[libc++] Fold __search_substring into _Traits::find in case the second string has length 1 #160076

philnik777 · 2025-09-22T11:27:27Z

Apple M4:

Benchmark                                                      Baseline    Candidate    Difference    % Difference
-----------------------------------------------------------  ----------  -----------  ------------  --------------
BM_string_literal/1024                                            16.99        16.79         -0.21           -1.21
BM_string_literal/128                                              3.44         3.34         -0.10           -2.88
BM_string_literal/16                                               1.80         1.69         -0.11           -5.93
BM_string_literal/2048                                            38.44        38.38         -0.07           -0.17
BM_string_literal/256                                              5.77         5.65         -0.12           -2.02
BM_string_literal/32                                               2.03         1.92         -0.11           -5.44
BM_string_literal/4096                                            73.92        73.74         -0.18           -0.25
BM_string_literal/512                                              9.49         9.41         -0.08           -0.84
BM_string_literal/64                                               2.59         2.45         -0.14           -5.38
BM_string_literal/8                                                1.79         1.69         -0.11           -5.90
BM_string_literal/8192                                           132.09       131.81         -0.28           -0.21

github-actions · 2025-09-22T11:31:38Z

✅ With the latest revision this PR passed the C/C++ code formatter.

llvmbot · 2025-09-23T10:22:12Z

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

Full diff: https://github.com/llvm/llvm-project/pull/160076.diff

3 Files Affected:

(modified) libcxx/include/__string/char_traits.h (+7)
(modified) libcxx/test/benchmarks/containers/string.bench.cpp (+39)
(modified) libcxx/test/std/strings/basic.string/string.ops/string_find/string_size.pass.cpp (+17)

diff --git a/libcxx/include/__string/char_traits.h b/libcxx/include/__string/char_traits.h
index 86c92477cbfeb..8292750919427 100644
--- a/libcxx/include/__string/char_traits.h
+++ b/libcxx/include/__string/char_traits.h
@@ -369,6 +369,13 @@ _LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR_SINCE_CXX14 const _CharT* __searc
   if (__len1 < __len2)
     return __last1;
 
+  if (__builtin_constant_p(__len2 == 1) && __len2 == 1) {
+    auto __res = _Traits::find(__first1, __len1, *__first2);
+    if (__res == nullptr)
+      return __last1;
+    return __res;
+  }
+
   // First element of __first2 is loop invariant.
   _CharT __f2 = *__first2;
   while (true) {
diff --git a/libcxx/test/benchmarks/containers/string.bench.cpp b/libcxx/test/benchmarks/containers/string.bench.cpp
index 966775d31a8cf..68f596b1be62a 100644
--- a/libcxx/test/benchmarks/containers/string.bench.cpp
+++ b/libcxx/test/benchmarks/containers/string.bench.cpp
@@ -60,6 +60,45 @@ static void BM_StringFindMatch2(benchmark::State& state) {
 }
 BENCHMARK(BM_StringFindMatch2)->Range(1, MAX_STRING_LEN / 4);
 
