C as an intermediate language

Here's a Forth program debugged in KDevelop – a graphical debugger without Forth support:

KDevelop used to debug Forth code

Cool stuff, not? The syntax highlighting for Forth files – that's someone else's work that comes with the standard KDevelop installation. But the rest – being able to run Forth under KDevelop, place breakpoints, and look at program state – all that stuff is something we'll develop below.

I'll show all the required code; we don't have to do very much, because we get a lot for free by using C as an intermediate language.

A high-level intermediate language is not unusual. A lot of compilers target an existing high-level platform instead of generating native code – for instance, by generating JVM bytecode or JavaScript source code. Why? Because of all the things you get for free that way:

  • Portability
  • Optimization
  • Some degree of library interoperability
  • Some degree of tools interoperability (IDEs, debuggers, etc.)

A few languages targeting high-level platforms are rather well-known: Scala and Clojure with compilers targeting the JVM, CoffeeScript and Dart which are compiled to JavaScript. (Then there's Java, which Google famously compiles to JavaScript – though that remains somewhat offbeat.)

Which languages of this kind are the most successful? Without doubt, today the answer is C++ and Objective-C – languages whose first compilers emitted C code.

I think C is an awesome intermediate language for a compiler to emit. It's extremely portable, it compiles in a snap, optimizes nicely, and you get interoperability with loads of stuff.

When I wanted to make a compiler for an interpreted language we developed internally, I actually thought about targeting a VM, not a source language. I planned to emit LLVM IR. It was GD who talked me out of it; and really, why LLVM IR?

After all, LLVM IR is less readable than C, less stable than the C standard – and less portable than C. It will likely always be, almost by definition.

Even if LLVM runs on every hardware platform, LLVM IR will only be supported by the LLVM tools – but not, say, the GNU tools, or Visual Studio. Whereas generating C code gives you great support by LLVM tools – and GNU, and Visual Studio. Debugging a program generated from LLVM IR in Visual Studio will probably always be inferior to debugging auto-generated C code compiled by the Visual Studio compiler.

"C as an intermediate language" is one of those things I wanted to write about for years. What prevented me was, I'd like to walk through an example – including some of the extra work that may be required for better debugging support. But I couldn't think of a blog-scale example ("web-scale" seems to be the new synonym for "loads of data"; I propose "blog-scale" to mean "small enough to fully fit into a blog post".)

Then it dawned on me: Forth! The minimalist language I've fallen out of love with that still has a warm place in my heart.

So, I'll do a Forth-to-C compiler. That'll fit in a blog post – at least a small Forth subset will – and Forth is different enough from C to be interesting. Because my point is, it doesn't have to be C with extensions like C++ or Objective-C. It can be something rather alien to C and you'll still get a lot of mileage out of the C tools supplied by your platform.

Without further ado, let's implement a toy Forth-to-C compiler. We shall:

Enough Forth to not be dangerous

To be dangerous, we'd have to support CREATE/DOES> or COMPILE or POSTPONE or something of the sort. We won't – we'll only support enough Forth to implement Euclid's GCD.

So here's our Forth subset – you can skip this if you know Forth:

  • Forth has a data stack.
  • Integers are pushed onto the stack. When you say 2 3, 2 is pushed and then 3.
  • Arithmetic operators pop operands from the stack and push the result. 6 2 / pops 6 and 2 and pushes 6/2=3. 2 3 = pushes 0 (false), because 2 is not equal to 3.
  • Stack manipulation words, well, manipulate the stack. DUP duplicates the top of the stack: 2 DUP is the same as 2 2. Swap: 2 3 SWAP is the same as 3 2. Tuck: 2 3 TUCK is the same as… errm… 3 2 3. As you can already imagine, code is more readable with less of these words.
  • New words are defined with : MYNAME …code… ; Then if you say MYNAME, you'll execute the code, and return to the point of call when you reach the semicolon. No "function arguments" are declared – rather, code pops arguments from the stack, and pushes results. Say, : SQUARE DUP * ; defines a squaring word; now 3 SQUARE is the same as 3 DUP * – it pops 3 and pushes 9.
  • Loops: BEGIN … cond UNTIL is like do { … } while(!cond), with cond popped by the UNTIL. BEGIN … cond WHILE … REPEAT is like while(1) { … if(!cond) break; … }, with cond popped by the WHILE.
  • Printing: 2 . prints "2 ". CR prints a newline.
  • Forth is case-insensitive.

