gdb, gdbserver, gdbsupport: include early header files with `-include`

The motivation for this change is for analysis tools and IDEs to be
better at analyzing header files on their own.

There are some definitions and includes we want to occur at the very
beginning of all translation units.  The way we currently do that is by
requiring all source files (.c and .cc files) to include one of defs.h
(for gdb), server.h (for gdbserver) of common-defs.h (for gdbsupport and
shared source files).  These special header files define and include
everything that needs to be included at the very beginning.  Other
header files are written in a way that assume that these special
"prologue" header files have already been included.

My problem with that is that my editor (clangd-based) provides a very
bad experience when editing header files.  Since clangd doesn't know
that one of defs.h/server.h/common-defs.h was included already, a lot of
things are flagged as errors.  For instance, CORE_ADDR is not known.
It's possible to edit the files in this state, but a lot of the power of
the editor is unavailable.

My proposal to help with this is to include those things we always want
to be there using the compilers' `-include` option.  Tom Tromey said
that the current approach might exist because not all compilers used to
have an option like this.  But I believe that it's safe to assume they
do today.

With this change, clangd picks up the -include option from the compile
command, and is able to analyze the header file correctly, as it sees
all that stuff included or defined by that -include option.  That works
because when editing a header file, clangd tries to get the compilation
flags from a source file that includes said header file.

This change is a bit self-serving, because it addresses one of my
frustrations when editing header files, but it might help others too.
I'd be curious to know if others encounter the same kinds of problems
when editing header files.  Also, even if the change is not necessary by
any means, I think the solution of using -include for stuff we always
want to be there is more elegant than the current solution.

Even with this -include flag, many header files currently don't include
what they use, but rather depend on files included before them.  This
will still cause errors when editing them, but it should be easily
fixable by adding the appropriate include.  There's no rush to do so, as
long as the code still compiles, it's just a convenience thing.

The changes are:

 - Add the appropriate `-include` option to the various Makefiles.

 - There is one particularity for gdbserver's Makefile: we do not want
   to include server.h when building `gdbreplay.o`, as `gdbreplay.cc`
   doesn't include it.  So we can't simply put the `-include` in
   `INTERNAL_CFLAGS`.  Add the `-include server.h` option to the
   `COMPILE` and `IPAGENT_COMPILE` variables, and added a special rule
   to compile `gdbreplay.o` with `-include gdbsupport/common-defs.h`.

 - Remove the `-include` option from the `check-headers` rule in
   gdb/Makefile.in, since it is already included in `INTERNAL_CFLAGS`.

Change-Id: If3e345d00a9fc42336322f1d8286687d22134340
Approved-By: Pedro Alves <pedro@palves.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index a5139ea..a9f641c 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -607,6 +607,7 @@ GDB_CFLAGS = \
 	-I. \
 	-I$(srcdir) \
 	-I$(srcdir)/config \
+	-include $(srcdir)/defs.h \
 	-DLOCALEDIR="\"$(localedir)\"" \
 	$(DEFS)
 
@@ -2050,7 +2051,7 @@ check-headers:
 	@echo Checking headers.
 	for i in $(CHECK_HEADERS) ; do \
 		$(CXX) $(CXX_DIALECT) -x c++-header -c -fsyntax-only \
-		$(INTERNAL_CFLAGS) $(CXXFLAGS) -include defs.h $(srcdir)/$$i ; \
+		$(INTERNAL_CFLAGS) $(CXXFLAGS) $(srcdir)/$$i ; \
 	done
 .PHONY: check-headers