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Summary: Import libraries are files ending in `.dll.a` and `.lib` depending on which compiler creates them (GCC, vs MSVC). Import Libraries are standard `archive` files that contain object files. These object files can have two different formats: 1) The normal COFF Object format for object files (contains all ascii data and very little program code, so do not try to execute.) 2) "short import" format which just contains a symbol name and the dll in which the symbol can be found. Import Libraries are useful for two things: 1) Allowing applications that don't support dynamic linking to link against the import lib (non-short format) which then makes calls into the DLL by loading it at runtime. 2) Allow linking of mutually recursive dlls. if `A.DLL` requires `B.DLL` and vice versa, import libs can be used to break the cycle as they can be created from the expected exports of the DLLs. A side effect of having these two capabilities is that Import libs are often used to hide specific versions of DLLs behind a non-versioned import lib. e.g. GCC_S.a (non-conventional import lib) will point to the correct `libGCC` DLL. With this support Windows Haskell files can now just link to `-lGCC_S` and not have to worry about what the actual name of libGCC is. Also third party libraries such as `icuuc` use import libs to forward to versioned DLLs. e.g. `icuuc.lib` points to `icuuc51.dll` etc. Test Plan: ./validate Two new tests added T11072gcc T11072msvc Two binary files have been added to the test folder because the "short" import library format doesn't seem to be creatable via `dlltool` and requires Microsoft's `lib.exe`. Reviewers: bgamari, RyanGlScott, erikd, goldfire, austin, hvr Reviewed By: RyanGlScott, erikd Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1696 GHC Trac Issues: #11072 (cherry picked from commit 97f2b164)
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