That's all – enough to implement GCD, and to scare and startle your infix-accustomed friends.

"Compilation strategy"

…A bit too dumb to be called a strategy; anyway, our blog-scale, blog-strength compiler will work as follows:

  • The stack is an array of "data"; data is a typedef for long.
  • Each user-defined word is compiled to a C function getting a data* pointing to the top of stack (TOS), and returning the new TOS pointer.
  • Where possible, built-in words can be used similarly to user-defined words: s=WORD(s).
  • Some words can't work that way: + can't become s=+(s) because that's not valid C. For such words, we do some trivial translation. 2 becomes PUSH(2), + becomes OP2(+). Similarly, control flow words are translated to do/while, if and break.
  • The stack grows downwards and shrinks upwards: if s[0] is the TOS, s[1] is the element below it, etc; s++ shrinks the stack by popping the TOS.

That's all; for instance, the gcd example from here:

: gcd begin dup while tuck mod repeat drop ;

…compiles to the C code below. As you can see, every word is translated in isolation – the compiler has almost no comprehension of "context" or "grammar":

data* gcd(data* s) { //: gcd
  do {               //begin
    s=dup(s);        //dup
    if(!*s++) break; //while
    s=tuck(s);       //tuck
    OP2(%);          //mod
  } while(1);        //repeat
  s=drop(s);         //drop
  return s;          //;

Here's the full Python source of the compiler (forth2c.py) – a bit redundant after all the explanations:

import sys
# "special" words - can't emit s=WORD(s)
special = {
  ':':'data* ',
  ';':'return s; }',
  'begin':'do {',
  'until':'} while(!*s++);',
  'repeat':'} while(1);',
  'while':'if(!*s++) break;',
# binary operators
binops='+ - * / = < > mod'.split()

def forth2c(inf,out):
  n = 0
  for line in inf:
    n += 1
    # emit line info for C tools (debuggers, etc.)
    # - a nice option C gives us
    print >> out,'n#line',n,'"%s"'%infile
    for token in line.lower().strip().split():
      if token in special:
        print >> out,special[token],
          num = int(token)
          print >> out, 'PUSH(%d);'%num,
        except ValueError:
          if token in binops:
            print >> out,'OP2(%s);'%op2c.get(token,token),
            if defining:
              print >> out,token+'(data* s) {',
            else: # call
              print >> out,'s=%s(s);'%token,
      defining = token == ':'

out = open('forth_program.c','w')
print >> out, '#include "forth_runtime.h"'
for infile in sys.argv[1:]:

And here's the "runtime library", if you can call it that (forth_runtime.h). It's the kind of stack-fiddling you'd expect: s++, s--, s[0]=something.

#include <stdio.h>
typedef long data;

#define PUSH(item) (--s, *s = (item))
#define OP2(op) (s[1] = s[1] op s[0], ++s)
#define PRINT() (printf("%ld ", s[0]), ++s)
#define cr(s) (printf("n"), s)
#define drop(s) (s+1)
#define dup(s) (--s, s[0]=s[1], s)
#define tuck(s) (--s, s[0]=s[1], s[1]=s[2], s[2]=s[0], s)

#define MAX_DEPTH 4096 //arbitrary
data stack[MAX_DEPTH];
data* forth_main(data*);
int main() { forth_main(stack+MAX_DEPTH); return 0; }

Note that our Forth dialect has an entry point word – forth_main – that is passed a pointer to an empty stack by C's main. Real Forth doesn't have an entry point – it's more like a scripting language that way; but the forth_main hack will do for our example.

That's all! A Forth dialect in under 60 LOC. By far the shortest compiler I ever wrote, if not the most useful.

