Using Windows Subsystem Linux for Windows build will fail due to -fPIC

[iaincoulter]

Following the steps to build a windows version with Windows Subsystem Linux installed causes -fPIC errors

clang: fatal error: unsupported option '-fPIC' for target 'x86_64-pc-windows-msvc'

Main issue is when you type 'bash' it starts up and instance of WSL. When running configure this logic is applied

        os_name=$(uname -s)
	os_vers=$(uname -r)
	echo "${script_name}: detected ${os_name} kernel version ${os_vers}."

	# Define a single variable off of which we can branch to tell if we are
	# building for Windows.
	is_win=no
	if [[ $os_name == MSYS* ]] || \
	   [[ $os_name == MINGW* ]]  || \
	   [[ $os_name == CYGWIN* ]] ; then
		is_win=yes
	fi

Thus is_win is not set to true, as when using WSL "uname -s" it returns "Linux" !!! (as it should as it is running linux)
Apart from manually modifying this not sure how this should be fixed.
A user with WSL installed could be using it to build a proper Linux version or a Windows version, but using "os_name" will not indicate what the user intended!!

[devinamatthews]

@iaincoulter do you mean the instructions here? And are you using striaght MSYS2, Mingw64, Cygwin, etc.? Lastly, are you specifically trying to build a .dll or just build BLIS at all (in the latter case you could just do everything in WSL)?

[iaincoulter]

I'm trying to build a .dll but don't see why i need to install MSYS2. Mingw64 or Cygwin as WSL Linux already provides all the tools (sed/bash/etc) they provide. For compiler I'm using windows LLVM as mentioned in that link.
modifying the configure script to set is_win=true. Code is compiling in this manner but has no completed yet. no Errors as of yet.

[devinamatthews]

That may work or it may not. MSYS2 is like WSL but it definitely knows and adapts to the fact that it is really on Windows. WSL on the other hand is literally Linux.

[iaincoulter]

I understand. The only fix I can see would be to pass a flag which sets that is_win variable. Just wanted to highlight this issue should other Windows users encounter it. I'll just stick to modifying the file for now

[devinamatthews]

If it does work let us know and we can note it in the docs.

[dnparikh]

The build system of BLIS was not designed to build a Windows .dll on a Linux system. If you are using the Windows Subsystem Linux, you should expect to build a library meant for Linux OS, not Windows.

[isuruf]

@iaincoulter was using Windows Subsystem Linux for bash only which is fine. It's good to have cross compiling support for BLIS. AFAIK, we can build a Windows .dll on a Linux system. See #90 (comment)

[devinamatthews]

OK, I guess the real fix for this problem would be to also check the predefined compiler macros for a window cross-compilation toolchain. @isuruf can you suggest something simple but robust (i.e. covering WSL-on-Windows as well as cross-compilation from Linux)?

[isuruf]

_WIN32 macro should be good for all windows compilers.

To avoid running executables, you can test whether the following compiles or not.

#ifdef _WIN32
int a;
#else
int a
#endif

[devinamatthews]

@iaincoulter please try this branch.

[devinamatthews]

@isuruf since you've also noticed a similar problem cross-compiling on Linux can you check if the linked PR fixes it?

[isuruf]

Yes, it works. There are more errors with that cross compiling build though.

1. Needs -fno-asynchronous-unwind-tables (we should check __MINGW64__ macro and add that flag). gcc bug link
2. Next error is,

x86_64-w64-mingw32-gcc -O2 -O3 -fomit-frame-pointer -mavx2 -mfma -mfpmath=sse -march=haswell -fno-asynchronous-unwind-tables -Wall -Wno-unused-function -Wfatal-errors -std=c99 -D_POSIX_C_SOURCE=200112L -Iinclude/x86_64 -I./frame/3/ -I./frame/ind/ukernels/ -I./frame/3/ -I./frame/1m/ -I./frame/1f/ -I./frame/1/ -I./frame/include -DBLIS_VERSION_STRING=\"0.8.1-17\" -DBLIS_IS_BUILDING_LIBRARY -Wl,--exclude-all-symbols -c kernels/haswell/3/sup/bli_gemmsup_rv_haswell_asm_d6x8m.c -o obj/x86_64/kernels/haswell/3/sup/bli_gemmsup_rv_haswell_asm_d6x8m.o
kernels/haswell/3/sup/bli_gemmsup_rv_haswell_asm_d6x8m.c: In function ‘bli_dgemmsup_rv_haswell_asm_6x6m’:
kernels/haswell/3/sup/bli_gemmsup_rv_haswell_asm_d6x8m.c:1763:1: error: bp cannot be used in asm here
 }
 ^
compilation terminated due to -Wfatal-errors.
Makefile:601: recipe for target 'obj/x86_64/kernels/haswell/3/sup/bli_gemmsup_rv_haswell_asm_d6x8m.o' failed
make: *** [obj/x86_64/kernels/haswell/3/sup/bli_gemmsup_rv_haswell_asm_d6x8m.o] Error 1