newlib: update build system generation documentation

Replace all of the individual autotool steps with a single autoreconf.
This simplifies the documentation greatly, and in the current system,
only takes ~10 seconds to regenerate everything.

Update the developer documentation to cover all the major components
of the current build system.  Hopefully this is a fairly complete road
map to everything.  I tried to include everything that I wish I knew
when I started hacking on this :P.

1 files changed, 171 insertions, 0 deletions

diff --git a/newlib/HOWTO b/newlib/HOWTO
index 668766096..e9af3a408 100644
--- a/newlib/HOWTO
+++ b/newlib/HOWTO
@@ -110,3 +110,174 @@ Level 4
      Full POSIX or Linux compliance.  Essentially these are functions that are
 present in a standard Linux kernel but are irrelevant to an embedded system.
 These functions do not form part of the EL/IX API.
+
+BUILD SYSTEM
+============
+
+     Newlib utilizes GNU Autotools (GNU Autoconf & GNU Automake) for its build
+system.  It can be broken down into two phases: configure & compile (make).
+NB: GNU Libtool is not used.  Newlib only produces static archives, and GNU
+Automake is capable of handling that.
+
+     As noted in the README file, you do not need to worry about which tools to
+run at any particular point.  Simply invoke `autoreconf` and it will do it all.
+
+     Tool versions are tightly controlled.  GNU Autoconf 2.69 exactly must be
+used.  Attempts to use any other version, newer or older, will be rejected.  If
+the current system does not include that version, it can be downloaded from the
+GNU Autoconf website and installed into a local directory tree (like ~/.local).
+GNU Automake 1.15.1 is required as a minimum.  Newer versions are allowed when
+running locally, but developers submitting patches must use 1.15.1 exactly.
+
+     The configure phase naturally starts with the configure script.  It merges
+multiple inputs to produce the final script.
+* aclocal.m4: Generated by aclocal; do not edit.  Copies all the various m4
+  macros used by configure from the system install into the local copy.  This
+  allows regenerating configure without needing all the same system packages.
+* autom4te.cache/: Generated by aclocal to cache system search results.  Safe
+  to delete at any time, but never edit anything in this directory.
+* iconv.m4: Generated by libc/iconv/ces/mkdeps.pl; do not edit.  Keeps the
+  set of dynamic iconv synced between the source files and configure.
+* acinclude.m4: Historically, a way of sharing configure logic with subdirs.
+  At some point, it should all be merged into configure.ac directly.
+* configure.ac: The main configure script for supporting all Newlib options.
+* configure.host: An index of every supported Newlib host and the settings to
+  use when compiling for it.
+* **/acinclude.m4: Subdirectories may include custom configure tests without
+  having to clutter the common configure.ac or acinclude.m4 file.  When adding
+  a new file, it must be manually included -- there is no mechanism to search
+  and automatically include every such file in the tree.  Look for m4_include
+  lines in configure.ac or existing acinclude.m4 files as examples.  Care must
+  be taken to not run excessive tests for incompatible targets, or to declare
+  generic names or symbols that can collide with other targets.  Any such file
+  should try to match $host (or similar) settings before running.  Settings
+  from configure.host (e.g. machine_dir or sys_dir) are guaranteed to be
+  available.  When creating names (Makefile variables, defines, etc...), try to
+  include the respective library and machine or sys name.  Do not confuse these
+  with the historical top-level acinclude.m4 file!
+
+     The configure script has to take special consideration to the fact that it
+usually runs with a toolchain that lacks a C library (since Newlib will provide
+it once it's compiled & installed).  Newlib uses the uncommon AC_NO_EXECUTABLES
+macro to tell GNU Autoconf to not automatically require a toolchain that can
+link executables.  As a result, Newlib itself cannot perform any link tests.
+It shouldn't really need to, but configure authors must keep this in mind and
+stick to the appropriate source-level test -- AC_PREPROC for testing behavior
+of the preprocessor and AC_COMPILE for testing compiler behavior.
+
+     The newlib.h header is created from newlib.hin during the configure phase.
+The newlib.hin file is managed by autoheader, so do not edit it directly.  Any
+symbol declared in configure via AC_DEFINE is automatically included.  The file
+declares all the Newlib-specific configure settings and will be installed as
+part of the standard set of headers.  Critically, all symbols are namespaced
+with a leading _ prefix to avoid collisions with user code, and any symbols not
+namespaced are automatically removed.  This is necessary due to some system m4
+macros automatically adding their own defines that we cannot disable.  It also
+means that, when using AC_DEFINE, the _ prefix must be manually added.
+
+     A single Makefile is created during the configure phase.  This will build
+all of Newlib using non-recursive make (i.e., there are no subdirectories with
+their own Makefiles).  The single Makefile.am file includes Makefile.inc files
+in its immediate subdirectories, and those further include Makefile.inc files
+in their subdirectories.  This means all variables and targets exist in a
+single global flat namespace, so care must be taken to not use generic names
+like "SOURCES" in any Makefile.inc file.  Instead, use %C%_ on all variables to
+get a unique prefix, and GNU Automake will expand it.  In order to use one of
+the standard GNU Automake variables (e.g., CLEANFILES), first declare it at the