A test

Let's test our forth2c using a few example source files. countdown.4th (from here):

    DUP .
    1 -
    DUP 0 =

gcd.4th (a multi-line version – to make single-stepping easier):

: gcd


: forth_main
  5 countdown
  10 6 gcd .
  35 75 gcd .
  12856 3248 gcd .

We can run the program with the shell script:

./forth2c.py countdown.4th gcd.4th main.4th
gcc -g -static -Wall -o forth_program forth_program.c

This should print:

5 4 3 2 1
2 5 8

So countdown manages to count down from 5 to 1, and gcd finds the gcd. Good for them.


Let's try our program with the trusty gdb:

$ gdb forth_program
There is NO WARRANTY!! etc. etc.
(gdb) b countdown # place breakpoint
Breakpoint 1: file countdown.4th, line 3.
(gdb) r # run
Breakpoint 1, countdown (s=0x804e03c) at countdown.4th:3
(gdb) bt # backtrace
#0  countdown (s=0x804e03c) at countdown.4th:3
#1  forth_main (s=0x804e03c) at main.4th:2
#2  main () at forth_runtime.h:14
(gdb) l # list source code
3	    DUP .
4	    1 -
5	    DUP 0 =
7	  DROP
8	;


Seriously, yay. We placed a breakpoint. We got a call stack – forth_main called countdown.  And we've seen the Forth source code of the function we debug. All that in a debugger that has no idea about Forth.

Partly it's due to compiling to high-level primitives, such as functions, that are supported by tools – we'd get that in every language. And partly it's due to #line – something we don't get everywhere.

#line is brilliant – all languages should have it. That's how we tell debuggers where our assembly code is really coming from – not the intermediate C code, but the real source.

Profilers and other tools

It's not just debuggers that understand #line – it's also profilers, program checkers, etc.

Here's KCachegrind showing the profile of our Forth program, obtained using Valgrind as follows:

valgrind --tool=callgrind ./forth_program
kcachegrind `ls -t callgrind* | head -1`

KCachegrind profiling Forth code

Now isn't that spiffy?

C compilers are also aware of #line – which we could try to abuse by pushing error reporting onto the C compiler. Say, if we use an undefined word do_something, we get an error at just the right source code line:

main.4th:3: implicit declaration of function ‘do_something

A very sensible error message – and we didn't have to do anything! Now let's try a BEGIN without a REPEAT – let's delete the REPEAT from countdown.4th:

gcd.4th:1: error: expected ‘while’ before ‘data’

Um, not a very good message. Perhaps this isn't such a good idea after all. If we want quality error reporting, we have to do the error checking at the source language level.

Custom data display: gdb pretty-printers

Things look rather nice. C tools – gdb, Valgrind, KCachegrind – are doing our bidding, aren't they?

Except for the data stack. Instead of the stack elements, gdb shows us the TOS pointer in hexadecimal: countdown (s=0x804e03c), forth_main (s=0x804e03c), which looks a bit lame.

It's not that the local variable s is useless – on the contrary, that's what we use to look at the data stack. This can be done using gdb commands such as p s[0] (prints the TOS), p s[1] (prints the element below TOS), etc. But we'd much rather look at the whole stack at a time, so that we can see how TUCK tucks our numbers into the stack as we single-step our code.

Can it be done?

  • The good news are that it's easy enough with gdb pretty-printers. (To me it became easy after Noam told me about the pretty-printers.)
  • The bad news are that, unlike the case with the universally supported #line, it's impossible to do custom pretty-printing in a uniform way for all tools. There's no "#line for pretty-printing" – quite a pity.
  • But then the good news are that gdb front-ends such as KDevelop or Eclipse support gdb pretty-printers – to an extent. (KDevelop apparently does a better job than Eclipse.) So you don't have to write a GUI plugin or something equally horrible.

Here's a gdb pretty printer for displaying our Forth data stack. It prints all the elements, from bottom to top (as the Forth stack contents is usually spelled). The TOS pointer is the data* you pass for printing – typically, some function's local variable s. The bottom of the stack is known statically – it's the pointer past the end of the global stack[] array. (If we had threads, that array would be thread-local, but anyway, you get the idea.)

