clang on a Windows host keeps \ in __FILE__ for a POSIX target under -ffile-reproducible: an ok image built on Windows differs from one built on Linux/macOS #850

Observation

An ok image (x86_64-openkal) built on a Windows host is not the bytes of the same image built on Linux or macOS. The difference is two __FILE__ strings from libc++abi's demangler:

Measured with a probe building one image (import std, an exception, std::source_location, [profile.release] trim_paths = true) with mcpp release/2026.10.27.1 and llvm@23.1.3 on ubuntu-24.04, macos-15 and windows-2022 (mcpplibs/openkal-platform, branch probe/image-bytes). The Linux and macOS images share one sha256; the Windows one differs in 3,921 bytes, all following from those two strings (.rodata, then .text and .rela.dyn offsets).

Cause (upstream)

cxa_demangle.cpp includes "demangle/ItaniumDemangle.h", found relative to the includer's directory; clang joins that directory and the spelled name with the host's separator (llvm::sys::path::append, native style), so on Windows the path is …/src\demangle/ItaniumDemangle.h. mcpp maps the package's prefix (-ffile-prefix-map) and, since #847, passes -ffile-reproducible with trim_paths. -ffile-reproducible is documented as using the target's separator in __FILE__, but for a non-Windows target Preprocessor::processPathForFileMacro only calls remove_dots(…, Style::posix), which does not treat \ as a separator, so the host's separator stays. For a Windows target the flag does normalise (to \).

Consequence

• mcpp pack --merge correctly refuses a package whose images were built on Windows together with one built elsewhere (two digests for one path).
• A universal package's images must be built on Linux or macOS for one image per architecture. mcpp builds every system's runner over the platform's hosted arrangement on any machine (2026.10.27.1: platforms (SPEC-013) -- ok images need no declaration; packaging and distribution (SPEC-014) -- three axes, universal packages, single files, okb #847), so a Linux or macOS machine packs the runners of all three systems; mcppls and mcpp-ide pack there.

Workaround in mcpp

None in the build: the joined directory is the includer's and depends on which files include which, so a prefix map per directory of every package would be needed, which the command-line length on Windows does not allow for packages of libc++'s size. The packaging spec states the rule (SPEC-014 §4.6) and the release notes do not claim host-independent images for Windows hosts.

To be reported to LLVM: -ffile-reproducible keeps the host's \ in __FILE__ for a POSIX target when the path was joined on a Windows host.