[Octave-patch-tracker] [patch #8919] Start of patch to enable visibility attributes for GCC in build system

[Markus Mützel]

Follow-up Comment #18, patch #8919 (project octave):

I tried to compile with visibility attributes for Windows using the attached
"part4".

But it looks like there is a compatibility issue with how we use (inline)
definitions in headers and the dllexport-dllimport mechanism used on Windows:
Symbols marked with the "dllimport" attribute cannot have definitions in the
same compilation unit. [1]
GCC fails with a very similar error in that case ("function X definition is
marked dllimport").

That means, we'd need to restrict us to *declarations* in headers we'd like to
use for symbol import.
We are probably using *definitions* in header file because we'd like to give
the compiler the opportunity to inline those functions.
But to be able to use mark symbols as dllimports, we'd need to *separate*
declaration and definition (or use separate headers for import?). However,
this might lead to the compiler producing less efficient instructions.

With the changes from "part5" compilation succeeds. But linking
liboctinterp-8.dll fails with a whole lot of errors similar to this one:

/home/osboxes/Documents/Repositories/Octave/mxe-octave/tmp-default-octave/octave-7.0.0/libinterp/octave-value/ov-bool.cc:200:
undefined reference to `double octave::read_value<double>(std::istream&)'


But that symbol *is* exported by liboctave-8.dll:

$ nm -C
./tmp-default-octave/octave-7.0.0/.build/liboctave/.libs/liboctave-8.dll |
grep "octave::read_value<double>"
000000006ceb0910 T double octave::read_value<double>(std::istream&)


I've currently no idea how to fix that.

(file #50572, file #50573)
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Additional Item Attachment:

File name: patch8919_part4_visibility_Windows.patch Size:2 KB
   
<https://file.savannah.gnu.org/file/patch8919_part4_visibility_Windows.patch?file_id=50572>

File name: patch8919_part5_visibility_Windows_fixes.patch Size:5 KB
   
<https://file.savannah.gnu.org/file/patch8919_part5_visibility_Windows_fixes.patch?file_id=50573>



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