So here's gdb_forth.py:

class StackPrettyPrinter(object):
  bottom = None
  bot_expr = 'stack + sizeof(stack)/sizeof(stack[0])'
  def __init__(self,s):
    self.s = s
  def to_string(self):
    if not self.bottom:
      self.bottom = gdb.parse_and_eval(self.bot_expr)
    s = self.s
    i = 0
    words = []
    while s[i].address != self.bottom:
      i += 1
    return ' '.join(reversed(words))

def stack_lookup_func(val):
  if str(val.type) == 'data *':
    return StackPrettyPrinter(val)

print 'loaded Forth extensions'

gdb.pretty_printers is a list of functions which may decide to wrap gdb.Value they're passed in a pretty-printer class object. Typically they decide based on the value's type. gdb.parse_and_eval returns a gdb.Value representing the value of the given C expression. A gdb.Value has a type, an address, etc. If the gdb.Value is a pointer, you can index it as a Python list: val[ind], and if it's a struct, you can use it as a dict: val['member_name'] (we don't have an example of that one here).

If you're interested in gdb pretty printers – a couple of notes:

  • Getting values from other values is much faster than parse_and_eval which just grinds the CPU to a halt; so learning the not-that-Pythonic-while-also-not-that-C-like syntax of gdb.Value access is very worthwhile.
  • You can return "maps" or "arrays" instead of plain strings. Pretty-printers for STL maps and vectors work that way. This can be used to pretty-print high-level data structures such as dynamic objects (if you're lucky and your design matches gdb's view of things – I found a lot of devil in the details.)

Let's test-drive our pretty printer – but first, let's register it in our ~/.gdbinit:

python execfile('/your/path/to/gdb_forth.py')

And now:

$ gdb forth_program
There is NO WARRANTY!!
loaded Forth extensions # aha, that's our printout!
(gdb) b gcd
(gdb) r
Breakpoint 1, gcd (s=10 6) # and that's the stack
(gdb) python for i in range(4): gdb.execute('c')
# continue 4 times
Breakpoint 1, gcd (s=6 4)
Breakpoint 1, gcd (s=4 2)
Breakpoint 1, gcd (s=2 0)
# these were the steps of gcd(10,6)
Breakpoint 1, gcd (s=35 75)
3           dup
# new inputs - gcd(35,75). let's single-step:
(gdb) s # step (execute DUP)
4         while
(gdb) p s # print s
$1 = 35 75 75 # so DUP duplicated the 75.
(gdb) s # step (execute WHILE)
5           tuck
(gdb) p s
$2 = 35 75 # ...and WHILE popped that 75.
(gdb) s # and now we execute the mighty TUCK:
6           mod
(gdb) p s
$3 = 75 35 75 # YES! Tucked the 75 right in!
(gdb) bt # and how's main's data stack looking?
#0  gcd (s=75 35 75) at gcd.4th:6
#1  forth_main (s=75 35) at main.4th:5
# well, it looks shorter - somewhat sensibly.
# (main TOS pointer is unchanged until gcd returns.)

I think it's rather nice. 10 6, 6 4, 4 2, 2 0 – a concise enough walk through Euclid's algorithm in Forth.

KDevelop with the stack display

And, finally, here's how it looks in KDevelop (where the stack displayed in the GUI changes upon every step, as one would expect from a good debugger). The stack is in the variables view on the left.

There is nothing special to be done to make things work in KDevelop – except for the standard procedure of convincing it to debug a user-supplied executable, as you might do with a C program.

Features that don't map naturally to C

Some features of a source language map more naturally to C than others. If we tried to tackle a language different from Forth – or to tackle all of Forth instead of a small subset – some features would pose hard problems.

Some such features can be handled straightforwardly with another intermediate language. For example, dynamic data attributes map to JavaScript much more nicely than they do to C.

But in many cases you don't pick your intermediate language because it's the most suitable one for your source language. If you want to target browsers, then it pretty much has to be JavaScript, even if it's a poor fit for your source language. Similarly, in other situations, it has to be C – or JVM, or .NET, etc.

