5 ns         25.4 ns
bm_mismatch<char>/2048          26.8 ns         27.4 ns         24.0 ns
bm_mismatch<char>/2049          26.7 ns         27.3 ns         25.5 ns
bm_mismatch<char>/4095           112 ns         64.7 ns         48.7 ns
bm_mismatch<char>/4096          53.0 ns         54.2 ns         46.8 ns
bm_mismatch<char>/4097          52.7 ns         54.2 ns         48.4 ns
bm_mismatch<char>/8191           160 ns          118 ns         98.4 ns
bm_mismatch<char>/8192           107 ns          108 ns         96.0 ns
bm_mismatch<char>/8193           106 ns          108 ns         97.2 ns
bm_mismatch<char>/16383          283 ns          234 ns          215 ns
bm_mismatch<char>/16384          227 ns          223 ns          217 ns
bm_mismatch<char>/16385          221 ns          221 ns          215 ns
bm_mismatch<char>/32767          547 ns          499 ns          488 ns
bm_mismatch<char>/32768          495 ns          492 ns          492 ns
bm_mismatch<char>/32769          491 ns          489 ns          488 ns
bm_mismatch<char>/65535         1028 ns          979 ns          971 ns
bm_mismatch<char>/65536          976 ns          970 ns          974 ns
bm_mismatch<char>/65537          970 ns          965 ns          971 ns
bm_mismatch<char>/131071        2031 ns         1948 ns         2005 ns
bm_mismatch<char>/131072        1973 ns         1955 ns         1974 ns
bm_mismatch<char>/131073        1989 ns         1932 ns         2001 ns
bm_mismatch<char>/262143        4469 ns         4244 ns         4223 ns
bm_mismatch<char>/262144        4443 ns         4183 ns         4243 ns
bm_mismatch<char>/262145        4400 ns         4232 ns         4246 ns
bm_mismatch<char>/524287       10169 ns         9733 ns         9592 ns
bm_mismatch<char>/524288       10154 ns         9664 ns         9843 ns
bm_mismatch<char>/524289       10113 ns         9641 ns        10003 ns
bm_mismatch<short>/1            1.86 ns         2.53 ns         2.32 ns
bm_mismatch<short>/2            2.57 ns         2.77 ns         2.55 ns
bm_mismatch<short>/3            3.26 ns         3.00 ns         2.79 ns
bm_mismatch<short>/4            3.95 ns         3.39 ns         3.15 ns
bm_mismatch<short>/5            4.83 ns         3.97 ns         3.72 ns
bm_mismatch<short>/6            5.43 ns         4.34 ns         4.03 ns
bm_mismatch<short>/7            6.11 ns         4.73 ns         4.44 ns
bm_mismatch<short>/8            6.84 ns         5.02 ns         4.79 ns
bm_mismatch<short>/15           11.5 ns         7.12 ns         6.50 ns
bm_mismatch<short>/16           13.9 ns         1.87 ns         2.11 ns
bm_mismatch<short>/17           14.0 ns         3.00 ns         2.47 ns
bm_mismatch<short>/31           23.1 ns         7.87 ns         2.47 ns
bm_mismatch<short>/32           23.8 ns         2.57 ns         2.81 ns
bm_mismatch<short>/33           24.5 ns         3.70 ns         2.94 ns
bm_mismatch<short>/63           44.8 ns         9.37 ns         3.46 ns
bm_mismatch<short>/64           2.32 ns         2.57 ns         2.64 ns
bm_mismatch<short>/65           2.52 ns         3.02 ns         3.51 ns
bm_mismatch<short>/127          45.6 ns         9.97 ns         5.18 ns
bm_mismatch<short>/128          3.85 ns         3.93 ns         3.94 ns
bm_mismatch<short>/129          3.82 ns         4.20 ns         4.70 ns
bm_mismatch<short>/255          50.4 ns         12.6 ns         8.07 ns
bm_mismatch<short>/256          7.23 ns         6.91 ns         6.98 ns
bm_mismatch<short>/257          7.24 ns         7.19 ns         7.55 ns
