# Understanding Separate Compilation

The TADS 3 compiler has a feature called "separate compilation" that can dramatically speed up the edit/compile/test cycle during game development. Separate compilation is easy to use once you understand how it works. This article describes how the feature works and how to take advantage of it.

## The TADS 2 approach: #include

In TADS 2, the compilation process is fairly easy to understand: you create your source file, and run your source file through the compiler to create the ".gam" file. The TADS 2 compiler reads through your entire source file from start to finish to create the executable version of your game.

In practice, it's not usually quite that simple, because it's usually inconvenient to put your entire game's source code into a single file. At the very least, you certainly don't want to copy the contents of the adv.t and std.t library files into your source file; you want instead to include the contents of those files into your game without actually copying the contents into your source files. In addition, though, you often want to separate your own source code into several files, because smaller files are easier to work with and allow for better organization of your code.

TADS 2 handled multi-file games using the "#include" directive. The way #include works is very simple: when the compiler sees a #include in your source code, it acts as though the entire contents of the referenced file had appeared in place of the #include. In other words, the compiler simply inserts each included file where its #include occurs. (The compiler doesn't modify your original source file to do this; the insertion is "virtual.")

The important thing to understand about the #include mechanism is that it operates purely at the source file level. This means that if you change something in an included file, you have to recompile the entire game, because the compiler has to re-read the entire source code to pick up a change in any part of the game.

## The TADS 3 approach: separate compilation

The TADS 3 compilation process isn't quite as simple as the TADS 2 approach. The TADS 2 compiler just reads your source file and produces the executable game file; the TADS 3 compiler adds an intermediate step.

Instead of turning a source file directly into an executable game file, the TADS 3 compiler turns the source file into something called an "object file." The terminology can be a little confusing, because it doesn't have anything to do with objects in the object-oriented programming sense; the term is the traditional name for the output file that a compiler generates, and usually applies (as it does here) to a compiler output file that still needs to go through some additional steps before it becomes an executable program.

Once the compiler has turned the source file into an object file, there's one extra step, called "linking." The linking process reads the object file and turns it into an executable game file.

Why go through all this extra trouble? Why not just turn the source code directly into an executable file, the way TADS 2 does? To a certain extent, this is the wrong question, because the TADS 2 compiler actually has to perform pretty much the same series of steps internally; the only difference is that TADS 2 takes care of the intermediate step invisibly, but TADS 3 makes it more obvious by writing out the object file and then reading it back in during the linking step. A better question is: why would we want to make the intermediate step visible, rather than hiding it the way TADS 2 does? The answer is that it makes separate compilation possible.

One important thing to know is that the linking step is capable of doing something you might guess from its name: it can combine multiple object files, linking them together into a single executable program file. ("Link" isn't used here in the sense of establishing a relationship among the files, as in linking your ATM card to your checking account; the executable game file doesn't depend on the object files once linking is completed, because the linker copies all of the necessary information from the object files to the final executable game file.)

Another important thing to know about the two steps - compiling and linking - is that compiling step is by far the more time-consuming part of the job. The linking step, where we turn the object file into an executable game file, is a much smaller portion of the work.

So now we can see what separate compilation is good for, apart from generating more intermediate files. Since the linking step lets us take a bunch of separate object files and combine them into a single executable game, we don't have to recompile the entire game every time we change a source file - we only have to recompile the object files affected by the source change. We don't have to compile the unaffected object files; we can just re-use the same ones from the previous build. Because the compile step is usually so much more time-consuming than the link step, this can save a lot of time in the development process.

For the typical author's working style, this is a huge advantage. Most authors write games incrementally: you implement a new bit of geography, then compile and run it to make sure it's working properly, fix a few things and compile and test again, flesh out some objects, compile and test some more, and so on. Even after the game is more or less fully implemented, and you're mostly fixing bugs, you usually use the same kind of edit/compile/test cycle to fix individual bugs and test that the fix did what you expected. When you're working like this, most of the edits you make during one of these edit/compile/test cycles will be isolated to one or two parts of the game. If you've organized your code into multiple sources files roughly along the lines of the game's geography, you'll usually only be changing one or two source files on each cycle of your edit/compile/test process. TADS 3's separate compilation capability will only recompile the files you actually change on each cycle; the compiler won't have to recompile the rest of your source files, and it won't have to recompile any of the TADS library.

