Bug 1866562 - Make MSVC-style architecture constants consistent in mingwclang builds. r=tjr

This patch makes mingwclang builds compile with -include _mingw.h. This
makes MSVC-style constants defined from the start of C++ files rather
than at the point where they start including files that depend on
_mingw.h. Thus, mingwclang builds will behave more closely to regular
builds, potentially avoiding future backouts for mingwclang build
failures and/or successful but incorrect mingwclang builds.

Differential Revision: https://phabricator.services.mozilla.com/D195662

browser/config/mozconfigs/win32/mingwclang

@@ -43,6 +43,8 @@ CXX="$MOZ_FETCHES_DIR/clang/bin/i686-w64-mingw32-clang++"
 CXXFLAGS="-fms-extensions"
 CFLAGS="$CFLAGS -fcrash-diagnostics-dir=${UPLOAD_PATH}"
 CXXFLAGS="$CXXFLAGS -fcrash-diagnostics-dir=${UPLOAD_PATH}"
+CFLAGS="$CFLAGS -include _mingw.h"
+CXXFLAGS="$CXXFLAGS -include _mingw.h"
 
 # We want to make sure we use binutils and other binaries in the tooltool
 # package.

browser/config/mozconfigs/win64/mingwclang

@@ -43,6 +43,8 @@ CXX="$MOZ_FETCHES_DIR/clang/bin/x86_64-w64-mingw32-clang++"
 CXXFLAGS="-fms-extensions"
 CFLAGS="$CFLAGS -fcrash-diagnostics-dir=${UPLOAD_PATH}"
 CXXFLAGS="$CXXFLAGS -fcrash-diagnostics-dir=${UPLOAD_PATH}"
+CFLAGS="$CFLAGS -include _mingw.h"
+CXXFLAGS="$CXXFLAGS -include _mingw.h"
 
 # We want to make sure we use binutils and other binaries in the tooltool
 # package.

mfbt/tests/TestWinArchDefs.cpp

@@ -0,0 +1,58 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This code tests the consistency of architecture-specific predefined macros
+// inherited from MSVC, before and after windows.h inclusion. See
+// https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros for a
+// list of such macros.
+
+// If this test compiles, it is successful. See bug 1866562 for an example
+// where mingwclang builds were failing to compile this code.
+
+#if defined(_M_IX86)
+constexpr auto kIX86 = _M_IX86;
+#endif
+
+#if defined(_M_X64)
+constexpr auto kX64 = _M_X64;
+#endif
+
+#if defined(_M_AMD64)
+constexpr auto kAMD64 = _M_AMD64;
+#endif
+
+#if defined(_M_ARM)
+constexpr auto kARM = _M_ARM;
+#endif
+
+#if defined(_M_ARM64)
+constexpr auto kARM64 = _M_ARM64;
+#endif
+
+#include <windows.h>
+
+#if defined(_M_IX86)
+static_assert(kIX86 == _M_IX86);
+#endif
+
+#if defined(_M_X64)
+static_assert(kX64 == _M_X64);
+#endif
+
+#if defined(_M_AMD64)
+static_assert(kAMD64 == _M_AMD64);
+#endif
+
+#if defined(_M_ARM)
+static_assert(kARM == _M_ARM);
+#endif
+
+#if defined(_M_ARM64)
+static_assert(kARM64 == _M_ARM64);
+#endif
+
+// If this test compiles, it is successful.
+int main() { return 0; }

mfbt/tests/moz.build

@@ -80,6 +80,13 @@ if CONFIG["OS_ARCH"] != "WASI":
         ]
     )
 
+if CONFIG["OS_ARCH"] == "WINNT":
+    CppUnitTests(
+        [
+            "TestWinArchDefs",
+        ]
+    )
+
 # Not to be unified with the rest, because this test
 # sets MOZ_PRETEND_NO_JSRUST, which changes the behavior
 # of the included headers.

testing/cppunittest.ini

@@ -92,5 +92,7 @@ skip-if = os != 'win'
 [TestVariant]
 [TestVector]
 [TestWeakPtr]
+[TestWinArchDefs]
+skip-if = os != 'win'
 [TestWrappingOperations]
 [TestXorShift128PlusRNG]