[libcxx-commits] [libcxx] has `basic_string` call `basic_string_view`'s assume-valid constructor (PR #105441)

[Christopher Di Bella via libcxx-commits]

https://github.com/cjdb created https://github.com/llvm/llvm-project/pull/105441

`basic_string` frequently calls `basic_string_view(data(), size())`, which accounts for ~15% of the observed overhead when hardening is enabled. This commit removes unnecessary checks when `basic_string` is known to already have valid data, by bypassing the public constructor, so that we eliminate that overhead.

>From 89870ab1a7921ac69314d250ea2f4cbf6ac166b9 Mon Sep 17 00:00:00 2001
From: Christopher Di Bella <cjdb at google.com>
Date: Tue, 20 Aug 2024 21:55:12 +0000
Subject: [PATCH] has `basic_string` call `basic_string_view`'s assume-valid
 constructor

`basic_string` frequently calls `basic_string_view(data(), size())`,
which accounts for ~15% of the observed overhead when hardening is
enabled. This commit removes unnecessary checks when `basic_string` is
known to already have valid data, by bypassing the public constructor,
so that we eliminate that overhead.
---
 libcxx/include/string      | 12 ++++++------
 libcxx/include/string_view |  3 +++
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/libcxx/include/string b/libcxx/include/string
index 6e93a6230cc2c0..cdc1afedbdf52f 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1213,7 +1213,7 @@ public:
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 operator __self_view() const _NOEXCEPT {
-    return __self_view(data(), size());
+    return __self_view(typename __self_view::__assume_valid(), data(), size());
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS basic_string&
@@ -1822,7 +1822,7 @@ public:
 
 #if _LIBCPP_STD_VER >= 20
   constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(__self_view __sv) const noexcept {
-    return __self_view(data(), size()).starts_with(__sv);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).starts_with(__sv);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(value_type __c) const noexcept {
@@ -1834,7 +1834,7 @@ public:
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(__self_view __sv) const noexcept {
-    return __self_view(data(), size()).ends_with(__sv);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).ends_with(__sv);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(value_type __c) const noexcept {
@@ -1848,15 +1848,15 @@ public:
 
 #if _LIBCPP_STD_VER >= 23
   constexpr _LIBCPP_HIDE_FROM_ABI bool contains(__self_view __sv) const noexcept {
-    return __self_view(data(), size()).contains(__sv);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).contains(__sv);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool contains(value_type __c) const noexcept {
-    return __self_view(data(), size()).contains(__c);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).contains(__c);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool contains(const value_type* __s) const {
-    return __self_view(data(), size()).contains(__s);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).contains(__s);
   }
 #endif
 
diff --git a/libcxx/include/string_view b/libcxx/include/string_view
index 2a03ee99e9ab52..c468dc6fc0ab7c 100644
--- a/libcxx/include/string_view
+++ b/libcxx/include/string_view
@@ -689,6 +689,9 @@ private:
 
   const value_type* __data_;
   size_type __size_;
+
+  template<class, class, class>
+  friend class basic_string;
 };
 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(basic_string_view);