Re: [dpdk-dev] [PATCH 6/6] doc: guide for Windows build using MinGW-w64

[Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>]

> Remembered another issue: thread-local storage (TLS) with shared libraries.
> Windows PE doesn't support TLS via special sections, so compilers use TLS
> emulation layer. With static libraries, there are no issues described below.
> 
> The first aspect is a build-time issue of MinGW. When linking to DPDK shared
> libraries, errors occur:
> 
> 	undefined reference to `__emutls_v.per_lcore__rte_errno'
> 	undefined reference to `__emutls_v.per_lcore__rte_lcore_id'
> 
> DPDK declares per_lcore__XXX in a map file, but GCC places __thread symbols
> in __emutls_v section, so the proper name to export becomes __emutls_v.XXX.
> This can be worked around by using an additional version script with MinGW,
> as I do in my port [0], however, the proper solution would be fixing the bug
> on MinGW side [1]. MinGW already converts TLS variable names when generating
> DEF files with `-Wl,--output-def` option (not used by DPDK, just a hint).

Did some research and AFAICT, there is not effortless solution for
efficient per-lcore variables on Windows. While MinGW-w64 has aforementioned
issues (actually, GCC on Windows does), Clang with default TLS options just
generates wrong results when exporting variables from dynamic libraries.
Demo: https://github.com/PlushBeaver/tlstest

Thread [0] claims this is a fundamental problem with PE-COFF executable
format, but I honestly lack expertise to tell if this is valid. Microsoft
docs [1] suggests that exporting __thread variables won't just work. Can
someone from Microsoft or from UNH Lab comment further?

[0]: https://sourceforge.net/p/mingw-w64/mailman/message/31777672/
[1]:
https://docs.microsoft.com/en-us/windows/win32/dlls/using-thread-local-storage-in-a-dynamic-link-library

-- 
Dmitry Kozlyuk