Finding the mingw-w64 runtime library code

It is difficult to guarantee that a compiled Python extension will be able to find the correct version of runtime DLLs – see: Windows DLL notes.

One method to avoid this problem is to include any necessary runtime code in the compiled extension. This can be done by:

  • Compiling all extensions (DLLs) with -static-libgcc, -static-libstdc++ etc flags;
  • Using a gcc compiler toolchain that does static linking to runtimes by default. This is a “static toolchain”. The mingw-builds set of scripts builds such a toolchain by passing the --static-gcc argument (see: mingw-build build file;

Using these approaches, all the necessary object code from the GCC runtimes will be statically linked into the binaries. As a consequence the binary size will be increased in comparison to the standard toolchains / dynamic linking. The advantage is, that there will be no dependency to external GCC runtime libraries, so the deployment of Python extensions is greatly improved.

However, exception heavy C++ programs such as QT should be compiled with shared runtimes to avoid problems with exception handling over DLL boundaries.

For building typically Python extensions a customized static GCC toolchain is the best compromise IMHO.

Note from njs: This definitely sounds like the right thing to try initially, but it’s possible (likely?) that we’ll run into the same problem that Steve Dower ran into with trying to set up MSVC to statically link the runtime code, i.e., TLS keys are a limited resource, so if libgcc allocates any TLS keys at startup, and every extension module has its own copy of libgcc, then this puts a hard cap on how many Python extension modules can be loaded into a single process. Which is really bad (consider projects like Sage that make heavy use of Cython, and each Cython source file creates a separate extension...). Or at least, it’s really bad IF it’s a problem at all. In fact I have no idea whether libgcc allocates any TLS keys at startup – introspecting a handy copy of libgcc*.dll shows that it does contain references to TlsAlloc, but I don’t know whether they’re called under ordinary circumstances. So we should start with static linking, but have a fallback plan in case it doesn’t work.

Worst case, I think our fallback plan is to switch back to dynamic linking, and require projects to use one of the standard dynamic linking strategies that we’ve discussed elsewhere. Possibly this means the mingwpy project will eventually want to start shipping a mingw-w64-runtime wheel, similar to what we’ve discussed doing with BLAS dlls, that projects could declare a dependency on.

MS / gcc ABI incompatibilities

There is a win32 default stack alignment incompatibility: GCC code provides (and assumes) 16 (2^4) byte stack alignment since GCC4.6, but MSVC uses 4 (2^2) byte stack alignment.

Win64 X86_64 is not affected. This issue is the major cause for segment faults on 32bit systems.

The solution is to use the -mincoming-stack-boundary=2 flag for compiling.


Stack alignment discussion: GCC upstream agrees the default should be compatible with MSVC, however it’s not. GCC will likely not accept a patch to go back to the pre-4.6 stack alignment. Needs follow-up with Kai Tietz (he asked for a test case). Note: 32-bit only problem.

Some googling suggests that the case where this problem will arise specifically is when you combine functions that contain SSE instructions (which are less forgiving of bad alignment than most of the x86 ISA), then called from code that doesn’t guarantee the right stack alignment – online hints suggest that this tends to happen with windows threads. This is consistent with the observation that OpenBLAS in particular blows up without this flag (b/c OpenBLAS is a heavy user of SSE + threads).

(See: 1, 2)

It’s likely that a sufficient test case to give to Kai would be a program that spawns a bunch of threads running the following function:

DWORD WINAPI thread(LPVOID param) {
    int x;
    intptr_t address = (intptr_t) &x;
    if (address % 16 != 0) {
        printf("Thread stack %p is not 16-byte aligned!\n", &x);

and if this prints any output then we have our proof that -mincoming-stack-boundary=2 is necessary for Windows/MSVC compatibility.

Choice of MSVC runtime

All code in a single process should use one single version of the MSVC runtime (see MSDN article).

The Python that gets installed from downloading from is build with MSVC. Therefore Python extensions for Python installed from these installers must use the same MSVC CRT.

Specifically (see: Python and MSVC versions):

Python version VC++ version C runtime
2.7.6 9.0 / 2008 MSVCR90.DLL
3.2.3 9.0 / 2008 MSVCR90.DLL
3.3.5 10.0 / 2010 MSVCR100.DLL
3.4.0 10.0 / 2010 MSVCR100.DLL
3.5.0 14.0 / 2015 UCRTBASE.DLL / VCRUNTIME140.DLL

By default, mingw-w64 links to the MSVCRT.DLL. This is a CRT dating from MSVC 4.2, but updated to contain the runtimes for MSVC 6.0, plus some more recent (>6.0) API calls (see these comments on using MSVCRT.DLL from Mingw-w64).