+top of the Makefile.am by assigning it (e.g., CLEANFILES =), and then always
+append to it in Makefile.inc (e.g. CLEANFILES += ...).  Again, GNU Automake
+will take care of combining all of this logic together to produce a portable
+Makefile.
+
+     Developers used to changing to a subdirectory and running make commands
+to iterate quickly on a smaller section of Newlib might be surprised that this
+does not work -- there is no Makefile in any subdirectory.  Instead, make has
+to be run from the top-level, and the full path to the target is specified.
+So instead of running `cd subdir && make foo.o`, run `make subdir/foo.o`.  This
+applies to any target, not just objects.  Some common targets:
+make libm.a
+make libc.a
+
+     Conditionals are allowed in Makefile.inc files, but using them to elide
+an entire subdir Makefile.inc should be avoided.  Instead, GNU Automake
+conditionals should be used only within a single Makefile.inc file to control
+settings specific to that directory.  See the documentation section below for
+more information.
+
+     Per-directory compiler settings are allowed.  Similar to the syntax used
+above, the _%C% suffix can be used.  See the Makefile.am file for the current
+set of flags (e.g. CFLAGS, CPPFLAGS, etc...) that are supported.
+AM_CFLAGS_%C% = ...
+libm_a_CFLAGS_%C% = ...
+libc_a_CFLAGS_%C% = ...
+
+     Per-file compiler settings are allowed.  Similar to the syntax used
+above, the _%C% suffix can be used.  See the Makefile.am file for the current
+set of flags (e.g. CFLAGS, CPPFLAGS, etc...) that are supported.
+AM_CFLAGS_%C%_moo.c = ...
+libm_a_CFLAGS_%C%_foo.c = ...
+libc_a_CFLAGS_%C%_bar.c = ...
+
+     An important aspect of the build system is allowing machine-specific
+directories to override common implementations.  For example, Newlib provides
+generic C implementations of string & memory functions in libc/string/ that
+perform adequately for any target, but many architectures include hand written
+assembly files for smaller & faster execution.  Those files live under the
+corresponding libc/machine/<ARCHITECTURE>/ directory.  Both versions are
+compiled when building Newlib, but when generating the libc.a archive, the
+common object is added first, followed by the machine-specific one.  This works
+because the object names follow a consistent convention: libm_a-<filename>.o
+for the math library and libc_a-<filename>.o for the C library.  If the common
+code stores a function foo in foo.c, then it can be overridden by another foo.c
+or foo.S or foo.s file since they all produce foo.o.  For this reason, all
+machine Makefile.inc files must be included last after all common Makefile.inc
+files.
+
+     Note that the Makefiles do not use VPATH to search source files across
+multiple source directories.  VPATH is used to match a single build directory
+to the corresponding source directory, but that is it.  For more details on
+how machine directories override functions in common directories, see the
+previous section.
+
+     Keep in mind that machine overrides only work at the file level, not at
+the symbol level.  If a common file defines multiple symbols, then the machine
+override must define all the same symbols.  This is why most common code stores
+only one function per source file.
+
+     The documentation, both the manual and man pages and header references,
+should be consistent regardless of what configure options are used (including
+the target system).  This makes it easy to get a complete documentation build
+at all times.  If inclusion of chapters depended on specific configure options,
+it would be difficult to find the set of options necessary to produce the
+complete manual, if it were even possible in the first place (as some configure
+options are incompatible).  Since documentation chapters & APIs are declared in
+the respective directories and not in a centralized location, it is important
+to not use any GNU Automake conditional to control whether Makefile.inc files
+are included.  Documentation that covers optional APIs should note that it is
+not guaranteed to be included in every build of Newlib for every target.  The
+level of detail is left up to the author's discretion.
+
+     Newlib automatically builds itself for all multilib configurations that
+the active toolchain supports.  This logic is provided by common code from the
+combined toolchain source tree, but bolts itself into the GNU Autoconf and
+Automake files.  The critical parts to keep in mind:
+* ../config/multi.m4: AM_ENABLE_MULTILIB adds the --{en,dis}able-multilib
+  options to configure for users, and takes care of calling ../config-ml.in
+  for the Makefile in the current directory.  While not applicable to Newlib
+  anymore (since Newlib only produces a single Makefile now), this does not
+  automatically run for any subdir Makefiles.  Instead, it must be run manually
+  in AC_CONFIG_FILES's commands argument.
+* ../config-ml.in: Rewrites the specified Makefile to include the various
+  multilib variables and the multi-do command.  Notably, this only makes the
+  multi-do rule available, it does not cause it to be used anywhere.
+* ../multilib.am: Manually included in Newlib's Makefile.am.  This adds calls
+  to multi-do for the common important rules: all (i.e. compiling), install,
+  clean, etc...
+* FLAGS_TO_PASS: Newlib's Makefile.am declares all the variables to pass to
+  each multilib configuration via the multi-do rule.  While the multi-do rule
+  includes many common flags automatically, Newlib passes along a few extra
+  that are unique to it.
+
+    The testsuite is not integrated at all currently.  It is not really well
+tested in general.  The README file has more information.  Hopefully someone
+someday will step up to integrate it into the existing framework.