[libc++] Replace modulus operations in std::seed_seq::generate with conditional checks.

Abseil benchmarks suggest that the conditional checks result in faster code (4-5x)
as they are compiled into conditional move instructions (cmov on x86).

Reviewed By: #libc, philnik, Mordante

Spies: pengfei, Mordante, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D125329

diff --git a/libcxx/benchmarks/random.bench.cpp b/libcxx/benchmarks/random.bench.cpp
new file mode 100644 (file)
index 0000000..e437849
--- /dev/null
+++ b/libcxx/benchmarks/random.bench.cpp
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <functional>
+#include <random>
+
+#include "benchmark/benchmark.h"
+
+constexpr std::size_t MAX_BUFFER_LEN = 256;
+constexpr std::size_t MAX_SEED_LEN = 16;
+
+static void BM_SeedSeq_Generate(benchmark::State& state) {
+  std::array<std::uint32_t, MAX_BUFFER_LEN> buffer;
+  std::array<std::uint32_t, MAX_SEED_LEN> seeds;
+  {
+    std::random_device rd;
+    std::generate(std::begin(seeds), std::begin(seeds) + state.range(0), [&]() { return rd(); });
+  }
+  std::seed_seq seed(std::begin(seeds), std::begin(seeds) + state.range(0));
+  for (auto _ : state) {
+    seed.generate(std::begin(buffer), std::begin(buffer) + state.range(1));
+  }
+}
+BENCHMARK(BM_SeedSeq_Generate)->Ranges({{1, MAX_SEED_LEN}, {1, MAX_BUFFER_LEN}});
+
+BENCHMARK_MAIN();

diff --git a/libcxx/include/__random/seed_seq.h b/libcxx/include/__random/seed_seq.h
index 8640cd1..330537f 100644 (file)
--- a/libcxx/include/__random/seed_seq.h
+++ b/libcxx/include/__random/seed_seq.h
@@ -109,39 +109,63 @@ seed_seq::generate(_RandomAccessIterator __first, _RandomAccessIterator __last)
             __first[__q] += __r;
             __first[0] = __r;
         }
+        // Initialize indexing terms used with if statements as an optimization to
+        // avoid calculating modulo n on every loop iteration for each term.
+        size_t __kmodn = 0;          // __k % __n
+        size_t __k1modn = __n - 1;   // (__k - 1) % __n
+        size_t __kpmodn = __p % __n; // (__k + __p) % __n
+        size_t __kqmodn = __q % __n; // (__k + __q) % __n
+
         for (size_t __k = 1; __k <= __s; ++__k)
         {
-            const size_t __kmodn = __k % __n;
-            const size_t __kpmodn = (__k + __p) % __n;
-            result_type __r = 1664525 * _Tp(__first[__kmodn] ^ __first[__kpmodn]
-                                           ^ __first[(__k - 1) % __n]);
-            __first[__kpmodn] += __r;
-            __r +=  __kmodn + __v_[__k-1];
-            __first[(__k + __q) % __n] += __r;
-            __first[__kmodn] = __r;
+          if (++__kmodn == __n)
+            __kmodn = 0;
+          if (++__k1modn == __n)
+            __k1modn = 0;
+          if (++__kpmodn == __n)
+            __kpmodn = 0;
+          if (++__kqmodn == __n)
+            __kqmodn = 0;
+
+          result_type __r = 1664525 * _Tp(__first[__kmodn] ^ __first[__kpmodn] ^ __first[__k1modn]);
+          __first[__kpmodn] += __r;
+          __r += __kmodn + __v_[__k - 1];
+          __first[__kqmodn] += __r;
+          __first[__kmodn] = __r;
         }
         for (size_t __k = __s + 1; __k < __m; ++__k)
         {
-            const size_t __kmodn = __k % __n;
-            const size_t __kpmodn = (__k + __p) % __n;
-            result_type __r = 1664525 * _Tp(__first[__kmodn] ^ __first[__kpmodn]
-                                           ^ __first[(__k - 1) % __n]);
-            __first[__kpmodn] += __r;
-            __r +=  __kmodn;
-            __first[(__k + __q) % __n] += __r;
-            __first[__kmodn] = __r;
+          if (++__kmodn == __n)
+            __kmodn = 0;
+          if (++__k1modn == __n)
+            __k1modn = 0;
+          if (++__kpmodn == __n)
+            __kpmodn = 0;
+          if (++__kqmodn == __n)
+            __kqmodn = 0;
+
+          result_type __r = 1664525 * _Tp(__first[__kmodn] ^ __first[__kpmodn] ^ __first[__k1modn]);
+          __first[__kpmodn] += __r;
+          __r += __kmodn;
+          __first[__kqmodn] += __r;
+          __first[__kmodn] = __r;
         }
         for (size_t __k = __m; __k < __m + __n; ++__k)
         {
-            const size_t __kmodn = __k % __n;
-            const size_t __kpmodn = (__k + __p) % __n;
-            result_type __r = 1566083941 * _Tp(__first[__kmodn] +
-                                              __first[__kpmodn] +
-                                              __first[(__k - 1) % __n]);
-            __first[__kpmodn] ^= __r;
-            __r -= __kmodn;
-            __first[(__k + __q) % __n] ^= __r;
-            __first[__kmodn] = __r;
+          if (++__kmodn == __n)
+            __kmodn = 0;
+          if (++__k1modn == __n)
+            __k1modn = 0;
+          if (++__kpmodn == __n)
+            __kpmodn = 0;
+          if (++__kqmodn == __n)
+            __kqmodn = 0;
+
+          result_type __r = 1566083941 * _Tp(__first[__kmodn] + __first[__kpmodn] + __first[__k1modn]);
+          __first[__kpmodn] ^= __r;
+          __r -= __kmodn;
+          __first[__kqmodn] ^= __r;
+          __first[__kmodn] = __r;
         }
     }
 }