It is possible, using spec files, to ask mingw-w64 to link to other versions of the MSVC runtimes. This could induce bad behavior if there is any API mismatch between the implementations in the mingw-w64 headers (tuned to MSVCRT.DLL) and those in the newer C runtime.

What workarounds are necessary to use MSVC 9.0 / 2008 (Python 2.7)?

The VS 14 / 2015 runtime

Matters get more confusing for the latest (at time of writing) MSVC, version 14 (MSVS 2015).

Firstly, the CRT is now two files:

  • ucrtbase.dll;
  • vcruntime140.dll;

of which the first will be kept with a backwards-compatible API / ABI across new VS releases.

Second, linking correctly to these new 2015 libraries requires some careful handling of the DLL import library (the ”.lib” or ”.a” file that’s used for actually linking to a .dll on Windows). We don’t want to use the standard mingw-w64 trick of dumping the symbols from ucrtbase.dll and using them to generate a import library; that will do the wrong thing. To understand why and what the right thing is, read this:

So that’s how to handle ucrtbase.dll. 99% of the things we care about are in the safe unversioned ucrtbase.dll. There remains some question of what to do with vcruntime140.dll. The ideal would be that our toolchain simply doesn’t link to it at all. It looks like the symbols it provides are:

[Ordinal/Name Pointer] Table
      [   0] _CreateFrameInfo
      [   1] _CxxThrowException
      [   2] _FindAndUnlinkFrame
      [   3] _IsExceptionObjectToBeDestroyed
      [   4] _SetWinRTOutOfMemoryExceptionCallback
      [   5] __AdjustPointer
      [   6] __BuildCatchObject
      [   7] __BuildCatchObjectHelper
      [   8] __C_specific_handler
      [   9] __C_specific_handler_noexcept
      [  10] __CxxDetectRethrow
      [  11] __CxxExceptionFilter
      [  12] __CxxFrameHandler
      [  13] __CxxFrameHandler2
      [  14] __CxxFrameHandler3
      [  15] __CxxQueryExceptionSize
      [  16] __CxxRegisterExceptionObject
      [  17] __CxxUnregisterExceptionObject
      [  18] __DestructExceptionObject
      [  19] __FrameUnwindFilter
      [  20] __GetPlatformExceptionInfo
      [  21] __NLG_Dispatch2
      [  22] __NLG_Return2
      [  23] __RTCastToVoid
      [  24] __RTDynamicCast
      [  25] __RTtypeid
      [  26] __TypeMatch
      [  27] __current_exception
      [  28] __current_exception_context
      [  29] __intrinsic_setjmp
      [  30] __intrinsic_setjmpex
      [  31] __processing_throw
      [  32] __report_gsfailure
      [  33] __std_exception_copy
      [  34] __std_exception_destroy
      [  35] __std_terminate
      [  36] __std_type_info_compare
      [  37] __std_type_info_destroy_list
      [  38] __std_type_info_hash
      [  39] __std_type_info_name
      [  40] __telemetry_main_invoke_trigger
      [  41] __telemetry_main_return_trigger
      [  42] __unDName
      [  43] __unDNameEx
      [  44] __uncaught_exception
      [  45] __uncaught_exceptions
      [  46] __vcrt_GetModuleFileNameW
      [  47] __vcrt_GetModuleHandleW
      [  48] __vcrt_InitializeCriticalSectionEx
      [  49] __vcrt_LoadLibraryExW
      [  50] _get_purecall_handler
      [  51] _get_unexpected
      [  52] _is_exception_typeof
      [  53] _local_unwind
      [  54] _purecall
      [  55] _set_purecall_handler
      [  56] _set_se_translator
      [  57] longjmp
      [  58] memchr
      [  59] memcmp
      [  60] memcpy
      [  61] memmove
      [  62] memset
      [  63] set_unexpected
      [  64] strchr
      [  65] strrchr
      [  66] strstr
      [  67] unexpected
      [  68] wcschr
      [  69] wcsrchr
      [  70] wcsstr

So, numbers 0 through 56 appear to me (= njs) to be internal machinery used for C++ exceptions and RTTI. I’m pretty sure this stuff is all irrelevant to us, because libgcc should already be providing the equivalent machinery needed for any C++ code that’s compiled using gcc, and trying to achieve C++ ABI compatibility between MSVC and gcc is outside the scope of this project.

