Treat 32-bit compiles on 64-bit Windows as native

It's a terrible user experience to force people building 32-bit
applications on 64-bit Windows to use a cross-info file when every other
tool treats it as a 'native' compilation -- it satisfies all the
requirements for a native compile.

This commit also fixes the platform detection on Windows which would
cause the 'native cpu' to be detected as 32-bit if you installed 32-bit
Python on 64-bit Windows, or if you were building with a 32-bit
toolchain on 64-bit Windows.

Doesn't support MinGW yet -- the next commits will add that since the
changes required for that are more involved.

2 files changed, 80 insertions, 13 deletions

diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 341e5e8..79c09ba 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -57,26 +57,90 @@ def detect_ninja():
         if p.returncode == 0 and mesonlib.version_compare(version, ">=1.6"):
             return n
 
-def detect_cpu_family():
+def detect_native_windows_arch():
+    """
+    The architecture of Windows itself: x86 or amd64
+    """
+    # These env variables are always available. See:
+    # https://msdn.microsoft.com/en-us/library/aa384274(VS.85).aspx
+    # https://blogs.msdn.microsoft.com/david.wang/2006/03/27/howto-detect-process-bitness/
+    arch = os.environ.get('PROCESSOR_ARCHITEW6432', '').lower()
+    if not arch:
+        try:
+            # If this doesn't exist, something is messing with the environment
+            arch = os.environ['PROCESSOR_ARCHITECTURE'].lower()
+        except KeyError:
+            raise InterpreterException('Unable to detect native OS architecture')
+    return arch
+
+def detect_windows_arch(compilers):
+    """
+    Detecting the 'native' architecture of Windows is not a trivial task. We
+    cannot trust that the architecture that Python is built for is the 'native'
+    one because you can run 32-bit apps on 64-bit Windows using WOW64 and
+    people sometimes install 32-bit Python on 64-bit Windows.
+
+    We also can't rely on the architecture of the OS itself, since it's
+    perfectly normal to compile and run 32-bit applications on Windows as if
+    they were native applications. It's a terrible experience to require the
+    user to supply a cross-info file to compile 32-bit applications on 64-bit
+    Windows. Thankfully, the only way to compile things with Visual Studio on
+    Windows is by entering the 'msvc toolchain' environment, which can be
+    easily detected.
+
+    In the end, the sanest method is as follows:
+    1. Check if we're in an MSVC toolchain environment, and if so, return the
+       MSVC toolchain architecture as our 'native' architecture.
+    2. If not, check environment variables that are set by Windows and WOW64 to
+       find out the architecture that Windows is built for, and use that as our
+       'native' architecture.
+    """
+    os_arch = detect_native_windows_arch()
+    if os_arch != 'amd64':
+        return os_arch
+    # If we're on 64-bit Windows, 32-bit apps can be compiled without
+    # cross-compilation. So if we're doing that, just set the native arch as
+    # 32-bit and pretend like we're running under WOW64. Else, return the
+    # actual Windows architecture that we deduced above.
+    for compiler in compilers.values():
+        # Check if we're using and inside an MSVC toolchain environment
+        if compiler.id == 'msvc' and 'VCINSTALLDIR' in os.environ:
+            # 'Platform' is only set when the target arch is not 'x86'.
+            # It's 'x64' when targetting x86_64 and 'arm' when targetting ARM.
+            platform = os.environ.get('Platform', 'x86').lower()
+            if platform == 'x86':
+                return platform
+        if compiler.id == 'gcc':
+            # TODO: Implement this
+            pass
+    return os_arch
+
+def detect_cpu_family(compilers):
     """
     Python is inconsistent in its platform module.
     It returns different values for the same cpu.
     For x86 it might return 'x86', 'i686' or somesuch.
     Do some canonicalization.
     """
-    trial = platform.machine().lower()
+    if mesonlib.is_windows():
+        trial = detect_windows_arch(compilers)
+    else:
+        trial = platform.machine().lower()
     if trial.startswith('i') and trial.endswith('86'):
         return 'x86'
     if trial.startswith('arm'):
         return 'arm'
-    if trial == 'amd64':
+    if trial in ('amd64', 'x64'):
         return 'x86_64'
     # Add fixes here as bugs are reported.
     return trial
 
-def detect_cpu():
-    trial = platform.machine().lower()
-    if trial == 'amd64':
+def detect_cpu(compilers):
+    if mesonlib.is_windows():
+        trial = detect_windows_arch(compilers)
+    else:
+        trial = platform.machine().lower()
+    if trial in ('amd64', 'x64'):
         return 'x86_64'
     # Add fixes here as bugs are reported.
     return trial
@@ -846,10 +910,12 @@ class CrossBuildInfo():
         return 'host_machine' in self.config
 
     def need_exe_wrapper(self):
-        if self.has_host() and detect_cpu_family() == 'x86_64' and \
+        # Can almost always run 32-bit binaries on 64-bit natively if the host
+        # and build systems are the same. We don't pass any compilers to
+        # detect_cpu_family() here because we always want to know the OS
+        # architecture, not what the compiler environment tells us.
+        if self.has_host() and detect_cpu_family({}) == 'x86_64' and \
            self.config['host_machine']['cpu_family'] == 'x86' and \
            self.config['host_machine']['system'] == detect_system():
-            # Can almost always run 32-bit binaries on 64-bit natively if the
-            # host and build systems are the same
             return False
         return True
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 0f00c5a..4412ffe 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -375,7 +375,8 @@ class GeneratedListHolder(InterpreterObject):
         self.held_object.add_file(a)
 
 class BuildMachine(InterpreterObject):
-    def __init__(self):
+    def __init__(self, compilers):
+        self.compilers = compilers
         InterpreterObject.__init__(self)
         self.methods.update({'system' : self.system_method,
                              'cpu_family' : self.cpu_family_method,
@@ -384,10 +385,10 @@ class BuildMachine(InterpreterObject):
                             })
 
     def cpu_family_method(self, args, kwargs):
-        return environment.detect_cpu_family()
+        return environment.detect_cpu_family(self.compilers)
 
     def cpu_method(self, args, kwargs):
-        return environment.detect_cpu()
+        return environment.detect_cpu(self.compilers)
 
     def system_method(self, args, kwargs):
         return environment.detect_system()
@@ -1099,7 +1100,7 @@ class Interpreter():
         self.variables = {}
         self.builtin = {}
         self.parse_project()
-        self.builtin['build_machine'] = BuildMachine()
+        self.builtin['build_machine'] = BuildMachine(self.coredata.compilers)
         if not self.build.environment.is_cross_build():
             self.builtin['host_machine'] = self.builtin['build_machine']
             self.builtin['target_machine'] = self.builtin['build_machine']