[libcxx-commits] [libcxx] [libc++] Optimize string operator[] for known large inputs (PR #69500)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Ilya Tocar (TocarIP)

<details>
<summary>Changes</summary>

If we know that index is larger than SSO size, we know that we can't be in SSO case, and should access the pointer. This removes extra check from operator[] for inputs known at compile time to be larger than SSO.

---
Full diff: https://github.com/llvm/llvm-project/pull/69500.diff


1 Files Affected:

- (modified) libcxx/include/string (+6) 


``````````diff
diff --git a/libcxx/include/string b/libcxx/include/string
index 91935162f02383a..cf9f0c847eb43af 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1198,11 +1198,17 @@ public:
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference operator[](size_type __pos) const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
+    if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
+      return *(__get_long_pointer() + __pos);
+    }
     return *(data() + __pos);
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](size_type __pos) _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
+    if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
+      return *(__get_long_pointer() + __pos);
+    }
     return *(__get_pointer() + __pos);
   }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/69500