[libcxx-commits] [libcxx] [libc++] Add a design document for Windows support (PR #202668)

[via libcxx-commits]

https://github.com/siradam7th updated https://github.com/llvm/llvm-project/pull/202668

>From 35f53b7d7a52991da9e2d5a2006448670c6e12e2 Mon Sep 17 00:00:00 2001
From: siradam7th <oss at afterback.gq>
Date: Sun, 12 Jul 2026 23:04:24 +0100
Subject: [PATCH] [libc++] add design document for Windows support (MSVC Mode)

---
 libcxx/docs/DesignDocs/WindowsSupport.rst | 22 ++++++++++++++++++++++
 libcxx/docs/index.rst                     |  1 +
 2 files changed, 23 insertions(+)
 create mode 100644 libcxx/docs/DesignDocs/WindowsSupport.rst

diff --git a/libcxx/docs/DesignDocs/WindowsSupport.rst b/libcxx/docs/DesignDocs/WindowsSupport.rst
new file mode 100644
index 0000000000000..7e1ec23a5852a
--- /dev/null
+++ b/libcxx/docs/DesignDocs/WindowsSupport.rst
@@ -0,0 +1,22 @@
+===============
+Windows support
+===============
+
+**This document only applies to the MSVC Mode.**
+
+Motivation:
+-----------
+
+Currently libc++ needs to link to the MSVC STL ``msvcprt`` library to implement part of the functionality on Windows. This dependency is counterintuitive since one standard library is depending on another standard library. Our goal is to remove this dependency by implementing the required functionality in libc++ itself.
+
+To achieve this, we have decided on the following:
+
+Goals:
+------
+
+- ``VCRuntime`` is the underlying ABI layer that libc++ will be using to implement the required functionality for Windows support.
+
+Non-Goals:
+----------
+
+- Interoperability with the MSVC STL in the case where both libraries are linked in the same binary is not a goal. For example if you link both libc++ and the MSVC STL in the same binary, a call to a function that needs to maintain internal state like ``std::set_new_handler`` from the libc++ side should not be expected to also change the MSVC STL internal state (since they are separate). We will be maintaining our own internal state for such functions, therefore any such functions should only be expected to have an effect on the libc++ side.
diff --git a/libcxx/docs/index.rst b/libcxx/docs/index.rst
index 38f77580d4530..796da1136584f 100644
--- a/libcxx/docs/index.rst
+++ b/libcxx/docs/index.rst
@@ -229,6 +229,7 @@ Design Documents
    DesignDocs/UnspecifiedBehaviorRandomization
    DesignDocs/VisibilityMacros
    DesignDocs/TimeZone
+   DesignDocs/WindowsSupport
 
 
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