+static void BM_string_literal(benchmark::State& state) {
+  std::string s;
+
+  for (int i = 0; i < state.range(0); i++)
+    s += 'a';
+
+  s += 'b';
+
+  benchmark::DoNotOptimize(s.data());
+  benchmark::ClobberMemory();
+  size_t pos;
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(pos = s.find("b")); // "b" is a string literal, it should be longer
+    benchmark::ClobberMemory();
+  }
+}
+
+BENCHMARK(BM_string_literal)->RangeMultiplier(2)->Range(8, 8 << 10);
+
+static void BM_char_literal(benchmark::State& state) {
+  std::string s;
+
+  for (int i = 0; i < state.range(0); i++)
+    s += 'a';
+
+  s += 'b';
+
+  benchmark::DoNotOptimize(s.data());
+  benchmark::ClobberMemory();
+  size_t pos;
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(pos = s.find('b')); // 'b' is a char literal, it should be faster
+    benchmark::ClobberMemory();
+  }
+}
+BENCHMARK(BM_char_literal)->RangeMultiplier(2)->Range(8, 8 << 10);
+
 static void BM_StringCtorDefault(benchmark::State& state) {
   for (auto _ : state) {
     std::string Default;
diff --git a/libcxx/test/std/strings/basic.string/string.ops/string_find/string_size.pass.cpp b/libcxx/test/std/strings/basic.string/string.ops/string_find/string_size.pass.cpp
index 60ed469ce991b..4aa4a3f339142 100644
--- a/libcxx/test/std/strings/basic.string/string.ops/string_find/string_size.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.ops/string_find/string_size.pass.cpp
@@ -43,14 +43,17 @@ TEST_CONSTEXPR_CXX20 void test0() {
   test(S(""), S("abcdeabcde"), 1, S::npos);
   test(S(""), S("abcdeabcdeabcdeabcde"), 1, S::npos);
   test(S("abcde"), S(""), 0, 0);
+  test(S("abcde"), S("a"), 0, 0);
   test(S("abcde"), S("abcde"), 0, 0);
   test(S("abcde"), S("abcdeabcde"), 0, S::npos);
   test(S("abcde"), S("abcdeabcdeabcdeabcde"), 0, S::npos);
   test(S("abcde"), S(""), 1, 1);
+  test(S("abcde"), S("a"), 1, S::npos);
   test(S("abcde"), S("abcde"), 1, S::npos);
   test(S("abcde"), S("abcdeabcde"), 1, S::npos);
   test(S("abcde"), S("abcdeabcdeabcdeabcde"), 1, S::npos);
   test(S("abcde"), S(""), 2, 2);
+  test(S("abcde"), S("a"), 2, S::npos);
   test(S("abcde"), S("abcde"), 2, S::npos);
   test(S("abcde"), S("abcdeabcde"), 2, S::npos);
   test(S("abcde"), S("abcdeabcdeabcdeabcde"), 2, S::npos);
@@ -59,58 +62,72 @@ TEST_CONSTEXPR_CXX20 void test0() {
   test(S("abcde"), S("abcdeabcde"), 4, S::npos);
   test(S("abcde"), S("abcdeabcdeabcdeabcde"), 4, S::npos);
   test(S("abcde"), S(""), 5, 5);
+  test(S("abcde"), S("a"), 5, S::npos);
   test(S("abcde"), S("abcde"), 5, S::npos);
   test(S("abcde"), S("abcdeabcde"), 5, S::npos);
   test(S("abcde"), S("abcdeabcdeabcdeabcde"), 5, S::npos);
   test(S("abcde"), S(""), 6, S::npos);
+  test(S("abcde"), S("a"), 6, S::npos);
   test(S("abcde"), S("abcde"), 6, S::npos);
   test(S("abcde"), S("abcdeabcde"), 6, S::npos);
   test(S("abcde"), S("abcdeabcdeabcdeabcde"), 6, S::npos);
   test(S("abcdeabcde"), S(""), 0, 0);
+  test(S("abcdeabcde"), S("a"), 0, 0);
   test(S("abcdeabcde"), S("abcde"), 0, 0);
   test(S("abcdeabcde"), S("abcdeabcde"), 0, 0);
   test(S("abcdeabcde"), S("abcdeabcdeabcdeabcde"), 0, S::npos);
   test(S("abcdeabcde"), S(""), 1, 1);
+  test(S("abcdeabcde"), S("a"), 1, 5);
   test(S("abcdeabcde"), S("abcde"), 1, 5);
   test(S("abcdeabcde"), S("abcdeabcde"), 1, S::npos);
   test(S("abcdeabcde"), S("abcdeabcdeabcdeabcde"), 1, S::npos);
   test(S("abcdeabcde"), S(""), 5, 5);
+  test(S("abcdeabcde"), S("a"), 5, 5);
   test(S("abcdeabcde"), S("abcde"), 5, 5);
   test(S("abcdeabcde"), S("abcdeabcde"), 5, S::npos);
   test(S("abcdeabcde"), S("abcdeabcdeabcdeabcde"), 5, S::npos);
   test(S("abcdeabcde"), S(""), 9, 9);
+  test(S("abcdeabcde"), S("a"), 9, S::npos);
   test(S("abcdeabcde"), S("abcde"), 9, S::npos);
   test(S("abcdeabcde"), S("abcdeabcde"), 9, S::npos);
   test(S("abcdeabcde"), S("abcdeabcdeabcdeabcde"), 9, S::npos);
   test(S("abcdeabcde"), S(""), 10, 10);
+  test(S("abcdeabcde"), S("a"), 10, S::npos);
   test(S("abcdeabcde"), S("abcde"), 10, S::npos);
   test(S("abcdeabcde"), S("abcdeabcde"), 10, S::npos);
   test(S("abcdeabcde"), S("abcdeabcdeabcdeabcde"), 10, S::npos);
   test(S("abcdeabcde"), S(""), 11, S::npos);
+  test(S("abcdeabcde"), S("a"), 11, S::npos);
   test(S("abcdeabcde"), S("abcde"), 11, S::npos);
   test(S("abcdeabcde"), S("abcdeabcde"), 11, S::npos);
   test(S("abcdeabcde"), S("abcdeabcdeabcdeabcde"), 11, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S(""), 0, 0);
+  test(S("abcdeabcdeabcdeabcde"), S("a"), 0, 0);
   test(S("abcdeabcdeabcdeabcde"), S("abcde"), 0, 0);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcde"), 0, 0);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcdeabcdeabcde"), 0, 0);
   test(S("abcdeabcdeabcdeabcde"), S(""), 1, 1);
+  test(S("abcdeabcdeabcdeabcde"), S("a"), 1, 5);
   test(S("abcdeabcdeabcdeabcde"), S("abcde"), 1, 5);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcde"), 1, 5);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcdeabcdeabcde"), 1, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S(""), 10, 10);
+  test(S("abcdeabcdeabcdeabcde"), S("a"), 10, 10);
   test(S("abcdeabcdeabcdeabcde"), S("abcde"), 10, 10);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcde"), 10, 10);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcdeabcdeabcde"), 10, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S(""), 19, 19);
+  test(S("abcdeabcdeabcdeabcde"), S("a"), 19, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcde"), 19, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcde"), 19, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcdeabcdeabcde"), 19, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S(""), 20, 20);
+  test(S("abcdeabcdeabcdeabcde"), S("a"), 20, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcde"), 20, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcde"), 20, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcdeabcdeabcde"), 20, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S(""), 21, S::npos);
+  test(S("abcdeabcdeabcdeabcde"), S("a"), 21, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcde"), 21, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcde"), 21, S::npos);
   test(S("abcdeabcdeabcdeabcde"), S("abcdeabcdeabcdeabcde"), 21, S::npos);