bm_mismatch<short>/511          52.3 ns         17.8 ns         14.0 ns
bm_mismatch<short>/512          13.6 ns         13.7 ns         13.6 ns
bm_mismatch<short>/513          13.9 ns         13.8 ns         18.5 ns
bm_mismatch<short>/1023         60.9 ns         30.9 ns         26.3 ns
bm_mismatch<short>/1024         26.7 ns         27.7 ns         25.7 ns
bm_mismatch<short>/1025         27.7 ns         27.6 ns         25.3 ns
bm_mismatch<short>/2047         88.4 ns         58.0 ns         51.6 ns
bm_mismatch<short>/2048         52.8 ns         55.3 ns         50.6 ns
bm_mismatch<short>/2049         55.2 ns         54.8 ns         48.7 ns
bm_mismatch<short>/4095          153 ns          113 ns          102 ns
bm_mismatch<short>/4096          105 ns          110 ns          101 ns
bm_mismatch<short>/4097          110 ns          110 ns         99.1 ns
bm_mismatch<short>/8191          277 ns          219 ns          206 ns
bm_mismatch<short>/8192          226 ns          214 ns          250 ns
bm_mismatch<short>/8193          226 ns          207 ns          208 ns
bm_mismatch<short>/16383         519 ns          492 ns          488 ns
bm_mismatch<short>/16384         494 ns          492 ns          492 ns
bm_mismatch<short>/16385         492 ns          488 ns          489 ns
bm_mismatch<short>/32767        1007 ns          968 ns          964 ns
bm_mismatch<short>/32768         977 ns          972 ns          970 ns
bm_mismatch<short>/32769         972 ns          962 ns          967 ns
bm_mismatch<short>/65535        1978 ns         1918 ns         1956 ns
bm_mismatch<short>/65536        1940 ns         1927 ns         1970 ns
bm_mismatch<short>/65537        1937 ns         1922 ns         1959 ns
bm_mismatch<short>/131071       4524 ns         4193 ns         4304 ns
bm_mismatch<short>/131072       4445 ns         4196 ns         4306 ns
bm_mismatch<short>/131073       4452 ns         4278 ns         4311 ns
bm_mismatch<short>/262143       9801 ns        10188 ns         9634 ns
bm_mismatch<short>/262144       9738 ns        10151 ns         9651 ns
bm_mismatch<short>/262145       9716 ns        10171 ns         9715 ns
bm_mismatch<short>/524287      19944 ns        20718 ns        20044 ns
bm_mismatch<short>/524288      21139 ns        20647 ns        20008 ns
bm_mismatch<short>/524289      21162 ns        19512 ns        20068 ns
bm_mismatch<int>/1              1.40 ns         1.84 ns         1.87 ns
bm_mismatch<int>/2              1.87 ns         2.08 ns         2.09 ns
bm_mismatch<int>/3              2.36 ns         2.31 ns         2.87 ns
bm_mismatch<int>/4              3.06 ns         2.72 ns         2.95 ns
bm_mismatch<int>/5              3.66 ns         3.37 ns         3.42 ns
bm_mismatch<int>/6              4.55 ns         3.65 ns         3.73 ns
bm_mismatch<int>/7              5.03 ns         3.93 ns         3.94 ns
bm_mismatch<int>/8              5.67 ns         1.86 ns         1.87 ns
bm_mismatch<int>/15             9.89 ns         4.41 ns         2.34 ns
bm_mismatch<int>/16             10.1 ns         2.33 ns         2.34 ns
bm_mismatch<int>/17             10.2 ns         3.34 ns         2.86 ns
bm_mismatch<int>/31             17.2 ns         5.54 ns         3.28 ns
bm_mismatch<int>/32             2.16 ns         2.15 ns         2.58 ns
bm_mismatch<int>/33             2.36 ns         3.01 ns         3.28 ns
bm_mismatch<int>/63             17.7 ns         6.50 ns         4.93 ns
bm_mismatch<int>/64             3.81 ns         3.58 ns         3.90 ns