In fact, many language designers made it their key constraint to play nicely with their platform – the intermediate language. Some advertise the harmonious integration with the platform in the language name: C++, Objective-C, CoffeeScript, TypeScript. They know that many users are more likely to choose their language because of its platform than for any other reason, because they're locked into the platform.

With that said, here are a few features that don't map straightforwardly to C, and possible ways to handle them.

  • Dynamic binding: easy enough. Generate something like call(object,method_id), or call(object,"method_name"), multidispatch(method,obj1,obj2) – whatever you want. Should work nicely.
  • Dynamic code generation – eval, etc.: C doesn't have eval – but it does have fopen("tmp.c"), fwrite, system("gcc -o tmp.so tmp.c …"), dlopen("tmp.so"), and dlsym. This can give you something a lot like eval. You need non-portable code to make it work, but not much of it. Alternatively, if a relatively slow eval with relatively poor debugger support is OK (often it is), you can implement eval using an interpreter, which can reuse the parser of your static compiler. We use both methods in a few places and it works fine.
  • Dynamic data: Code generation is easy – access(object,"attr_name") or whatever. The ugly part is viewing these objects in debuggers. One approach is pretty-printers or similar debugger scripting. Another approach – for "static enough data" – is to generate C struct definitions at runtime, compile them to a shared library with debug info, and load the library, as you'd do to implement eval.
  • Exceptions: I use setjmp/longjmp for that; I think that's what cfront did.
  • Garbage collection: AFAIK it can work fine, funnily enough. There are garbage collectors for C, such as the Boehm collector. Do they work for C programs? Not really – they can mistake a byte pattern that happens to look like a pointer for an actual pointer, causing a leak. But you don't have that problem – because your C program is generated from code that your compiler can analyze and then tell the collector where to look for pointers. So while gc doesn't truly work for C, it can work fine for languages implemented on top of C!
  • Generics, overloading, etc.: most of this is handled by your compiler, the only problem being name mangling. You can mangle names in a way that you find least ugly, or you can mangle them according to the C++ mangling convention, so that debuggers then demangle your names back into collection<item> or some such. (The C++ mangling convention is not portable so you might need to support several.)
  • Multiple return values: we pass pointers to all return values except the first; I think returning a struct might make things look nicer in debuggers. Of course the calling convention will be less efficient compared to a compiler emitting assembly rather than C. But it will still be more efficient than your actual competition, such as a Python tuple.
  • Program reduction (as in, transform (+ (* 2 3) 5) to (+ 6 5), then to 11): ouch. I guess it could make sense to write an interpreter to do that in C, but you wouldn't get that much mileage out of the C optimizer, and no mileage at all from C debuggers, profilers, etc. The basic model of computation has to be close enough to C to gain something from the C toolchain besides the portability of your C code implementing your language. (I'm not saying that you absolutely have to create a memory representation of (+ 6 5) at runtime – just that if you want to create it, then C tools won't understand your program.)

What about using C++ instead of C?

You could, and I did. I don't recommend it. You get exceptions for free instead of fiddling with setjmp/longjmp. Then they don't work when you throw them from one shared library and catch in another (I think RTTI has similar problems with shared libraries). You get templates for free. Then your auto-generated code compiles for ages.

Eli Bendersky writes about switching from C to C++ to implement a language VM. He says it's easier to use C++ features when they match your VM's needs then to reimplement them in C.

Here's how it worked out for me. I once implemented a VM and an interpreter in C++. Then we wrote a compiler for the language. Because the VM was in C++, it was much easier for the compiler to emit C++ code interfacing with the VM than C code. And then with this auto-generated C++ code, we bumped into shared-library-related problems, and build speed problems, etc. – and we have them to this day.

But of course, YMMV. One reason to use C++ is that debuggers get increasingly better at displaying STL collections and base class object pointers (which they know to downcast to the real runtime type and show the derived class members). If your language constructs map to these C++ features, you can get better debug-time data display in a portable way.

Anything you'd like to add?

