Makefile: New 'reusevm' target & bootstrap-skipping method

This is a "less magic" and "more helpful" way to handle bootstrapping
vs not bootstrapping the VM.

The default behavior of "make" is reverted to its original behavior.
i.e. it generates code for the VM using DynASM (requiring LuaJIT.)

If LuaJIT is not available then this error message is printed:

    Error: Missing dependency (luajit) for bootstrapping the VM.

    Here are your options to build RaptorJIT:
      make reusevm     # copy reference VM from reusevm/
      (install luajit) # satisfy the dependency
      nix-build        # use nix to satisfy the dependencies

    Warning:
      Only reusevm when running a pristine copy of RaptorJIT from
      a repository that keeps the reference VM up-to-date.
      (Otherwise you might mix some stale code into your build.)

So then the user can choose between installing LuaJIT or running 'make
reusevm' to use the in-tree generated code at their own risk.

In practice it should be safe to 'reusevm' when building a pristine
master branch because CI is ensuring that the code is always
up-to-date. However, other branches or working copies may have stale
code in reusevm/ that is not safe to use.

README.md

@@ -70,26 +70,25 @@ The [AnandTech review of the Haswell microarchitecture](http://www.anandtech.com
 
 ### Compilation for users
 
-Building a standard RaptorJIT from source is easy:
+Simple build:
 
+```shell
+$ make  # requires LuaJIT (2.0 or 2.1) to run DynASM
 ```
-$ make
-$ ls -l src/raptorjit{,.a,.o} src/libraptorjit.so 
--rwxr-xr-x 1 luke users 553592 Jul  7 08:22 src/raptorjit
--rw-r--r-- 1 luke users 843116 Jul  7 08:21 src/raptorjit.a
--rw-r--r-- 1 luke users  18072 Jul  7 08:22 src/raptorjit.o
--rwxr-xr-x 1 luke users 566288 Jul  7 08:22 src/libraptorjit.so
+
+Alternative if you don't have LuaJIT available and you are building a
+pristine copy from the master branch:
+
+```shell
+$ make reusevm  # Reuse reference copy of the generated VM code
+$ make          # Does not require LuaJIT now
 ```
 
 ### Compilation for VM hackers
 
-If you are hacking the virtual machine definition then you need a few
-additional software dependencies for the build. For example, you need
-LuaJIT to run DynASM in order to regenerate the virtual machine sources.
-
-You can install these dependencies yourself or you can use the
-reference [Nix](http://nixos.org/nix/) build environment provided with
-RaptorJIT.
+RaptorJIT uses [Nix](http://nixos.org/nix/) to provide a reference
+build environment. You can use Nix to build/test/benchmark RaptorJIT
+with suitable versions of all dependencies provided.
 
 Note: Building with nix will be slow the first time because it
 downloads the exact reference versions of the toolchain (clang, etc)

raptorjit.nix

@@ -9,9 +9,6 @@ mkDerivation rec {
   name = "raptorjit-${version}";
   inherit version;
   src = source;
-  preBuild = ''
-    make bootstrapclean
-  '';
   buildInputs = [ luajit ];  # LuaJIT to bootstrap DynASM
   installPhase = ''
     mkdir -p $out/bin

src/Makefile

@@ -242,9 +242,6 @@ LJCORE_O= lj_gc.o lj_err.o lj_char.o lj_bc.o lj_obj.o lj_buf.o \
 LJVMCORE_O= $(LJVM_O) $(LJCORE_O)
 LJVMCORE_DYNO= $(LJVMCORE_O:.o=_dyn.o)
 
-LIB_VMDEF= jit/vmdef.lua
-LIB_VMDEFP= $(LIB_VMDEF)
-
 LUAJIT_O= raptorjit.o
 LUAJIT_A= raptorjit.a
 LUAJIT_SO= libraptorjit.so
@@ -253,7 +250,7 @@ LUAJIT_T= raptorjit
 ALL_T= $(LUAJIT_T) $(LUAJIT_A) $(LUAJIT_SO) $(HOST_T)
 ALL_HDRGEN= lj_bcdef.h lj_ffdef.h lj_libdef.h lj_recdef.h lj_folddef.h \
 	    host/buildvm_arch.h
-ALL_GEN= $(LJVM_S) $(ALL_HDRGEN) $(LIB_VMDEFP)
+ALL_GEN= $(LJVM_S) $(ALL_HDRGEN)
 WIN_RM= *.obj *.lib *.exp *.dll *.exe *.manifest *.pdb *.ilk
 ALL_RM= $(ALL_T) *.o host/*.o $(WIN_RM)
 
@@ -263,7 +260,7 @@ ALL_RM= $(ALL_T) *.o host/*.o $(WIN_RM)
 
 TARGET_O= $(LUAJIT_A)
 TARGET_T= $(LUAJIT_T) $(LUAJIT_SO)
-TARGET_DEP= $(LIB_VMDEF) $(LUAJIT_SO)
+TARGET_DEP= $(LUAJIT_SO)
 
 Q= @
 E= @echo
@@ -278,10 +275,13 @@ default all:	$(TARGET_T)
 
 clean:
 	$(HOST_RM) $(ALL_RM)
-
-bootstrapclean: clean
 	$(HOST_RM) $(ALL_GEN)
 
+reusevm:
+	cd reusevm; \
+	cp -r * ../
+	@echo "Copied reference VM. Ready to build."
+
 libbc:
 	./$(LUAJIT_T) host/genlibbc.lua -o host/buildvm_libbc.h $(LJLIB_C)
 	$(MAKE) all
@@ -300,13 +300,28 @@ depend:
 	  test -s $$file || $(HOST_RM) $$file; \
 	  done
 
-.PHONY: default all clean libbc depend
+.PHONY: default all clean libbc depend reusevm
 
 ##############################################################################
 # Rules for generated files.
 ##############################################################################
 
 host/buildvm_arch.h: $(DASM_DASC) $(DASM_DEP) $(DASM_DIR)/*.lua
+	@command -v $(DASM) 2>/dev/null || { \
+	  echo "Error: Missing dependency (luajit) for bootstrapping the VM."; \
+	  echo "";                                                             \
+	  echo "Here are your options to build RaptorJIT:";                    \
+	  echo "  make reusevm     # copy reference VM from reusevm/";          \
+	  echo "  (install luajit) # satisfy the dependency";                   \
+	  echo "  nix-build        # use nix to satisfy the dependencies";      \
+	  echo "";                                                             \
+	  echo "Warning:";                                                     \
+	  echo "  Only reusevm when running a pristine copy of RaptorJIT from";\
+	  echo "  a repository that keeps the reference VM up-to-date.";       \
+	  echo "  (Otherwise you might mix some stale code into your build.)"; \
+	  echo;                                                                \
+	  exit 1;                                                              \
+	}
 	$(E) "DYNASM    $@"
 	$(Q)$(DASM) -o $@ $(DASM_DASC)
 
@@ -336,10 +351,6 @@ lj_recdef.h: $(BUILDVM_T) $(LJLIB_C)
 	$(E) "BUILDVM   $@"
 	$(Q)$(BUILDVM_X) -m recdef -o $@ $(LJLIB_C)
 
-$(LIB_VMDEF): $(BUILDVM_T) $(LJLIB_C)
-	$(E) "BUILDVM   $@"
-	$(Q)$(BUILDVM_X) -m vmdef -o $(LIB_VMDEFP) $(LJLIB_C)
-
 lj_folddef.h: $(BUILDVM_T) lj_opt_fold.c
 	$(E) "BUILDVM   $@"
 	$(Q)$(BUILDVM_X) -m folddef -o $@ lj_opt_fold.c