'guix build --target=' handling questions

[Sergei Trofimovich <slyfox@inbox.ru>]

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Hia!

Before asking my questions I'll setup the scene first.

The scene:

I'm playing with cross-compiler support in guix.

[ x86_64 -> alpha target as an example with
  this tiny patch: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=25799 ]

I've stumbled upon the two cases: working[1] and non-working[2].

[1] working:
  $ ./pre-inst-env guix build --check re2c --target=alpha-unknown-linux-gnu --no-grafts
  ...
  /gnu/store/15ibxf5xzcqzfx0xgb5s6qa5z2ah3avg-re2c-0.16
  $ file /gnu/store/15ibxf5xzcqzfx0xgb5s6qa5z2ah3avg-re2c-0.16/bin/re2c 
  /gnu/store/15ibxf5xzcqzfx0xgb5s6qa5z2ah3avg-re2c-0.16/bin/re2c: ELF 64-bit LSB executable,
Works as expected.

[2] non-working:
  $ ./pre-inst-env guix build --check re2 --target=alpha-unknown-linux-gnu --no-grafts
  ...
  /gnu/store/mg474j0gx56fdl6wrpaxxxwsdknl7k2i-re2-2017-01-01
  $ file /gnu/store/mg474j0gx56fdl6wrpaxxxwsdknl7k2i-re2-2017-01-01/lib/libre2.so.0.0.0 
  /gnu/store/mg474j0gx56fdl6wrpaxxxwsdknl7k2i-re2-2017-01-01/lib/libre2.so.0.0.0: ELF 64-bit LSB shared object, x86-64

It seems that re2 derivation ignores --target
and builds it's library with host compiler, not target compiler.

Relevant re2 package snippet from
    http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/regex.scm#n27

     (build-system gnu-build-system)
     (arguments
      `(#:test-target "test"
        ;; There is no configure step, but the Makefile respects a prefix.
        #:make-flags (list (string-append "prefix=" %output))
        #:phases
        (modify-phases %standard-phases
          (delete 'configure)

As we see here definition skips ./configure step (that's ok)
and does not pass CXX=${target}-g++ to make flag.

I wonder what are the possible options to override CXX here?

Makefile for re2 is very straightforward:
    CXX?=g++
    ...
    obj/%.o: %.cc $(HFILES)
    <tab>@mkdir -p $$(dirname $@)
    <tab>$(CXX) -c -o $@ $(CPPFLAGS) $(RE2_CXXFLAGS) $(CXXFLAGS) -DNDEBUG $*.cc
    ...
    obj/so/libre2.$(SOEXT): $(SOFILES)
    <tab>@mkdir -p obj/so
    <tab>$(MAKE_SHARED_LIBRARY) -o obj/so/libre2.$(SOEXTVER) $(SOFILES)
    <tab>ln -sf libre2.$(SOEXTVER) $@

Question time:

- Is there a way to run 'guix environment --target=' in the same way as 'guix build --target='
  sets it up? I'd like to see how both compilers are supposed to be present in there.

- Why default g++ in PATH is the host g++ and not target g++?
  Target seems to make most sense if no explicit compiler is specified.

- How to actually set CXX to point to target g++?
  It looks like implicitly there already both host (through native-inputs)
  and target (through build-inputs) compilers available.
  I would expect something like
      #:make-flags (list (string-append "CXX=" <.?.>)) 
  What should be in place of that "<.?.>" to refer to target g++?

Thank you!

-- 

  Sergei

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