Cannot use dynamic runtime (/MD) in Windows

[javidcf]

Looking at msvc_tools.py.tpl, it does not seem to be possible to compile on Windows using the dynamic runtime (that is, using /MD / /MDd and LIBCMT). Even if one adds /MD to the copts of some rule (because passing --copt="/MD" does not even get correctly processed, but that's a different issue), the Python wrapper will add /MT or /MTd, overriding the option. I am trying to compile TensorFlow + some additional code as static libraries to integrate in a bigger project that uses /MD, and with the current behavior it's not possible.

The obvious fix would be to add an additional check to the script before self.AddOpt('/MT'), but I don't really know if there are any other reasons behind this. It seems to me that, if anything, /MD should be favored as the default option (which is what Visual Studio does nowadays).

I am using Bazel 0.4.0 (installed through Chocolatey) in x64 machine with Win10. My build command line looks like:

$ bazel build -c opt --cpu=x64_windows_msvc --host_cpu=x64_windows_msvc --verbose_failures --experimental_ui <target...>

[dslomov]

@meteorcloudy, wdyt?

[meteorcloudy]

I think only using /MT is just a legacy behavior after I created the wrapper script, I don't see anything special reason behind this. So, I think we can just add an additional check before adding /MT.

[javidcf]

Sorry I forgot to mention this before, but I also noted that msvc_cl.py translates the copt -g to /MTd. I guess ideally it should translate to either /MTd or /MDd depending on the existing options, but that would require further changes I guess.

[javidcf]

I have sent a PR to better handle /MT and /MD, but I have just realized msvc_link.py also seems to link to libcmt.lib / libcmtd.lib by default. I'm not sure how important this is, though, because I am being able to create a DLL containing TensorFlow and use it in another application that uses /MD using my PR. However maybe the linker script should be adapted too somehow.

[steven-johnson]

AFAICT, this still isn't workable in 0.4.5; I'm building against a library that requires /MD set, and though I am building all of my code with /MD as well, the code in msvc_link.py still assumes we want to link against a libcmt variant (rather than an msvcrt variant); in my case, this produces lots of c-runtime-related link errors.

I experimentally hacked my msvc_link.py to link against the MSVCRT.lib instead, and the link errors went away... but I can't find a way to do this without such hackage. Is there a way to fix this otherwise? Is it fixed for 0.5? If not, can it be?

[javidcf]

@steven-johnson I have recently been able to make it work for TensorFlow building Bazel from source and then, when compiling TensorFlow, passing --copt=/MD to bazel build.

[steven-johnson]

javidcf@ That's good to know, but "building Bazel from source" should not be a prerequisite for being able to make this use case work :-)

[steven-johnson]

I think this should perhaps be reopened to fix the linker issue -- it's quite possible I'm doing something wrong, but I can't get a cc_binary to link reliably when specifying /MD and linking against another (non-Bazel) Windows library that is also compiled with /MD.

[fire]

USE_DYNAMIC_CRT=1 Is the way I worked around this, but I don't know how to encode it into the BUILD.

[wangjia184]

For bazel v5.3, use --features=static_link_msvcrt parameter