## When to keep using #include

Even with separate compilation, you do still need to use #include in some cases. In particular, if you're using the standard library, each and every source file in your game should have these #include's near the top:

       #include <adv3.h>
       #include <en_us.h>

You might be wondering why you need to do this. For that matter, you might wonder why TADS 3 allows #include at all, if separate compilation is so much better. The answer isn't just compatibility with TADS 2; if it were, none of the system or library files would need to use #include, which they do use.

The reason #include is used in TADS 3 is that it serves a purpose that's complementary to separate compilation. Even with the separate compilation feature, #include still comes in handy for some uses. To understand why, you have to understand that the link step (where multiple separately-compiled object files are combined into a single executable game) operates purely on the object files.

You can think of an object file as containing a bunch of little blobs of data, each blob representing an object (or a class or a function) from the source file, and each one tagged with the name you specified in the source code. The compiler's function is to parse the source code syntax and boil it down to those named blobs of data. The linker's function is to put all those blobs together. Now, since an object in one source file can refer to an object in separate source file, and since the compiler looks at each source file in isolation, it's up to the linker to piece together all of those references from one file to another; that's why the names of the data blobs are stored in the object files.

So, the linker makes it possible for you to divide up your game among several source files. The compiler allows a source file to refer to objects, classes, and functions that are never defined in that source file, because it knows the linker will sort out those mysterious references later. (The real name for those mytery references is "external references," because they refer to things defined outside of the source file.)

However, that's all the linker does. There are a number of things it can't do:

- The linker can't make macros definitions from one source file visible to another. A macro definition is simply a "#define" directive.
- The linker can't allow one source file to see templates defined in another file. A template defines custom object creation syntax - for example, from en_us.h:

         Room template 'name' "desc";

- The linker can't make "intrinsic class" statements visible across source files. (An "intrinsic class" statement defines the interface to a built-in class, such as List or Vector. You'll probably never write one of these definitions yourself, because you'll virtually always want to use the ones in the system headers provided with the compiler.)

All of these things are outside of the control of the linker, because they all apply strictly to the source code. Only the compiler can process these constructs. But sometimes - frequently, in fact - it's useful to create a macro or a template once and then use it in multiple source files. This is where #include comes in.

Whenever you write a "#define" directive or "template" statement, you should consider whether you're likely to need it only in a single source file or in several source files. If you might need the same definition in several source files, then you should put it in a "header" file. A header file is simply a bit of source code that you want to make common to several source files. (They're called "header" files because they're usually #include'd near the top, or head, of the source files that use them.)

By convention, TADS header files are given names that end in the ".h" suffix. This is the same convention that C and C++ programmers use. This naming convention makes it easier to see at a glance whether a file contains ordinary source code or common header code that's meant to be included in multiple source files.

Be careful about what you put in headers. Macro definitions and template definitions are always suitable for header files. Most other code is not. In particular, unless you're doing something really clever and know exactly what you're doing, you should never put an object, class, or function definition in a header file. Think about what would happen if you did define an object in a header file: if you included that same header file from multiple source files, then that same exact object definition would appear in each source file, because the contents of the header file are essentially inserted into each including source file. The compiler would happily compile each source file, so each corresponding object module would have the same object definition. When the linker tries to put those object files together into a game, it would find that the same object was defined in several object files, which isn't allowed - the linker would have no way of knowing which was the "real" object with the given name, so it would fail with an error.

So, to summarize, if you're defining a macro, object template, or intrinsic class interface, put it in a ".h" file; if you're defining anything else (an object, class, function, etc.), put it in a ".t" file. In #include directives, never include anything but ".h" files.

Put these in header (.h) files:

- #define directives
- "template" statements

Put these in source (.t) files:

- objects
- classes
- functions
- grammar rules (VerbRule, "grammar", etc.)

Here's a sample of what a .h file might look like. As described above, .h files should include your macro and "template" definitions.

       // myHeader.h

       #define addthree(a, b, c) ((a) + (b) + (c))
       #define ONE   1
       #define TWO   2
       #define THREE 3

       Room template 'name' "desc";
       Thing template 'vocabWords_' 'name' "desc";

And here's a sample of what a .t file might look like. As described earlier, .t files should include all of your object, class, and function definitions.