If you have experience with using C as an intermediate language, I'll gladly add your comments. I'm more experienced with some things than others in this department, and so a lot of the potential issues would likely never occur to me.


C is a great intermediate language, and I'd be thrilled to see more new languages compiling to C. I don't live in either Java or JavaScript these days; surely I'm not alone. A language compiling to C could be fast, portable, have nice tools and library support – and immediately usable to many in the land of C, C++ and Objective-C.


#1 Miroslav Bajtoš (@bajtos) on 09.30.12 at 8:53 am

Hello Yossi, have you heard about Vala? It's compiling C#-like language to C, using GObject from Gtk+ for OOP.


#2 Steve on 09.30.12 at 3:05 pm

I'm also pretty interested in seeing how simple typed (or untyped I suppose) functional languages might be able to leverage C++11's new lambda support as a natural target, as well as deal with C++'s automatic memory management via unique_ptr and shared_ptr instead of relying on garbage collection. It would be interesting to see if a fairly high-level functional language could map to C++11's model and thereby achieve similar performance. I agree that your library-related problems could show up, but maybe the right thing is to just make sure not to rely on C++'s exception mechanism. For example, using std::pair return values might help achieve something similar to Haskell's Either type, which can be used to implement a monadic exception model.

#3 philip andrew on 09.30.12 at 7:26 pm

I asked this question here about how to dynamically reload compiled c/c++ code quickly, its related http://stackoverflow.com/questions/12319329/how-to-dynamically-load-often-re-generated-c-code-quickly/12322672

#4 Yossi Kreinin on 10.01.12 at 1:05 am

@Steve: if you're generating code (as opposed to writing it by hand), then there seems to be relatively little benefit in generating C++ that undergoes rather complicated (and slow) transformations to become something not unlike C inside compared to generating C directly. How is std::pair better than a C struct? It doesn't look better in debuggers and it compiles more slowly. Similarly for lambdas which become pairs of an anonymous function + an anonymous struct. There can be some benefit in generating C++ maps or class hierarchies as I mentioned above – namely debugger support – but I don't see it with every C++ feature (perhaps mistakenly for some of such features which might become better-supported in the future in ways that I don't currently imagine).

#5 Ivan Tikhonov on 10.01.12 at 2:39 am

I am doing exactly the same (Forth to C in Python) and here is my quick summary of things:


#6 Araq on 10.01.12 at 5:35 am

You just described how Nimrod works. Please check it out. ;-)

#7 Yossi Kreinin on 10.01.12 at 7:38 am

@Miroslav, @Araq: cool stuff, heard something about it :)

@philip: looks like they didn't quite answer how to reload code… I'd think dlclose and then dlopen could do the trick, especially if the names were different every time, but I dunno.

@Ivan: you did surprise me :) Is this recreational or do you have concrete plans to use it?

#8 No One on 10.01.12 at 8:16 am

Useful tool: libtcc (from Bellard's tinycc.org) lets you compile directly to memory without any intermediate files /dlopen / dlsym. While it has no optimizer to speak of, it IS comparable to -O0 code from gcc, and it is incredibly fast.

Is there any compiler that implements a #line+column? e.g. it would be nice if your forth example could indicate which parts of the C code correspond to which input part of the line – so you wouldn't have to (unnaturally) insert newlines in the code just to make it a breakpoint target.

#9 Alexey on 10.01.12 at 8:55 am

@Steve Cannot say a thing about C++ lambdas, but I would expect horrible performance from shared_ptr. Reference counting have nonzero overhead and functional language tend to allocate a lot of short lived objects (few GB/s is normal allocation rate for haskell). Generational GC is probably better in this case.

Also C is not good target language for functional languages. Tail call elimination is essential but there is no way to do it in portable C. You have either to use compiler-specific extensions and lose some portability or use trampolies. This is because C doesn't give you any control over stack.

Another case when C is not appropriate choice is when you want to control what machine code is generated for specific language constructs.

#10 Yossi Kreinin on 10.01.12 at 9:02 am

@No One: I think that in any environment, you basically want line-level single-stepping to be line-level, and the fact that your formatting of the code affects the meaning of a "single step" is a feature, not a bug.

