rt1 — a one-shot clang-cl / lld overlay for ARM32 Windows (LLVM 18.1.8)

rt1 is a small binary overlay onto the official LLVM 18.1.8 win64 package, for people compiling C and C++ for ARM32 Windows (thumbv7) with clang-cl and lld-link. It fixes four code-generation and linker defects that produce silently wrong ARM32 binaries, and four narrower ones — see the patch table in RELEASE-NOTES.md. If you do not target ARM32 Windows, you do not need it.

It is not a maintained fork: released once, no rebases, no version bumps, no updates planned. Base is llvmorg-18.1.8 (3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff).

Install

Install the official LLVM 18.1.8 win64 package, then copy the release archive's bin/ and lib/clang/18/include/ over it. Nothing else in the installation is touched.

Every binary embeds the build commit 7653a13d8d28189c7cef1401b934ec30d98ea353:

findstr /M /C:"7653a13d8d28189c7cef1401b934ec30d98ea353" <binary>     (cmd.exe)
grep -ac 7653a13d8d28189c7cef1401b934ec30d98ea353 <binary>           (git-bash/WSL)

findstr prints the filename and exits 0; grep -ac prints 2. Both were run against all nine shipped binaries and against stock 18.1.8 as a control, which both correctly reject. stage/MODIFICATIONS.txt carries the full per-family recipe.

What is here

RELEASE-NOTES.md	known differences from MSVC, known limitations, the porting checklist, provenance
DISPOSITION-TABLE.md	one verdict for every defect examined during this work — including the ones not fixed, and why
patches/	the eight patches as readable diffs
stage/MODIFICATIONS.txt	the Apache-2.0 §4(b) modification notice, and the package inventory
tools/, scratch/	the scanners, instruments and per-defect evidence the two documents cite

The scratch/ paths look internal because they are: they are preserved exactly as the documents cite them, so every citation in DISPOSITION-TABLE.md resolves to a file you can open. A verdict whose instrument you cannot inspect is an assertion, not evidence.

Rebuilding

stage/MODIFICATIONS.txt (SOURCE section) carries the configure command line that produced these binaries and the two ways to obtain the source. One flag is not obvious and is required: -DLLVM_ENABLE_DIA_SDK=OFF.

Upstream

Nothing has been filed upstream. Where DISPOSITION-TABLE.md says a filing is prepared but not yet submitted, that is exact: drafts exist outside this repository and no issue or pull request has been submitted. Issue numbers that do appear are other people's existing upstream discussions, cited as context.

AI assistance

This work was heavily AI-assisted and human-directed. The direction, the standard of evidence, the corrections and the validation are the author's, and the author is responsible for every patch and every claim in this repository. Upstream contributions, if and when they are made, will carry disclosure per LLVM's policy.

Licence

Apache License v2.0 with LLVM Exceptions, as LLVM itself. See LICENSE.TXT in the release archive and the modification notice in stage/MODIFICATIONS.txt.