bm_mismatch<int>/65             3.74 ns         4.36 ns         4.45 ns
bm_mismatch<int>/127            19.5 ns         9.56 ns         7.74 ns
bm_mismatch<int>/128            7.30 ns         6.41 ns         6.85 ns
bm_mismatch<int>/129            7.09 ns         7.04 ns         7.06 ns
bm_mismatch<int>/255            24.7 ns         14.8 ns         13.3 ns
bm_mismatch<int>/256            14.0 ns         12.1 ns         12.3 ns
bm_mismatch<int>/257            13.8 ns         12.7 ns         12.8 ns
bm_mismatch<int>/511            34.3 ns         26.3 ns         24.8 ns
bm_mismatch<int>/512            27.6 ns         23.6 ns         23.9 ns
bm_mismatch<int>/513            27.3 ns         24.4 ns         25.1 ns
bm_mismatch<int>/1023           62.5 ns         50.9 ns         48.3 ns
bm_mismatch<int>/1024           54.4 ns         46.1 ns         46.6 ns
bm_mismatch<int>/1025           54.2 ns         48.4 ns         47.5 ns
bm_mismatch<int>/2047            116 ns         97.8 ns         94.1 ns
bm_mismatch<int>/2048            108 ns         92.6 ns         92.4 ns
bm_mismatch<int>/2049            108 ns          104 ns         94.0 ns
bm_mismatch<int>/4095            233 ns          222 ns          205 ns
bm_mismatch<int>/4096            226 ns          223 ns          225 ns
bm_mismatch<int>/4097            221 ns          219 ns          210 ns
bm_mismatch<int>/8191            499 ns          485 ns          488 ns
bm_mismatch<int>/8192            496 ns          490 ns          495 ns
bm_mismatch<int>/8193            491 ns          485 ns          488 ns
bm_mismatch<int>/16383           982 ns          962 ns          964 ns
bm_mismatch<int>/16384           974 ns          971 ns          971 ns
bm_mismatch<int>/16385           971 ns          961 ns          968 ns
bm_mismatch<int>/32767          2003 ns         1959 ns         1920 ns
bm_mismatch<int>/32768          1996 ns         1947 ns         1928 ns
bm_mismatch<int>/32769          1990 ns         1945 ns         1926 ns
bm_mismatch<int>/65535          4434 ns         4275 ns         4312 ns
bm_mismatch<int>/65536          4437 ns         4267 ns         4321 ns
bm_mismatch<int>/65537          4442 ns         4261 ns         4321 ns
bm_mismatch<int>/131071         9673 ns         9648 ns         9465 ns
bm_mismatch<int>/131072         9667 ns         9671 ns         9465 ns
bm_mismatch<int>/131073         9661 ns         9653 ns         9464 ns
bm_mismatch<int>/262143        20595 ns        19605 ns        19064 ns
bm_mismatch<int>/262144        19894 ns        19572 ns        19009 ns
bm_mismatch<int>/262145        19851 ns        19656 ns        18999 ns
bm_mismatch<int>/524287        39556 ns        39364 ns        38131 ns
bm_mismatch<int>/524288        39678 ns        39573 ns        38183 ns
bm_mismatch<int>/524289        40168 ns        39301 ns        38121 ns
```

Author: philnik777

libcxx/benchmarks/algorithms/mismatch.bench.cpp

@@ -10,6 +10,15 @@
 #include <benchmark/benchmark.h>
 #include <random>
 
+void BenchmarkSizes(benchmark::internal::Benchmark* Benchmark) {
+  Benchmark->DenseRange(1, 8);
+  for (size_t i = 16; i != 1 << 20; i *= 2) {
+    Benchmark->Arg(i - 1);
+    Benchmark->Arg(i);
+    Benchmark->Arg(i + 1);
+  }
+}
+
 // TODO: Look into benchmarking aligned and unaligned memory explicitly
 // (currently things happen to be aligned because they are malloced that way)
 template <class T>
@@ -24,8 +33,8 @@ static void bm_mismatch(benchmark::State& state) {