#11 Ivan Tikhonov on 10.01.12 at 9:19 am

@Yossi: i hope it's serious, but i think it's 10th my attempt at Forth for my personal use. But they get better each time :)

I like C and mostly write programs in it, but it is quite verbose and some limitations (which can be solved by proper macros) really annoy me.

But i am worrying a bit it may be just an unconscious excuse to procrastinate instead of working toward my goals.

#12 Yossi Kreinin on 10.01.12 at 9:22 am

@Ivan: I have a tendency to counter my wish to procrastinate through meta-programming in some brutal way of, "OK, don't do that, show me (myself, that is) some progress"; basically making myself feel as if there was a deadline and that it has to be done, doesn't matter how. Works to an extent.

#13 Ivan Tikhonov on 10.02.12 at 8:49 am

Thanks, that helps. :-)

#14 Alex on 10.02.12 at 9:26 am

@Alexey: "You have either to use compiler-specific extensions and lose some portability or use trampolies. This is because C doesn't give you any control over stack."

…or you could just have a compiler switch to choose between "optimised" vs. "portable fallback"?

It's a bit of a pain, but actually implementing proper tailcalls in a platform-specific way really isn't that hard, and at least you can still guarantee safety for the platforms you don't care about enough to optimise.

#15 Ivan Tikhonov on 10.02.12 at 10:24 am

@Alex: …or you could just have a compiler switch to choose between "optimised" vs. "portable fallback"?

Tail call is not only about speed and optimization. It's about having recursion without blowing stack.

Implementing tail calls just for the sake of optimization is meaningless in case of Forth, as you gain more from automatic inling than from tail calls.

#16 Alex on 10.02.12 at 2:14 pm

True, but implementing tail calls *slowly* isn't too hard: you can do it fairly easily with some combination of thunks and runtime type checking. So C's lack of built in support for tail calls is really more of an optimisation question, in that you need to get into the non-portable stuff mainly for the sake of making them work *well*.

#17 Ivan Tikhonov on 10.02.12 at 11:42 pm

@Alex "True, but implementing tail calls *slowly* isn't too hard: you can do it fairly easily with some combination of thunks and runtime type checking."

I am not sure what you mean, but won't it not only slow down tail calls, but whole code? Seems like it will involve a lot of indirect calls and prevent inlining.

#18 Alex on 10.03.12 at 10:40 am

Indeed it would, but it also prevents the stack from overflowing, and it's the easiest way to do it in portable, standard-compliant C.

That's what I meant by a compiler switch: one option to be non-portable, relying on asm tweaks or compiler settings, that get the behaviour you actually want ("real" tail calls); and one option that is portable but slows down the whole code, for platforms you haven't written the non-portable version for yet (indirect calls).

All I'm saying is that you shouldn't necessarily feel you have to exclusively choose between ISO C and machine code output: there is still a flexible middle ground for you to take advantage of when your language doesn't match C quite so well.

#19 oren on 10.04.12 at 11:41 pm

The GHC Haskell compiler has a mode in which it outputs C code, and another one where it then compiles the C code using GCC.


Now Haskell is *the* most popular pure functional language today, also it has a default of lazy evaluation, which means many, many thunks are created around from an innocent-looking line of code.

While the docs say this mode is now deprecated in favor of using direct native code generator or LLVM, this does makes Yossi's arguments about using C as an intermediate language much stronger.

#20 Dmitry on 10.06.12 at 1:51 am

Прикольно, я не знал про #line и про то, что все инструменты, основанные на gcc, можно приделать к другому языку через C, спасибо за ликбез.

В среде Racket, кстати, есть аналогичные средства, и вместо сгенерированного текста с #line там няшные макросы, порождающие синтаксические объекты (родные для Racket) с привязками к произвольным локациям в исходном файле. И возможности ВМ позволяют легко реализовать любой язык на свете. Но, действительно, использовать всё это потом можно только в Racket, и о разных там Eclipse и Valgrind остаётся только мечтать.