Of the rest, set_unexpected and unexpected can be ignored, because they’re yet more C++-related nonsense that apparently isn’t even used anymore), but the others are real standard C functions, and a quick check reveals that most of them are not available in ucrtbase.dll, but only here in vcruntime140.dll.

These can be broken down further into two categories:

  1. Simple string functions: memchr, memcmp, memcpy, memmove, memset, strchr, strrchr, strstr, wcschr, wcsrchr, wcsstr. These are crucial functions that we definitely need to support, but there’s no particular advantage to using the implementation from vcruntime140.dll. They can just be reimplemented inside mingw-w64. (Basic working versions are trivial; making them fast takes more work, but could be lifted from BSD libc or whatever.)

  2. setjmp / longjmp: Reimplementing these from scratch would be a huuuuge hassle. Fortunately, they’re a very advanced and tricky feature that’s rarely useful! ...But unfortunately, both numpy and scipy actually use them. Numpy’s usage might be optional (it has fallback code for if they’re not available, and the only cost would be that you couldn’t hit control-C to interrupt a long-running inner-loop – and this may not even work on Windows in the first place), but scipy uses them for actual flow control D-:. It’s possible this could just be fixed in scipy if necessary.

    setjmp/longjmp may also be needed for exception handling to work in 32-bit mingw-w64-compiled C++ code. Though I’m actually a bit confused on this point, since the copy of libgcc_s_sjlj-1.dll that I have on my Debian box doesn’t actually seem to contain any references to the setjmp or longjmp symbols in the CRT?

    On further investigation it looks like maybe the reason libgcc does not import setjmp/longjmp from the CRT is that it is built to use the gcc builtins __builtin_setjmp / __builtin_longjmp instead? If these exist and are functional in mingw-w64 builds then that is an even better solution – we just need to set up the headers so that user code that tries to call setjmp/longjmp are redirected to the builtins. Maybe. Need to check with mingw-w64 folks about this.

Math precision issues

(Quoting Nathaniel Smith on email):

mingw-w64’s libm implementations are the borrowed from those used on BSDs, Linux, etc., and assume – consistent with the ABI on those other platforms – that the x87 FPU will be configured to use 80 bit precision for intermediate results. MSVC’s ABI, though, configures the x87 FPU into 64 bit precision mode, and we don’t want to override that because who knows what would break. The result is that when run in MSVC-compatibility mode, mingw-w64’s libm code currently assumes that it has higher internal precision than it actually has, and doesn’t necessarily produce the right answers. In particular the trig functions currently are just thin wrappers around the x87 fsin / fcos / etc. instructions. These are pretty sloppy and inaccurate, which doesn’t matter too much if you’re going to throw away all the low-order bits anyway... but if you’re going to keep those bits then it becomes more of an issue.


Investigating sleef library for needed functions. Nathaniel suggested libm implementation from bionic (Android’s libc).

Bionic’s libm is here: Basically what would be needed is just copying these files into mingw-w64 plus build system updates.

Distutils issues

Mingw via Python distutils needs to link extension code to the correct Python library, e.g. libpython27.dll. To do this, mingw needs a library definition file libpython27.a. It is possible to create this file using the mingw-w64 tools.

Query from njs: can’t we just use the python27.lib that’s shipped with CPython itself?

numpy distutils currently needs patching to pass correct flags to compiler / linker.

How to return correct flags to mingw-w64, from Python built with MSVC?

Manifest resources

Solution is to extend the GCC toolchain with the Manifest resource files and ensure linkage with the help of the ‘specs’ file.

If this is the solution, then what’s the problem? Carl, could you add some text here elaborating? (and then delete this comment :-)) -njs

BLAS / LAPACK libraries

There is no silver bullet for the problem of finding fast, reliable BLAS and LAPACK routines with a suitable license.

See BLAS / LAPACK on Windows.

Partial to-do list

  • Implement linking to MSVC 2015 CRT libraries from mingw-w64;
    • work out correct method;
    • discuss with MS developers;
    • merge to mingw-w64;
  • Implement high-precision libm;
    • decide on library;
    • merge to mingw-w64;
  • Develop “runtime agility” for mingw-w64 - method of adapting to different MSVC CRT libraries dynamically. Maybe 50-60K of developer work / 2 man months. On back-burner for mingw-w64 project.
  • Create buildbot / appveyor scripts to build mingwpy library;
  • Create test rig for OpenBLAS maybe via numpy, implement on buildbots with different processors or gcc compile farm.