Generate separate DWARF debug info

The build now produces a ".dwo" file containing DWARF debug
information. This contains a binary encoding of internal information
about RaptorJIT e.g. the layout of all its internal data structures.

The DWARF information is for use by debugging tools that need to
understand JIT internals.

The DWARF file is currently generated with gcc. I would prefer to use
clang, for the sake of consistency with the rest of the build, but I
seem to get _much_ more debug information from gcc so I stick with
that for now.

The nix build expression outputs the debug information as
lib/raptorjit.dwo

raptorjit.nix

@@ -9,10 +9,14 @@ mkDerivation rec {
   name = "raptorjit-${version}";
   inherit version;
   src = source;
-  buildInputs = [ luajit ];  # LuaJIT to bootstrap DynASM
+  buildInputs = [
+      luajit                # LuaJIT to bootstrap DynASM
+      gcc6                  # GCC for generating DWARF info
+    ];
+  dontStrip = true;         # No extra stripping (preserve debug info)
   installPhase = ''
-    mkdir -p $out/bin
-    cp src/luajit $out/bin/raptorjit
+    install -D src/luajit $out/bin/raptorjit
+    install -D src/lj_dwarf.dwo $out/lib/raptorjit.dwo
   '';
 
   enableParallelBuilding = true;  # Do 'make -j'

src/Makefile

@@ -475,6 +475,8 @@ LJCORE_O= lj_gc.o lj_err.o lj_char.o lj_bc.o lj_obj.o lj_buf.o \
 	  lj_lib.o lj_alloc.o lib_aux.o \
 	  $(LJLIB_O) lib_init.o
 
+DWARF_DWO= lj_dwarf.dwo
+
 LJVMCORE_O= $(LJVM_O) $(LJCORE_O)
 LJVMCORE_DYNO= $(LJVMCORE_O:.o=_dyn.o)
 
@@ -566,7 +568,7 @@ E= @echo
 # Make targets.
 ##############################################################################
 
-default all:	$(TARGET_T)
+default all:	$(TARGET_T) $(DWARF_DWO)
 
 clean:
 	$(HOST_RM) $(ALL_RM)
@@ -655,6 +657,11 @@ $(HOST_O): %.o: %.c
 	$(E) "HOSTCC    $@"
 	$(Q)$(HOST_CC) $(HOST_ACFLAGS) -c -o $@ $<
 
+$(DWARF_DWO): %.dwo: %.c
+	$(E) "CC(debug) $@"
+#       GCC because clang does not seem to produce decent debug info (?)
+	$(Q)gcc -g3 -gdwarf-4 -fno-eliminate-unused-debug-types -gsplit-dwarf -c $<
+
 include Makefile.dep
 
 ##############################################################################

src/lj_dwarf.c

@@ -0,0 +1,24 @@
+/*
+** Compilation unit for DWARF debug information.
+*/
+
+#include "lj_obj.h"
+#include "lj_gc.h"
+#include "lj_err.h"
+#include "lj_debug.h"
+#include "lj_str.h"
+#include "lj_frame.h"
+#include "lj_state.h"
+#include "lj_bc.h"
+#include "lj_ir.h"
+#include "lj_jit.h"
+#include "lj_iropt.h"
+#include "lj_mcode.h"
+#include "lj_trace.h"
+#include "lj_snap.h"
+#include "lj_gdbjit.h"
+#include "lj_record.h"
+#include "lj_asm.h"
+#include "lj_dispatch.h"
+#include "lj_vm.h"
+#include "lj_target.h"