Вместо "news are", кажется, правильнее "news is".

#21 ann on 10.10.12 at 9:02 pm

thanks this

#22 oliver on 02.19.13 at 7:58 am

Haxe (http://haxe.org/doc/features) is a language that only supports compilation to other existing languages (C++, Javascript, Java…). The concept seems neat, but I'm not sure whether the language itself is actually well-designed.

#23 Leo on 07.17.13 at 5:17 am

I'm finding C to be a wonderful language even if you *do* work in the JavaScript space. We've been building Superconductor, a system for real-time visualization of big data sets, and under-the-hood, compile to OpenCL (a C variant) to use the GPU. Most browsers don't have OpenCL bindings (WebCL), so we also generate JavaScript.

Two performance benefits arise: we found the JavaScript that looks like C to be much faster than normal looking JavaScript, and if you use the Emscripten compiler (C–LLVM–>JS), newer browsers basically run it as if it were C (asm.js).

#24 vs on 07.17.13 at 6:20 am

There exist a language/compiler named 'Harbour', which is able to generate both pseudo-code ("pcode") and ANSI C output. Pseudo-code output also uses C as an intermediate language, thus, pcode and ANSI C objects can be freely mixed, and both can be transparently compiled to binary using the tool 'hbmk2'. There is also an option to generate portable pcode binaries (.hrb).

#25 John Cowan on 10.17.14 at 8:43 pm

The Scheme language has several compilers that convert to higher-level languages. In particular, Gambit, Chicken, Bigloo, and Larceny are independent implementations that compile to C. On the JVM and CLR, it's more usual to compile to bytecode rather than Java or C# source: Kawa and Bigloo compile to JVM bytecode, and Larceny and IronScheme to CLR bytecode. Most other Schemes are bytecode compilers with or without JITs, native x86 compilers, or tree-walking interpreters. Links to these implementations can be found in the fairly complete list of Schemes. In addition, Common Lisp implementations include Embeddable Common Lisp and its descendent ManKai CL, which compile to C, as well as Armed Bear CL, which compiles to JVM bytecode.

Chicken is an especially interesting case, because it supports both proper tail calls and call with current continuation without extra expense using Cheney on the MTA, a technique whereby each Scheme procedure is compiled to a C procedure that calls its successor rather than returning, winding up the stack until it becomes large enough, at which point all live local data is flushed to the heap and the stack is reset with longjmp().

#26 Lewis Andrew Campbell on 11.23.14 at 12:29 am

I've been working on a stack based VM this weekend, and this blog post was really helpful. I would never have thought of just keeping a pointer to the stack and decrementing the address, instead of keeping an index to the top element. I did have many mysterious segfaults copying this technique though, until I realised you had to initialise the stack as pointing just past the last element.

#27 Lucio on 01.03.15 at 3:33 am

>>"C is a great intermediate language"
I've reached the same conclusion after coding a compile-to-c mode for a lang: LiteScript.
Initially designed to compile-to-js, I was pleased to see how good fit is C for an intermediate language.
Note: I did also used setjmp/longjmp for exceptions and the Boehm GC.
Check it at: https://github.com/luciotato/LiteScript

#28 alpha.tau on 08.07.15 at 12:49 am

All this talk about C being a "great/useful/suitable" intermediate language begs a simple question – is there
a library out there somewhere to simplify the development of a C backend/code-generator, instead of
having to "reinvent the wheel" each and every time?

#29 Sean B. Palmer on 09.20.15 at 6:46 pm

I'm not sure how this happened, but it did:


#30 Yossi Kreinin on 09.20.15 at 7:25 pm

I'm flattered, I guess…

I really should go over the blog and fix the shit done by the various WP upgrades over the years where the text gets fucked up and no longer compiles. Oh, WP, WP…

#31 none on 12.31.16 at 12:01 am

1) Boehm GC actually works pretty well in practice– see the literature about it.

2) Purescript (purescript.org) compiles to C++ in a useful way, basically using std::shared_ptr as a poor implementer's GC. Its usual back end is javascript though.

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