xdc: A hypothetical D cross-compiler and AST manipulation to

I'd like to present my vision for a new D compiler.  I call it xdc, a loose abbreviation for "Cross D Compiler" (if confused, see http://english.stackexchange.com/questions/37394/why-do-some-words-have-x-as-a-substitute).  It could also mean other fun things like "Crossbone D Compiler" (I imagine a logo with some crossbones having a metal D atop where the skull normally goes), "eXperimental D Compiler", or something sounding like "ectasy"

We usually think of language features as being rewritten into simpler features.  The simple features eventually get rewritten into machine instructions.  Compilers are, fundamentally, responsible for performing "lowering" operations.

It makes sense to me, then, to make a compiler whose internals /look/ like a bunch of these rewrites and lowering operations.  There should be a bunch of input patterns matched to the desired results.  This has the happy side-effect of giving us a pleasant way to do AST manipulations from within D code.

I've also had a long-standing desire to see D on many more platforms.  It should make an appearance on console platforms and on smartphones.  I've tried doing this with a retargetable compiler like GCC before, and the work was surprisingly large.  Even if the compiler already emits code for the target system's CPU, there are still a large number of details involving calling conventions, executable format, and any number of CPU/OS specific tweaks to object file output.  It makes a lot more sense to me to just output C/C++ code and feed that to a native toolchain.  That would skip a lot of the platform-specific nonsense that creates a barrier to entry for people who, say, just want to write a simple game for Android/iPhone/PS(3|4)/etc in D, and don't want to become compiler experts first.  Ideally, some day, this compiler would also emit code or bytecode for Javascript, AS3/Flash, Java, and any other popular targets that the market desires.  This can probably be done with DMD, but I'd like to make the process more approachable, and make backend authoring as easy as possible.  It should be possible (and easy) to tell the compiler exactly what lowerings should be applied before the AST hits the backend.

xdc should bring all of that cross-platform targeting together with a compiler infrastructure that can blow everything else away (I hope!).

xdc is my dream for a D compiler that gives us our first step (of few) towards having what haXe has already (http://haxe.org/) : a compiler that can land code just about anywhere.

What follows is a collection of my thoughts written down as notes.

== Ideal Outcomes ==

.- D to C/C++ compiler that can easily reach target platforms that are
.    currently either unsupported or poorly supported by current D
.    compilers.
.   - Useful for game developers wishing to write D code on the
.       next big console platform.
.   - Useful for embedded systems developers wishing to write D code
.       on obscure or potentially proprietary microcontrollers.

.- Other backends (ex: llvm, Java bytecode, AS3/Flash bytecode, etc)
.    possible in the future.  Community desires considered when
.    selecting new backend targets.

.- Interpreter backend: a notable backend that would be implemented as
.    a necessity for making CTFE work.  A dedicated interpreter
.    backend would hopefully be much faster and easier on memory than
.    DMD's souped-up constant folder.  (Even though DMD has become
.    considerably better at this in the past year or two.)

.- Abstract Syntax Tree (AST) manipulation toolchain, possibly usable
.    in CTFE.  It would work like this:
.    (1) Pass some D code or Domain Specific Language (DSL) of your
.          choice (as text) into xdc at compile-time.
.    (2) xdc returns an AST.
.    (3) Use xdc's pattern-matching and substitution DSL to
.          manipulate the AST.
.    (4) xdc consumes the AST and emits modified D code.
.    (5) mixin(...) the result.
.   - If xdc is the compiler used to execute itself in CTFE, then
.       it might be possible to optimize this by having it expose
.       itself as a set of intrinsics.

.- Reference counting available by default on all platforms.
.   - Gets you into the action with minimal effort and little or no
.       compiler hacking. (More complete GC tends to require platform
.       specific ASM and/or operating system API support).

.- Full garbage collection available if supported.
.   - Ex: The C backends would default to ref-counting until the ASM
.       and OS-level code is written to support full GC.
.   - Ex: A Java backend would probably use the Java JVM by default.

.- Threading model determined by compiler configuration or available
.    platform hints.
.   - Ex: The user may have a posix-threads implementation available,
.       but know little other details about the target system.  It
.       should be possible for xdc to use pthreads to emulate the
.       TLS and synchronization mechanisms needed to make D tick.
.       (Or at least emulate as many features as possible.)
.   - Ex: Possible "no threading" target for people who don't need
.       threading but DO need other D features to be available NOW
.       on an alien platform.  Errors when the source code passed
.       into xdc assumes that threading features are present.

.- D compiler that is easy to hack on.
.   - "Looks like the problem it solves."
.       (To quote Walter's DConf2013 keynote.)
.   - Made of a bunch of patterns that describe
.       code rewrites/lowerings.
.   - Few or no null value checks necessary.
.      - null checks don't look like pattern matching or lowering.
.   - Few or no convoluted if-while-if-for-etc nests.
.      - These also don't look like pattern matching or lowering.
.   - It should be largely made of "pattern handlers" (see below).
.   - Each pattern handler will have one part that closely resembles
.       the AST fragment for the D code that it recognizes, and
.       another part that resembles the lowered form that it outputs.
.   - Dependency analysis that prevents your AST manipulation from
.       happening either too early or too late.
.   - Because the code that actually does lowerings is generated from
.       a DSL, it is possible to make it automate a lot of tedious
.       tasks, like updating the symbol table when nodes are added or
.       removed from the AST.
.   - This makes it easier to test experimental features.

