Don't modify generated libbc header if unchanged.

src/Makefile

@@ -559,7 +559,7 @@ clean:
 	$(HOST_RM) $(ALL_RM)
 
 libbc:
-	./$(LUAJIT_T) host/genlibbc.lua $(LJLIB_C) >host/buildvm_libbc.h
+	./$(LUAJIT_T) host/genlibbc.lua -o host/buildvm_libbc.h $(LJLIB_C)
 	$(MAKE) all
 
 depend:

src/host/genlibbc.lua

@@ -6,15 +6,29 @@
 -- Released under the MIT license. See Copyright Notice in luajit.h
 ----------------------------------------------------------------------------
 
-local function usage()
-  io.stderr:write("Usage: ", arg and arg[0] or "genlibbc", " lib_*.c\n")
+local function usage(arg)
+  io.stderr:write("Usage: ", arg and arg[0] or "genlibbc",
+		  " [-o buildvm_libbc.h] lib_*.c\n")
   os.exit(1)
 end
 
-local function read_source()
-  if not (arg and arg[1]) then usage() end
+local function parse_arg(arg)
+  local outfile = "-"
+  if not (arg and arg[1]) then
+    usage(arg)
+  end
+  if arg[1] == "-o" then
+    outfile = arg[2]
+    if not outfile then usage(arg) end
+    table.remove(arg, 1)
+    table.remove(arg, 1)
+  end
+  return outfile
+end
+
+local function read_files(names)
   local src = ""
-  for _,name in ipairs(arg) do
+  for _,name in ipairs(names) do
     local fp = assert(io.open(name))
     src = src .. fp:read("*a")
     fp:close()
@@ -36,33 +50,56 @@ local function find_defs(src)
   return defs
 end
 
-local function write_defs(fp, defs)
-  fp:write("/* This is a generated file. DO NOT EDIT! */\n\n")
-  fp:write("static const int libbc_endian = ",
-	   string.byte(string.dump(function() end), 5) % 2, ";\n\n")
+local function gen_header(defs)
+  local t = {}
+  local function w(x) t[#t+1] = x end
+  w("/* This is a generated file. DO NOT EDIT! */\n\n")
+  w("static const int libbc_endian = ")
+  w(string.byte(string.dump(function() end), 5) % 2)
+  w(";\n\n")
   local s = ""
   for _,name in ipairs(defs) do
     s = s .. defs[name]
   end
-  fp:write("static const uint8_t libbc_code[] = {\n")
+  w("static const uint8_t libbc_code[] = {\n")
   local n = 0
   for i=1,#s do
     local x = string.byte(s, i)
-    fp:write(x, ",")
+    w(x); w(",")
     n = n + (x < 10 and 2 or (x < 100 and 3 or 4))
-    if n >= 75 then n = 0; fp:write("\n") end
+    if n >= 75 then n = 0; w("\n") end
   end
-  fp:write("0\n};\n\n")
-  fp:write("static const struct { const char *name; int ofs; } libbc_map[] = {\n")
+  w("0\n};\n\n")
+  w("static const struct { const char *name; int ofs; } libbc_map[] = {\n")
   local m = 0
   for _,name in ipairs(defs) do
-    fp:write('{"', name, '",', m, '},\n')
+    w('{"'); w(name); w('",'); w(m) w('},\n')
     m = m + #defs[name]
   end
-  fp:write("{NULL,", m, "}\n};\n\n")
-  fp:flush()
+  w("{NULL,"); w(m); w("}\n};\n\n")
+  return table.concat(t)
 end
 
-local src = read_source()
+local function write_file(name, data)
+  if name == "-" then
+    assert(io.write(data))
+    assert(io.flush())
+  else
+    local fp = io.open(name)
+    if fp then
+      local old = fp:read("*a")
+      fp:close()
+      if data == old then return end
+    end
+    fp = assert(io.open(name, "w"))
+    assert(fp:write(data))
+    assert(fp:close())
+  end
+end
+
+local outfile = parse_arg(arg)
+local src = read_files(arg)
 local defs = find_defs(src)
-write_defs(io.stdout, defs)
+local hdr = gen_header(defs)
+write_file(outfile, hdr)
+