[libcxx-commits] [libcxx] [libc++] Optimize std::find_if (PR #167697)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,65 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MEMORY_VALID_RANGE_H
+#define _LIBCPP___MEMORY_VALID_RANGE_H
+
+#include <__algorithm/comp.h>
+#include <__assert>
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__memory/assume_aligned.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/is_constant_evaluated.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/remove_cvref.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Tp>
+_LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_SANITIZE("address") bool
+__is_valid_range(const _Tp* __first, const _Tp* __last) {
+  if (__libcpp_is_constant_evaluated()) {
+    // If this is not a constant during constant evaluation, that is because __first and __last are not
+    // part of the same allocation. If they are part of the same allocation, we must still make sure they
+    // are ordered properly.
+    return __builtin_constant_p(__first <= __last) && __first <= __last;
+  }
+
+  return !__less<>()(__last, __first);
+}
+
+// This functions allows the compiler to assume that [__first, __last) is a valid range to be given to an algortihm.
+// Specifically, this means that
+// - [__first, __last) is dereferenceable
+// - __first and __last are correctly aligned according to the language rules
+// This allows (curently only clang-based compilers) to auto-vectorize algorithms that contain early returns.
----------------
ldionne wrote:

```suggestion
// This function allows the compiler to assume that [__first, __last) is a valid range as defined by the C++ Standard.
// Specifically, this means that
// - [__first, __last) is dereferenceable
// - __last is reachable from __first
// - if __first and __last are contiguous iterators, the pointers they "decay to" are correctly aligned according to the language rules for pointers
//
// In practice, we only add explicit assumptions for properties (1) and (3).
// These assumptions allow (curently only clang-based compilers) to auto-vectorize algorithms that contain early returns.
```

https://github.com/llvm/llvm-project/pull/167697