[libcxx-commits] [libcxx] [libc++][ranges] optimize the performance of `ranges::starts_with` (PR #84570)

[Xiaoyang Liu via libcxx-commits]

https://github.com/xiaoyang-sde created https://github.com/llvm/llvm-project/pull/84570

## Abstract

This pull request optimizes the performance of `ranges::starts_with`.


>From 3a384218365b175e8884a91125efe4e07f9e500a Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Fri, 8 Mar 2024 13:43:39 -0800
Subject: [PATCH 1/2] [libc++][ranges] detect size mismatch in
 'ranges::starts_with'

---
 .../include/__algorithm/ranges_starts_with.h  | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/libcxx/include/__algorithm/ranges_starts_with.h b/libcxx/include/__algorithm/ranges_starts_with.h
index 90e184aa9bccc2..b39e1657a855ea 100644
--- a/libcxx/include/__algorithm/ranges_starts_with.h
+++ b/libcxx/include/__algorithm/ranges_starts_with.h
@@ -15,6 +15,7 @@
 #include <__functional/identity.h>
 #include <__functional/ranges_operations.h>
 #include <__iterator/concepts.h>
+#include <__iterator/distance.h>
 #include <__iterator/indirectly_comparable.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
@@ -50,6 +51,17 @@ struct __fn {
       _Pred __pred   = {},
       _Proj1 __proj1 = {},
       _Proj2 __proj2 = {}) const {
+    if constexpr (sized_sentinel_for<_Sent1, _Iter1> && sized_sentinel_for<_Sent2, _Iter2>) {
+      auto __n1 = ranges::distance(__first1, __last1);
+      auto __n2 = ranges::distance(__first2, __last2);
+      if (__n2 == 0) {
+        return true;
+      }
+      if (__n2 > __n1) {
+        return false;
+      }
+    }
+
     return __mismatch::__fn::__go(
                std::move(__first1),
                std::move(__last1),
@@ -69,6 +81,17 @@ struct __fn {
     requires indirectly_comparable<iterator_t<_Range1>, iterator_t<_Range2>, _Pred, _Proj1, _Proj2>
   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(
       _Range1&& __range1, _Range2&& __range2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) const {
+    if constexpr (sized_range<_Range1> && sized_range<_Range2>) {
+      auto __n1 = ranges::size(__range1);
+      auto __n2 = ranges::size(__range2);
+      if (__n2 == 0) {
+        return true;
+      }
+      if (__n2 > __n1) {
+        return false;
+      }
+    }
+
     return __mismatch::__fn::__go(
                ranges::begin(__range1),
                ranges::end(__range1),

>From 5f8773ea9dcedb1a7fbaf02fd7b145834c1b3cf3 Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Fri, 8 Mar 2024 13:44:51 -0800
Subject: [PATCH 2/2] [libc++][ranges] change the 'operator()' of
 'ranges::starts_with' to 'static'

---
 libcxx/include/__algorithm/ranges_starts_with.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/__algorithm/ranges_starts_with.h b/libcxx/include/__algorithm/ranges_starts_with.h
index b39e1657a855ea..93409d5b109137 100644
--- a/libcxx/include/__algorithm/ranges_starts_with.h
+++ b/libcxx/include/__algorithm/ranges_starts_with.h
@@ -43,14 +43,14 @@ struct __fn {
             class _Proj1 = identity,
             class _Proj2 = identity>
     requires indirectly_comparable<_Iter1, _Iter2, _Pred, _Proj1, _Proj2>
-  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr bool operator()(
       _Iter1 __first1,
       _Sent1 __last1,
       _Iter2 __first2,
       _Sent2 __last2,
       _Pred __pred   = {},
       _Proj1 __proj1 = {},
-      _Proj2 __proj2 = {}) const {
+      _Proj2 __proj2 = {}) {
     if constexpr (sized_sentinel_for<_Sent1, _Iter1> && sized_sentinel_for<_Sent2, _Iter2>) {
       auto __n1 = ranges::distance(__first1, __last1);
       auto __n2 = ranges::distance(__first2, __last2);
@@ -79,8 +79,8 @@ struct __fn {
             class _Proj1 = identity,
             class _Proj2 = identity>
     requires indirectly_comparable<iterator_t<_Range1>, iterator_t<_Range2>, _Pred, _Proj1, _Proj2>
-  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(
-      _Range1&& __range1, _Range2&& __range2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) const {
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr bool
+  operator()(_Range1&& __range1, _Range2&& __range2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) {
     if constexpr (sized_range<_Range1> && sized_range<_Range2>) {
       auto __n1 = ranges::size(__range1);
       auto __n2 = ranges::size(__range2);