LTCG issues with the WIN7/amd64 environment of WDK 7600

Now that Windows 7 is out, we all sooner or later have to upgrade to WDK 7600. I am still reluctant to move away from WDK 60006001 because of the dropped W2K support, but this is a different issue.

However, as one cfix user who has obviously already adopted WDK 7600 kindly pointed out to me, linking a kernel mode unit test against cfix using WDK 7600 and the WIN7/amd64 environment fails reproducibly with the following error message:

error fatal error C1047: The object or library file '...\lib\amd64\cfixkdrv.lib' was created with an older compiler than other objects; rebuild old objects and libraries

In contrast, building the same driver for WIN7/x86 works fine.

As the documentation for C1047 indicates, this error is usually related to inconsistent usage of Link Time Code Generation (LTCG): As soon as you use LTCG, all objects and libraries must be compiled with /GL – this normally is not a big deal, but as this WDK page rightfully explains, it means that libraries built this way are not suitable for redistribution because of their dependency on a specific compiler/linker version. But of couse, it also means that a library not built using /GL cannot be used easily when you build your program using LTCG.

Before Windows 7, all WDK build environment configurations I am aware of did not use LTCG. Neither did cfix, so everything worked fine even if your compiler/linker versions did not match the ones used for building cfix.

With WDK 7600, this situation changes: While WLH and other downlevel build environments still do not seem to use LTCG, the WIN7 environment, at least for amd64, enables LTCG by default.

What this means is that as soon as you link against a library which is not part of the WDK and therefore likely to be built using a different compiler version, you’ll get C1047. It is thus no surprise that attempting to link against cfixkdrv.lib, which is the library all kernel mode unit tests have to link against and which itself has been built using WDK 6000, leads to the error quoted above.

However, once you have figured this out, the workaround for this issue is trivial: Disable LTCG for your test driver by adding the following line to your SOURCES file:


Link time code generation is a very powerful optimization technique and I encourage everybody to make use of it if possible. However, for the compatibility reasons outlined above, I consider it a rather stupid idea to have the WDK enable LTCG by default. Rather, I had much preferred to see an opt-in switch for LTCG.

Anyway, for the next version, I will consider shipping both, a 7600-compatible LTCG-enabled and a non-LTCG-enabled version of cfixkdrv.lib. Until then, the workaround described above will do the trick.

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