[libc++] Avoid string reallocation in std::filesystem::path::lexically_relative

libcxx/src/filesystem/path.cpp

@@ -292,7 +292,9 @@ path path::lexically_relative(const path& base) const {
   // return a path constructed with 'n' dot-dot elements, followed by the
   // elements of '*this' after the mismatch.
   path Result;
-  // FIXME: Reserve enough room in Result that it won't have to re-allocate.
+  constexpr size_t ElemSize = 2; // ".."
+  constexpr size_t SeparatorSize = 1; // separator is always a single char
+  Result.__reserve(ElemCount * (ElemSize + SeparatorSize) + SeparatorSize + PP.Path.size());
   while (ElemCount--)
     Result /= PATHSTR("..");
   for (; PP; ++PP)

libcxx/test/benchmarks/filesystem.bench.cpp

@@ -171,4 +171,25 @@ BENCHMARK_CAPTURE(BM_LexicallyNormal, large_path, getRandomPaths, /*PathLen*/ 32
     ->Range(2, 256)
     ->Complexity();
 
+template <class GenInput>
+void BM_LexicallyRelative(benchmark::State& st, GenInput gen, size_t PathLen) {
+  using fs::path;
+  auto BasePath = gen(st.range(0), PathLen);
+  auto TargetPath = gen(st.range(0), PathLen);
+  benchmark::DoNotOptimize(&BasePath);
+  benchmark::DoNotOptimize(&TargetPath);
+  while (st.KeepRunning()) {
+    benchmark::DoNotOptimize(TargetPath.lexically_relative(BasePath));
+  }
+  st.SetComplexityN(st.range(0));
+}
+BENCHMARK_CAPTURE(BM_LexicallyRelative, small_path, getRandomPaths, /*PathLen*/ 5)
+    ->RangeMultiplier(2)
+    ->Range(2, 256)
+    ->Complexity();
+BENCHMARK_CAPTURE(BM_LexicallyRelative, large_path, getRandomPaths, /*PathLen*/ 32)
+    ->RangeMultiplier(2)
+    ->Range(2, 256)
+    ->Complexity();
+
 BENCHMARK_MAIN();