[Build] Linux→Windows cross-compile toolchain (clang-cl + xwin)

New CMake preset `cross-win-clangcl` for Ninja Multi-Config on
non-Windows hosts. Toolchain: clang-cl (MSVC-ABI), lld-link, xwin SDK/CRT
splat. Shader compilation routes through wine fxc.exe with FXC_PATH
env forwarding and unix→Windows path translation via `winepath -w`.
Plus per-file build fixes (constexpr→const, llvm-rc forward-slash,
zlib-ng AVX guard, /RTCsu MSVC-only). third_party/snappy bumped to
fabi/sylpheed-crossbuild for the target-aware POSIX-gate header.

Produces `xenia_canary.exe` (Debug MSVC) suitable for use as the
audit oracle under Wine for xenia-rs work. See
docs/CROSS_BUILD_SETUP.md for the full reproduction recipe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmake/toolchains/linux-to-win-msvc.cmake

@@ -0,0 +1,119 @@
+# cmake/toolchains/linux-to-win-msvc.cmake
+# Linux host -> Windows MSVC-ABI cross toolchain using clang-cl + lld-link
+# + xwin-supplied Win10 SDK/CRT. Driven by Ninja Multi-Config.
+
+set(CMAKE_SYSTEM_NAME      Windows)
+set(CMAKE_SYSTEM_PROCESSOR x86_64)
+
+if(NOT DEFINED XWIN_DIR)
+  if(DEFINED ENV{XWIN_DIR})
+    set(XWIN_DIR "$ENV{XWIN_DIR}")
+  else()
+    set(XWIN_DIR "$ENV{HOME}/.xwin/splat")
+  endif()
+endif()
+set(XWIN_DIR "${XWIN_DIR}" CACHE PATH "xwin splat root (contains crt/ and sdk/)")
+
+if(NOT EXISTS "${XWIN_DIR}/crt/include")
+  message(FATAL_ERROR "XWIN_DIR=${XWIN_DIR} missing crt/include - run xwin splat.")
+endif()
+
+set(CMAKE_C_COMPILER   clang-cl)
+set(CMAKE_CXX_COMPILER clang-cl)
+set(CMAKE_LINKER       lld-link)
+set(CMAKE_RC_COMPILER  llvm-rc)
+set(CMAKE_AR           llvm-lib)
+set(CMAKE_MT           llvm-mt)
+
+set(CMAKE_C_COMPILER_TARGET   x86_64-pc-windows-msvc)
+set(CMAKE_CXX_COMPILER_TARGET x86_64-pc-windows-msvc)
+
+set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+
+# Force /MD (release CRT) for every config. xenia's CMakeLists.txt does a
+# `string(REPLACE "/MDd" "/MD" ...)` on CMAKE_CXX_FLAGS_DEBUG, but with
+# clang-cl, the runtime selection comes from CMAKE_MSVC_RUNTIME_LIBRARY
+# (which expands to -MDd in dash form), so the substitution misses.
+# Pinning the policy here avoids the need for non-redistributable debug
+# CRT DLLs (MSVCP140D.dll etc) at runtime, which xwin doesn't ship.
+cmake_policy(SET CMP0091 NEW)
+set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDLL")
+
+# xwin's default splat is a flat layout (crt/include, sdk/include/{ucrt,um,shared,...}),
+# which clang-cl's /winsysroot does NOT understand. Use explicit -imsvc + -libpath:
+# instead. Re-running `xwin splat --use-winsysroot-style` would also work but
+# requires re-downloading ~600 MB.
+# Use SHELL: prefix so CMake doesn't deduplicate repeated -imsvc tokens.
+add_compile_options(
+  "SHELL:-imsvc \"${XWIN_DIR}/crt/include\""
+  "SHELL:-imsvc \"${XWIN_DIR}/sdk/include/ucrt\""
+  "SHELL:-imsvc \"${XWIN_DIR}/sdk/include/um\""
+  "SHELL:-imsvc \"${XWIN_DIR}/sdk/include/shared\""
+  "SHELL:-imsvc \"${XWIN_DIR}/sdk/include/winrt\""
+)
+add_link_options(
+  "/libpath:${XWIN_DIR}/crt/lib/x86_64"
+  "/libpath:${XWIN_DIR}/sdk/lib/ucrt/x86_64"
+  "/libpath:${XWIN_DIR}/sdk/lib/um/x86_64"
+)
+# xwin pulls the latest MSVC STL which now hard-asserts Clang >= 19. Our host
+# has Clang 18, which works fine in practice — opt out of the version check.
+# (See yvals_core.h STL1000 in $XWIN_DIR/crt/include.)
+add_compile_definitions(_ALLOW_COMPILER_AND_STL_VERSION_MISMATCH)
+# llvm-rc needs SDK headers explicitly + the resource file's own dir so the
+# RC's relative ICON path (../../../assets/icon/icon.ico) resolves.
+get_filename_component(_toolchain_dir "${CMAKE_CURRENT_LIST_FILE}" DIRECTORY)
+get_filename_component(_xenia_root "${_toolchain_dir}/../.." ABSOLUTE)
+set(CMAKE_RC_FLAGS_INIT
+    "/I \"${XWIN_DIR}/sdk/include/um\" /I \"${XWIN_DIR}/sdk/include/shared\" /I \"${_xenia_root}/src/xenia/app\"")
+
+# Quiet MSVC-STL false positives and xenia-specific warnings that clang-cl
+# emits but cl.exe doesn't (treated as errors under /WX).
+add_compile_options(
+  -Wno-microsoft-include
+  -Wno-unused-command-line-argument
+  -Wno-ignored-pragma-intrinsic
+  -Wno-nonportable-include-path
+  -Wno-pragma-pack
+  -Wno-tautological-pointer-compare
+  -Wno-microsoft-cast
+  -Wno-deprecated-declarations
+  # These are silenced for native Linux Clang in CMakeLists.txt's else() branch,
+  # but the if(MSVC) branch fires under clang-cl and skips them — so re-add.
+  -Wno-switch
+  -Wno-attributes
+  -Wno-deprecated-register
+  -Wno-deprecated-volatile
+  -Wno-deprecated-enum-enum-conversion
+  # cl.exe accepts __pragma(optimize("s",on)); clang-cl only knows the empty
+  # argument form. xenia gates XE_MSVC_OPTIMIZE_SMALL on _MSC_VER so the
+  # rejected pragma still emits from the clang-cl path. Treat as no-op.
+  -Wno-ignored-pragmas
+  # xenia decorates several `virtual` methods with XE_FORCEINLINE; clang-cl
+  # then complains that the inline body isn't visible in includer TUs
+  # (definitions live in command_processor.cc). cl.exe accepts this silently.
+  -Wno-undefined-inline
+  -Wno-sizeof-pointer-memaccess
+  # `'ZM'` (PE magic, stored little-endian as "MZ" in the binary) — cl.exe
+  # accepts the multi-char constant silently; clang-cl errors under /WX.
+  -Wno-multichar
+)
+# _mm_cvtsi64x_si128 is an MSVC-only alias for the standard _mm_cvtsi64_si128.
+# Used (gated on XE_PLATFORM_WIN32) in xenia/gpu/draw_util.cc.
+add_compile_definitions(_mm_cvtsi64x_si128=_mm_cvtsi64_si128)
+
+# Plumb FXC for tools/build/compile_shader_dxbc.py (wine fxc auto-prepend).
+# Use real Win10 SDK fxc.exe 10.x (supports SM 5_1, produces vkd3d-proton-
+# acceptable DXBC).
+if(DEFINED ENV{FXC_PATH})
+  set(ENV_FXC "$ENV{FXC_PATH}")
+else()
+  set(ENV_FXC "$ENV{HOME}/.local/share/xenia-cross/fxc/fxc.exe")
+endif()
+set(ENV{FXC_PATH} "${ENV_FXC}")
+
+set(CMAKE_FIND_ROOT_PATH "${XWIN_DIR}")
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

