Hitchikers Guide to Porting Phobos / D Runtime to other architectures

[Johannes Pfau]

Am Sun, 08 Apr 2012 21:08:52 +0200
schrieb "Iain Buclaw" <ibuclaw at ubuntu.com>:

> I got asked whether there are any porting hints for phobos on 
> other architectures the other day from the debian GCC 
> maintainers.  So I gathered this must be at least a dedicated 
> wiki or article to be written up on the subject. :)
> 
> I know there are a few working on porting gdc and associated 
> libraries over to ARM (with my assistance from the compiler 
> side).  So please tell, what are your experiences? Successes?  
> Failures?  What tips would you give to someone wanting to port to 
> their own architecture?
> 
> Regards
> Iain

(This is mostly about porting to a different C library. I don't
remember many issues when porting to a different CPU architecture)

Issues I hit with druntime:

* Adapting the core.stdc bindings to something different than the
  currently supported C libraries sucks: The version blocks are
  sometimes completely wrong. For example Android's bionic is a C
  library based on BSD code, but running on Linux. As a result
  sometimes the version(FreeBSD) blocks apply for bionic, but sometimes
  the version(linux) blocks are right. I basically had to rewrite
  the complete core.stdc bindings. This is an issue because druntime
  and phobos do not distinguish between OS/Kernel and C library.

* Wrong constants or macros in the C bindings are very hard to spot -
  you'll only notice those at runtime

* When statically linking the phobos/druntime library you are no warned
  about missing symbols - For shared libraries  -Wl,--no-undefined can
  be used, however, there are some issues with that as well:
  (http://stackoverflow.com/questions/2356168/force-gcc-to-notify-about-undefined-references-in-shared-libraries
  second answer)

* Bionic just implements some functions as macros and never exports
  those as functions (htons, etc). Because of the last point it's easy
  to miss that

Ideally all of the core.stdc bindings should be generated
automatically. This is possible if we can run code (using offsetof,
alignof, etc) but it's not that easy for cross compilation. I thought
about hooking into the GCC C frontend to do that, but I had no time to
look at it yet.

* All those issues also apply to phobos, where phobos uses custom C
  bindings / extern(C) declarations.

* I had to edit some stuff in std.stdio (because Android has no wide
  character/fwide support). Templates can be annoying in this case:
  some if(isOutputRange!T) chains hid an error in the IO code, it took
  me some time to find that problem. The reported error was completely
  misleading (cannot put dchar[] into LockingTextWriter or something)

* When adding new, system specific code to a module and using selective
  imports, that may affect other modules (can't remember which compiler
  bug this was). This means that adding an import in one module might
  break another module on another architecture.

* Porting the GC doesn't seem to be too difficult, but some care is
  needed to get stack scanning/TLS scanning right (If you have random
  crashes, it's either the GC not working(probably not scanning
  stack/tls) or fno-section-anchors missing)

* Always use "-fno-section-anchors". It's not needed for simple code,
  but I was chasing a weird bug in derelict, till I realized I didn't
  compile derelict with "-fno-section-anchors".

* Right now, issue 284 is a little annoying. At least unittest and
  phobos/druntime as shared libraries won't work at all till that's
  fixed.

* AFAIK the unittests cannot be run when cross-compiling right now?

* There might be more issues like this one where phobos is checking for
  a wrong status code:
  (https://github.com/D-Programming-Language/phobos/pull/487)

* For systems where long double isn't available, fixing core.stdc.math
  is annoying. I have to implement a proper solution which works
  for all systems without long double.

However, all that considered most issues are when interfacing C. The D
code most of the time 'just works'.