       // mySource.t

       // Make sure my header definitions are included.
       // Note that we use "quotes" rather than <angle-brackets> to name
       //   the file, because this is our header.  Angle brackets are for
       //   *system* header files.
       #include "myHeader.h"

       class MagicScroll
         spellName = nil
         performSpell() { /* nothing by default */ }
       ;

       blertzScroll: MagicScroll
         spellName = 'blertz'
         performSpell() { /* do my work here */ }
       ;

Going back to the very start of this section, we mentioned that you should usually #include <adv3.h> and <en_us.h> at the start of each source file in your program. Now you can see why: those header files contain the macro and template definitions for the standard library, so you have to #include the headers to make those macro and template definitions visible to your source files.

## How to take advantage of separate compilation

Here are some tips to help you take full advantage of TADS 3's separate compilation capability.

- Organize your source code into several files according to your working style. The goal is to minimize the number of source files you change on each edit/compile/test cycle. Exactly how you do this depends on how you like to work, but for most people, the game's geography is a good way to break things up: identify the major sections of your game's map, and give each major section its own file where you define the section's rooms and objects. A few especially complicated objects, such as major NPC's (non-player characters), might warrant their own separate files.
- Keep each source file relatively small. You want to minimize the amount of source code that the compiler needs to look at on each edit/compile/test cycle, to speed up each compilation. If a source file starts getting too large, consider if there's a logical basis for subdividing the file. But don't go overboard; each compile cycle will take a certain minimum amount of time because of the link step, so there's a point of diminishing returns in the quest for smaller source files. You'll be best served if you simply organize your code in a way that makes sense to you and that's convenient for you to work with. If you do this, you'll almost certainly have a good arrangement for separate compilation.
- Do not use #include directives to combine your source files. Instead, use the linker by listing your files in your project's makefile (the ".t3m" file that you use to control the compiler). If you're accustomed to the TADS 2 way of doing things, making the transition is usually very simple. In TADS 2, your game's main ".t" file - the one you specify on the compiler command line - might look like this:

  mygame.t:
      #include "adv.t"
      #include "std.t"
      #include "desert.t"
      #include "island.t"
      #include "boat.t"
      #include "caves.t"
      #include "castle.t"

      /* the rest of my main source code... */
  | The TADS 2 way— not ideal for TADS 3

  The TADS 3 equivalent is to create a .t3m file for your project, and list the .t files there, rather using #include to combine them:

  mygame.t3m:
      # (my other compile options go here)
      -lib system
      -lib adv3/adv3
      -source mygame
      -source desert
      -source boat
      -source caves
      -source castle
  | The better way for TADS 3

  Note that if you're using TADS Workbench on Windows, you won't have to edit the .t3m file file directly, since Workbench manages the file for you through the Workbench graphical interface. The equivalent with Workbench is to add each of your source files to your project's "Source Files" list in the project window.

## Some questions and answers

If I'm not using #include, how does the compiler know which files to include in my build?

As we saw above, you should list your source files in your project's .t3m file, which we call the makefile. When you compile, use a command line like this:

       t3make -f mygame.t3m

This tells the compiler to read the compilation instructions from the makefile. Since you've listed all of your source files in the makefile, the compiler knows which files you want to include.

I want to include a source file in my project conditionally - only when I build for debugging, say. How can I do this?

The project makefile format doesn't have any conditional features of its own, so there's not a direct way to do this. Fortunately, there's a simple trick you can use to include a source file only in certain builds. Refer to the Build Configurations article for a description of the technique.

Makefiles are too complicated! What if I just want to use the command-line compiler?

Actually, you are, even if you use a makefile. The makefile is simply a file containing command-line options. If you really wanted to, you could type the entire contents of your .t3m file on the command line with the "t3make" command, and the compiler would interpret the instructions the same way. It's a heck of a lot easier to use the makefile, though, because you don't have to type all of those options over and over, every time you compile.

Are these "makefiles" the same thing as the Unix "make" tool uses?