cmake/toolchains/xwin-case-symlinks.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python3
+"""Scan xenia source for #include <...> directives and ensure case-aliases
+exist in xwin's flat SDK/CRT include dirs. Idempotent — safe to re-run."""
+from __future__ import annotations
+import os, re, sys
+from pathlib import Path
+
+INCLUDE_RE = re.compile(r'^\s*#\s*include\s*<([A-Za-z][A-Za-z0-9_/.\-]*\.h)>',
+                        re.MULTILINE)
+SOURCE_ROOTS = ["src", "third_party/SDL2/src", "third_party/SDL2/include",
+                "third_party/discord-rpc/src", "third_party/fmt",
+                "third_party/imgui"]
+SOURCE_EXTS = {".h", ".hpp", ".inc", ".c", ".cc", ".cpp", ".cxx"}
+
+def collect_includes(repo_root: Path) -> set[str]:
+    seen: set[str] = set()
+    for root in SOURCE_ROOTS:
+        base = repo_root / root
+        if not base.exists(): continue
+        for path in base.rglob("*"):
+            if path.suffix.lower() not in SOURCE_EXTS: continue
+            try: txt = path.read_text(encoding="utf-8", errors="ignore")
+            except OSError: continue
+            for m in INCLUDE_RE.finditer(txt):
+                seen.add(m.group(1))
+    return seen
+
+def fix_dir(target_dir: Path, want: set[str]) -> int:
+    if not target_dir.is_dir(): return 0
+    idx = {e.name.lower(): e.name for e in target_dir.iterdir()}
+    created = 0
+    for name in want:
+        if "/" in name: continue
+        actual = idx.get(name.lower())
+        if actual is None or actual == name: continue
+        link = target_dir / name
+        if link.exists() or link.is_symlink(): continue
+        try:
+            link.symlink_to(actual)
+            created += 1
+        except OSError: pass
+    return created
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        sys.exit("usage: xwin-case-symlinks.py <xenia-canary-root> <xwin-splat-dir>")
+    repo = Path(sys.argv[1]).resolve()
+    xwin = Path(sys.argv[2]).resolve()
+    want = collect_includes(repo)
+    total = 0
+    for d in (xwin/"crt/include", xwin/"sdk/include/ucrt",
+              xwin/"sdk/include/um", xwin/"sdk/include/shared",
+              xwin/"sdk/include/winrt"):
+        total += fix_dir(d, want)
+    print(f"Created {total} case-symlinks across {len(want)} unique includes.")