     benchmark::DoNotOptimize(std::mismatch(vec1.begin(), vec1.end(), vec2.begin()));
   }
 }
-BENCHMARK(bm_mismatch<char>)->DenseRange(1, 8)->Range(16, 1 << 20);
-BENCHMARK(bm_mismatch<short>)->DenseRange(1, 8)->Range(16, 1 << 20);
-BENCHMARK(bm_mismatch<int>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_mismatch<char>)->Apply(BenchmarkSizes);
+BENCHMARK(bm_mismatch<short>)->Apply(BenchmarkSizes);
+BENCHMARK(bm_mismatch<int>)->Apply(BenchmarkSizes);
 
 BENCHMARK_MAIN();

libcxx/include/__algorithm/mismatch.h

@@ -64,7 +64,10 @@ __mismatch(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Pred& __pred, _Proj1& __
   constexpr size_t __unroll_count = 4;
   constexpr size_t __vec_size     = __native_vector_size<_Tp>;
   using __vec                     = __simd_vector<_Tp, __vec_size>;
+
   if (!__libcpp_is_constant_evaluated()) {
+    auto __orig_first1 = __first1;
+    auto __last2       = __first2 + (__last1 - __first1);
     while (static_cast<size_t>(__last1 - __first1) >= __unroll_count * __vec_size) [[__unlikely__]] {
       __vec __lhs[__unroll_count];
       __vec __rhs[__unroll_count];
@@ -84,8 +87,32 @@ __mismatch(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Pred& __pred, _Proj1& __
       __first1 += __unroll_count * __vec_size;
       __first2 += __unroll_count * __vec_size;
     }
+
+    // check the remaining 0-3 vectors
+    while (static_cast<size_t>(__last1 - __first1) >= __vec_size) {
+      if (auto __cmp_res = std::__load_vector<__vec>(__first1) == std::__load_vector<__vec>(__first2);
+          !std::__all_of(__cmp_res)) {
+        auto __offset = std::__find_first_not_set(__cmp_res);
+        return {__first1 + __offset, __first2 + __offset};
+      }
+      __first1 += __vec_size;
+      __first2 += __vec_size;
+    }
+
+    if (__last1 - __first1 == 0)
+      return {__first1, __first2};
+
+    // Check if we can load elements in front of the current pointer. If that's the case load a vector at
+    // (last - vector_size) to check the remaining elements
+    if (static_cast<size_t>(__first1 - __orig_first1) >= __vec_size) {
+      __first1 = __last1 - __vec_size;
+      __first2 = __last2 - __vec_size;
+      auto __offset =
+          std::__find_first_not_set(std::__load_vector<__vec>(__first1) == std::__load_vector<__vec>(__first2));
+      return {__first1 + __offset, __first2 + __offset};
+    } // else loop over the elements individually
   }
-  // TODO: Consider vectorizing the tail
+
   return std::__mismatch_loop(__first1, __last1, __first2, __pred, __proj1, __proj2);
 }
 

libcxx/test/std/algorithms/alg.nonmodifying/mismatch/mismatch.pass.cpp

@@ -184,5 +184,33 @@ int main(int, char**) {
     }
   }
 
+  { // check the tail of the vectorized loop
+    for (size_t vec_size = 1; vec_size != 256; ++vec_size) {
+      {
+        std::vector<char> lhs(256);
+        std::vector<char> rhs(256);
+
+        check<char*>(lhs, rhs, lhs.size());
+        lhs.back() = 1;
+        check<char*>(lhs, rhs, lhs.size() - 1);
+        lhs.back() = 0;
+        rhs.back() = 1;
+        check<char*>(lhs, rhs, lhs.size() - 1);
+        rhs.back() = 0;
+      }
+      {
+        std::vector<int> lhs(256);
+        std::vector<int> rhs(256);
+
+        check<int*>(lhs, rhs, lhs.size());
+        lhs.back() = 1;
+        check<int*>(lhs, rhs, lhs.size() - 1);
+        lhs.back() = 0;
+        rhs.back() = 1;
+        check<int*>(lhs, rhs, lhs.size() - 1);
+        rhs.back() = 0;
+      }
+    }
+  }
   return 0;
 }