libcxx/test/benchmarks/containers/string.bench.cpp

libcxx/include/__string/char_traits.h

…d string has length 1

…d string has length 1 (llvm#160076) Apple M4: ``` Benchmark Baseline Candidate Difference % Difference ----------------------------------------------------------- ---------- ----------- ------------ -------------- BM_string_literal/1024 16.99 16.79 -0.21 -1.21 BM_string_literal/128 3.44 3.34 -0.10 -2.88 BM_string_literal/16 1.80 1.69 -0.11 -5.93 BM_string_literal/2048 38.44 38.38 -0.07 -0.17 BM_string_literal/256 5.77 5.65 -0.12 -2.02 BM_string_literal/32 2.03 1.92 -0.11 -5.44 BM_string_literal/4096 73.92 73.74 -0.18 -0.25 BM_string_literal/512 9.49 9.41 -0.08 -0.84 BM_string_literal/64 2.59 2.45 -0.14 -5.38 BM_string_literal/8 1.79 1.69 -0.11 -5.90 BM_string_literal/8192 132.09 131.81 -0.28 -0.21 ```

bgra8 · 2025-10-15T14:25:00Z

@philnik777 we (at google) have bisected a performance regression caused by this change in the https://github.com/abseil/abseil-cpp/blob/master/absl/strings/string_view_benchmark.cc which benchmarks the standard std::string_view class.

Interestingly we only see the regressions on ARM servers:

Benchmark	Baseline (ns)	This revision (ns)	Diff (ns)	Regression
BM_find_string_view_len_one/1	5.17	9.88	4.71	91%
BM_find_string_view_len_one/8	5.17	9.88	4.71	91%

The regression is computed as: (exp - base) / base * 100

I don't have an Apple M4 (which is also ARM) readily available to check if the regression reproduces there. Would it be possible for you to check or maybe replicate the std::string_view benchmark? Maybe it would make sense for std::string_view to have a dedicated benchmark the same way as std::string already has?

LATER EDIT: to clarify, baseline is the revision previous to this one.

philnik777 force-pushed the optimize_string_find_single_char branch from dd1885a to f188376 Compare September 22, 2025 13:38

philnik777 marked this pull request as ready for review September 23, 2025 10:21

philnik777 requested a review from a team as a code owner September 23, 2025 10:21

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Sep 23, 2025

ldionne approved these changes Sep 23, 2025

View reviewed changes

[libc++] Fold __search_substring into _Traits::find in case the secon…

57ff008

…d string has length 1

philnik777 force-pushed the optimize_string_find_single_char branch from f188376 to 57ff008 Compare September 24, 2025 08:45

philnik777 merged commit a769608 into llvm:main Sep 25, 2025
78 checks passed

philnik777 deleted the optimize_string_find_single_char branch September 25, 2025 07:29

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[libc++] Fold __search_substring into _Traits::find in case the second string has length 1 #160076

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