[swift-dev] Handling Windows C library Matrix

Saleem Abdulrasool compnerd at compnerd.org
Wed Jun 29 09:44:37 CDT 2016


Windows has an interesting (different) model for handling the C library.
There are *four* different libraries and you select the library you want
across two axis.  Traditionally the driver controls the selected library
via flags (and options exposed to the developer via the IDE).  Im not sure
what is the best way to handle that with swift.

This table explains how the library selection works:

                           Debug         |      Release
statically linked |  libcmtd.lib (/MTd)  |  libcmt.lib (/MT)
dyamically linked |  msvcrtdl.lib (MDd)  |  msvcrt.lib (/MD)

Now, there are certain advantages to both linkage models.  The static
linkage means that you don't have any dependency on the VC runtime
redistributable at the cost of small binary.  The dynamic linkage means
that you have a dependency on the VC runtime redistributable, but you have
a smaller binary and future updates can update libc.

The library selection unfortunately goes further than just linkage.  The
flags control other flags which matter for the C library interfaces (DLL

The crux of the issue is, how to model this behavior in the swift driver,
since the only places where this matters will be the ClangImporter and the
standard library build, but the latter can be addressed much more easily as
it would be internal to the build and not exposed to the users.

If we are looking for precedent, the clang driver introduced a
`--dependent-lib` CC1 option (mapping /MTd, /MT, /MDd, /MD to it

Note that I am *NOT* advocating a second driver interface, because I think
that will just cause unnecessary work and confusion.  As long as we have
options or some means t control the interface, I think having a single
unified driver interface is a significantly better approach.

Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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