Rewrite std::filesystem::path iterators and parser

This patch entirely rewrites the parsing logic for paths. Unlike the previous
implementation this one stores information about the current state; For example
if we are in a trailing separator or a root separator. This avoids the need for
extra lookahead (and extra work) when incrementing or decrementing an iterator.
Roughly this gives us a 15% speedup over the previous implementation.

Unfortunately this implementation is still a lot slower than libstdc++'s.
Because libstdc++ pre-parses and splits the path upon construction their
iterators are trivial to increment/decrement. This makes libc++ lazy parsing
100x slower than libstdc++. However the pre-parsing libstdc++ causes a ton
of extra and unneeded allocations when constructing the string. For example
`path("/foo/bar/")` would require at least 5 allocations with libstdc++
whereas libc++ uses only one. The non-allocating behavior is much preferable
when you consider filesystem usages like 'exists("/foo/bar/")'.

Even then libc++'s path seems to be twice as slow to simply construct compared
to libstdc++. More investigation is needed about this.

llvm-svn: 285526

1 files changed, 2 insertions, 1 deletions

diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
index ed562e0cc63e..253fbd684711 100644
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -115,7 +115,8 @@ macro(add_benchmark_test name source_file)
   if (LIBCXX_BENCHMARK_NATIVE_STDLIB)
     set(native_target ${name}_native)
     add_executable(${native_target} EXCLUDE_FROM_ALL ${source_file})
-    add_dependencies(${native_target} google-benchmark-native)
+    add_dependencies(${native_target} google-benchmark-native
+                                      google-benchmark-libcxx)
     target_link_libraries(${native_target} -lbenchmark)
     if (LIBCXX_BENCHMARK_NATIVE_STDLIB STREQUAL "libstdc++")
       target_link_libraries(${native_target} -lstdc++fs)