No; they're related only in name and slightly in purpose. The Unix "make" tool is a very general system for declarative, dependency-based scripting; roughly speaking, it is to the Unix command shell as Prolog is to C, if that makes any sense. The Unix "make" tool can be difficult to learn and use, because it combines a somewhat inside-out conceptual model with a syntax based almost entirely on puncutation marks (and what's more, not only is whitespace significant, but different kinds of whitespace - specifically spaces and tabs - have different meanings).

The TADS 3 "makefile" is really nothing more than a file containing the compiler options. However, the name isn't entirely unjustified, because the TADS 3 compiler does do dependency-based builds (see the next question), although the dependency rules are all built in to the compiler.

How does the compiler know which source files need to be recompiled?

The TADS 3 compiler figures out which files need to be recompiled by looking at the modification dates on the object files, and comparing them to the dates on the source files. If a source file has been modified more recently than its corresponding object file, the compiler recompiles the source file; otherwise, the compiler just uses the existing object file, because it sees that the object file was compiled after the last time the corresponding source file was changed.

The compiler also keeps track of all of the header files that a source file references with #include. If any header file that a source file includes has changed more recently than the object file, the compiler is forced to recompile that entire source file, because the change to the header could affect the object file.

The compiler keeps track of a few other factors that can affect the outcome of a compilation as well. Any changes to -D or -U options (defining or undefining a macro symbol) require everything to be recompiled, because these can change the code selected by conditional compilation constructs (#if and the like), or can change the way that code that uses the macros is expanded by the preprocessor. Changes to the set of directories that the compiler searches for included files (-I options) require full recompilation, because these could change which copy of a header file is actually included in the compilation. Installing a new version of the compiler itself will force a full rebuild, because internal elements of the object file format sometimes change between compiler releases.

What if the compiler fails to recompile a file that I know needs to be recompiled?

For the most part, the compiler's dependency tracking mechanism is overly conservative: that is, the compiler will usually recompile more files than it really needs to. Changes in headers, for example, don't necessarily require every includer to be recompiled; but the compiler has no way to detect which includers are affected and which aren't, so it simply recompiles them all.

Even so, there are a few kinds of changes that you can make to one source file that will necessitate recompiling other source files. The compiler can't detect these kinds of dependencies; it assumes that each source file is completely independent of the others, which is mostly true, but not always. The kinds of situations where cross-source dependencies occur are often subtle, but sometimes involve re-purposing a symbol (changing a property name to a function name, for example). It's not usually obvious when one of these situations occurs, and they can sometimes cause compiler or linker errors that seem wrong (in other words, you're pretty sure the code is right even though the compiler is complaining).

If you think the compiler has missed something, and needs to recompile a file that it doesn't seem to think it needs to recompile, the easiest thing to do is to use the "build all" option, -a. This option tells the compiler to ignore dependencies and just recompile the entire program from scratch. To rebuild everything, simply add the -a option to the command line:

    t3make -a -f mygame.t3m

If you're using Workbench on Windows, use the "Full Recompile for Debugging" command in the "Build" menu; it does the same thing.

What about TADS 2's "precompiled header" feature? Isn't that the same thing as separate compilation?

Yes and no. It is essentially the same idea: it lets you group some code and compile it ahead of time, and then re-use the compiled representation on each subsequent build, avoiding the time needed to re-parse that same source code.

But precompiled headers aren't nearly as useful as the true separate compilation in TADS 3. The main problem with the TADS 2 scheme is it lets you use only a single precompiled header in a compilation, so you have to decide ahead of time what files you want to include in the precompiled section. In practice, authors move from section to section while working on a game, so the part of the game you want pre-compiled keeps changing. You can work around this limitation by setting up a master header for the pre-compiled part, and editing the master header as you shift your focus around the game, but this requires several manual steps every time you move to a new section - it's so much overhead, and so error-prone, that it's not worth the trouble for most authors.

TADS 3, in contrast, lets you have as many separate compilation units as you want. There's no need to decide in advance what to "precompile," since every module is essentially precompiled. There's no need to fuss with extra compiler options, since separate compilation is a natural part of the process. And there's absolutely no manual overhead as you shift your focus around the game, since the compiler automatically figures out what you've been changing and recompiles just those parts.

TADS 3 Technical Manual
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