Re: Supporting --enable-version-specific-runtime-libs

[markh at landmark dot com]

(I forgot to send this to libstdc++ the first time.  This is just a
duplicate of my earlier message.)

> 
> In addition, I kept USE_LIBDIR just because I didn't want libstdc++
> divergence with what is apparently a common idiom in makefile/configure
> scripts across the development toolchain (ie gcc/java/c++).
> 
> The resulting bits in src/Makefile.am look like:
> 
> # Cross compiler and multilib support.
> # Install a library built with a cross compiler in tooldir, not libdir.
> if USE_LIBDIR
> toolexeclibdir = $(libdir)$(MULTISUBDIR)
> else
> if VERSION_SPECIFIC_LIBS
> gcc_version = @gcc_version@
> toolexecdir = $(libdir)/gcc-lib/$(target_alias)
> toolexeclibdir = $(toolexecdir)/$(gcc_version)
> else
> toolexecdir = $(exec_prefix)/$(target_alias)
> toolexeclibdir = $(toolexecdir)/lib$(MULTISUBDIR)
> endif
> endif

What you have put in place is the same as my original (erroneous)
change, the one I made before submitting any patches, the one that
drove me a little crazy.

Consider the following scenario:

   You configure your source using ./configure
   --enable-version-specific-runtime-libs, but without using
   --with-cross-host, because you don't want to build a cross compiler.
   During configuration, configure will run the code that corresponds
   to the following line in configure.in:

AM_CONDITIONAL(USE_LIBDIR, test -z "$with_cross_host")

   Now, because you did not specify --with-cross-host, this test will
   cause USE_LIBDIR to become defined, and so, even though you have a
   specification of '--enable-version-specific-runtime-libs', the
   library and header files will NOT be installed in the
   version-specific directories.  Instead the "if USE_LIBDIR" clause
   will be used.

   Here's my reasoning about what should be the correct order of
   priority of processiong these options.  Feel free to dispute this.

    1) If a cross-compiler is specified, then all bets are off.  This
       is a special and unusual compiler that is going to be built and
       it is going to reside in a special directory tree.  Should we
       support version-specific directories for cross compilers also?
       I don't think it has been done in the past, but we could
       consider adding that also.

    2) If a cross-compiler has not been specified (i.e., the other 99%
       :) ), then

        a) by default, we allow the files to be installed in the
           "multilib" directory tree.

        b) if a version-specific directory structure has been
           requested, then this overrides the default.

   If this reasoning is correct, then we could see the following
   pseudo-code: 

     if cross-compiler then
       cross-compiler code;
     else
       if version-specific compiler then
         version-specific code;
       else
         default ("multilib") code;
       end-if
     end-fi

   Or, equivalently,

     if not-cross-compiler then
       if version-specific compiler then
         version-specific code;
       else
         default ("multilib") code;
       end-if
     else
       cross-compiler code;
     end-if

   (In this implementation, notice how the priority of the
   configuration switches has been turned on its head.)

   What you have written (and I made the same mistake) is:

     if not-cross-compiler then
         default ("multilib") code;
     else
       if version-specific compiler then
         version-specific code;
       else
         cross-compiler code;
       end-if
     end-if

   Why did we both make this mistake?  I think it is because USE_LIBDIR
   was poorly selected as a variable name (and has gone on for a long
   time with people cutting and pasting code), and because the original
   if-else clause in Makefile.am was poorly considered since it is
   based on the absence of a configuration switch, rather than a test
   for the presence of a configuration switch.

   If you insist on continuing the USE_LIBDIR idiom, please change the
   erroneous code above in src/Makefile.am to:

> if USE_LIBDIR
> if VERSION_SPECIFIC_LIBS
> gcc_version = @gcc_version@
> toolexecdir = $(libdir)/gcc-lib/$(target_alias)
> toolexeclibdir = $(toolexecdir)/$(gcc_version)
> else
> toolexeclibdir = $(libdir)$(MULTISUBDIR)
> endif
> else
> toolexecdir = $(exec_prefix)/$(target_alias)
> toolexeclibdir = $(toolexecdir)/lib$(MULTISUBDIR)
> endif

  But before you do this, please remember how easy it was for you to
  make this mistake, as I did.

-- 
## Mark Harig
## Landmark Systems, Reston, Virginia, USA
## Email: mharig@landmark.com