[libc++] Avoid string reallocation in std::filesystem::path::lexically_relative
#152964
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…ly_relative` ``` Benchmark Time CPU Time Old Time New CPU Old CPU New ------------------------------------------------------------------------------------------------------------------------------------------------ BM_LexicallyRelative/small_path/2 -0.2111 -0.2082 229 181 228 180 BM_LexicallyRelative/small_path/4 -0.2579 -0.2550 455 338 452 337 BM_LexicallyRelative/small_path/8 -0.2643 -0.2616 844 621 838 619 BM_LexicallyRelative/small_path/16 -0.2582 -0.2556 1562 1158 1551 1155 BM_LexicallyRelative/small_path/32 -0.2518 -0.2496 3023 2262 3004 2254 BM_LexicallyRelative/small_path/64 -0.2806 -0.2775 6344 4564 6295 4549 BM_LexicallyRelative/small_path/128 -0.2165 -0.2137 11762 9216 11683 9186 BM_LexicallyRelative/small_path/256 -0.2672 -0.2645 24499 17953 24324 17891 BM_LexicallyRelative/large_path/2 -0.3268 -0.3236 426 287 422 285 BM_LexicallyRelative/large_path/4 -0.3274 -0.3248 734 494 729 492 BM_LexicallyRelative/large_path/8 -0.3586 -0.3560 1409 904 1399 901 BM_LexicallyRelative/large_path/16 -0.3978 -0.3951 2764 1665 2743 1659 BM_LexicallyRelative/large_path/32 -0.3934 -0.3908 5323 3229 5283 3218 BM_LexicallyRelative/large_path/64 -0.3629 -0.3605 10340 6587 10265 6564 BM_LexicallyRelative/large_path/128 -0.3450 -0.3423 19379 12694 19233 12649 BM_LexicallyRelative/large_path/256 -0.3097 -0.3054 36293 25052 35943 24965 ```
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llvmbot
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@llvm/pr-subscribers-libcxx Author: Timothy Choi (tinnamchoi) ChangesImproves runtime by around 20 to 40%. Full diff: https://github.com/llvm/llvm-project/pull/152964.diff 2 Files Affected:
diff --git a/libcxx/src/filesystem/path.cpp b/libcxx/src/filesystem/path.cpp
index 9f7dc54fdf156..624cf785577eb 100644
--- a/libcxx/src/filesystem/path.cpp
+++ b/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)
diff --git a/libcxx/test/benchmarks/filesystem.bench.cpp b/libcxx/test/benchmarks/filesystem.bench.cpp
index dc6b0ac537f7e..f2e596238a922 100644
--- a/libcxx/test/benchmarks/filesystem.bench.cpp
+++ b/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();
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philnik777
reviewed
Aug 11, 2025
Forgot to run `clang-format` before.
philnik777
approved these changes
Aug 11, 2025
|
✅ With the latest revision this PR passed the C/C++ code formatter. |
Should've ran `git clang-format HEAD~2` instead.
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@philnik777 Workflow requires approval again 😅 |
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Is it "normal" for the CIs to time out? |
Yeah, there are currently some infrastructure issues. |
philnik777
requested changes
Aug 11, 2025
philnik777
reviewed
Aug 12, 2025
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Improves runtime by around 20 to 40%. (1.3x to 1.7x)