.- A step-by-step view of what the compiler is doing to your code.
.   - Since the semantic analysis of xdc would be composed of
.      "pattern handlers" (see below), then each time one of them
.      completes the compiler could output the result of calling
.      .toString() (or .toDCode() or whatever) on the entire AST.
.   - This could be attached to an ncurses interface that would be
.      activated by passing a flag to the compiler, which would then
.      proceed to show the AST at every stage of compilation.
.      Press ENTER to see the next step, etc.
.   - This could also be exposed as API functionality that IDEs could
.      use to show developers how the compiler sees their code.

.- D code analysis engine that might be usable to automatically
.    translate D1 code into D2 code, or maybe D2 into D3 in the far
.    future.

== Architectural Overview ==

.- xdc will largely consist of "pattern handlers" that recognize
.    patterns in its AST and replace them with AST fragments that
.    contain successively fewer high-level features (lowering).
.   - These pattern handlers would feature a DSL that should make
.       the whole task fairly easy.
.   - The DSL proposed would be similar to regular expressions in
.       semantics but different in syntax.
.      - It will have operators for choice, repetition, optional
.          matches, capturing, and so on.
.      - The DSL must support nested structures well.
.      - The DSL must support vertical layout of patterns well.
.      - Because of the vertical patterns, most operators will either
.          be prefix or will be written in block style:
.          some_block_header { block_stmt1; block_stmt2; etc; }
.      - Actions like entering and leaving nodes are given their own
.          special syntax.  The machine will treat them like tokens
.          that can be matched the same as any AST node.  Notably,
.          node-entry and node-exit do not require introducing
.          non-regular elements to the DSL.  node-entry and node-exit
.          may be subsumed into Deterministic Finite Automatons (DFAs).
.   - An example pattern handler might look like this:

const lowerWhileStatement =
{
    // Apologies in advance if this isn't actually valid D code:
    //   This is a design sketch and I currently don't have a way to compile it.
    //
    // The Pattern template, PatternMatch template, and PatternHandler class
    //   have not yet been written.  This is an example of how I might expect
    //   them to be used.
    //

    auto consumes = "while_statement";
    auto produces = "if_statement","goto","label");

    auto recognizer = Pattern!
        "WhileStatement has
        {
            // Capture the conditional expression (call it \"expr\") and
            //   capture the loop body (call it \"statement\").
            .expression $expr;
            .statement  $statement has
            {
                // Capture any continue/break statements.
                any_amount_of {
                    any_amount_of .; // Same as .* in regexes.
                    one_of
                    {
                        ContinueStatement $continues;
                        BreakStatement    $breaks;
                    }
                }
                any_amount_of .;
            }
        }";

    auto action = (PatternMatch!(recognizer) m)
    {
        m.captures.add("uniqLoopAgain", getUniqueLabel(syntaxNode.enclosingScope))
        m.captures.add("uniqExitLoop", getUniqueLabel(syntaxNode.enclosingScope))

        // The "recognizes" clause defines m.getCapture!"continues" with:
        //   "ContinueStatement $continues;"
        // That line appears in a repitition context ("any_amount_of") and is
        //   therefore typed as an array.
        foreach( ref node; m.getCapture!"continues" )
            node.replaceWith( m, "GotoStatement has $uniqLoopAgain" )

        // Ditto for m.getCapture!"breaks" and "BreakStatement $breaks;".
        foreach( ref node; m.getCapture!"breaks" )
            node.replaceWith( m, "GotoStatement has $uniqExitLoop" )
    };

    auto synthesizer = Pattern!
        "Label has $uniqLoopAgain
        IfStatement has
        {
            OpNegate has $expr
            GotoStatement has $uniqExitLoop
        }
        $statement
        GotoStatement has $uniqLoopAgain
        Label has $uniqExitLoop
        ";

    return new PatternHandler(produces, consumes, recognizer, action, synthesizer);
};

(Also available at: http://pastebin.com/0mBQxhLs )

.- Dispatch to pattern handlers is performed by the execution of a
.    DFA/Packrat hybrid instead of the traditional OOP inheritance
.    with method calls.
.   - Each pattern handler's recognizer gets treated like a regex
.       or Parsing Expression Grammar (PEG) fragment.
.   - All of the recognizers in the same semantic pass are pasted
.       together in an ordered-choice expression.  The ordering is
.       determined by dependency analysis.
.   - A recognizer's pattern handler is invoked when the recognizer's
.       AST expression is matched.
.   - Once any substitutions are completed, then the machine executing
.       the pattern engine will set its cursor to the beginning of
.       the newly substituted AST nodes and continue running.
.   - Executing potentially hundreds of pattern handlers in a single
.       ordered-choice expression would be obnoxious for a packrat
.       parser (packrat machine?).  Thankfully, ordered-choice is
.       possible in regular grammars, so it can be lowered into regex
.       operations and the whole thing turned into a DFA.
.   - If pattern recognizers end up needing recursive elements,
.       then they will probably not appear at the very beginning of
.       the pattern.  Patterns with enough regular elements at the
.       start will be able to merge those regular elements into the
.       DFA with the rest of the pattern recognizers, and it all
.       becomes very fast table lookups in small tables.

.- This compiler would involve the creation of a parser-generator
.    API that allows code to programmatically create grammars, and
.    to do so without a bunch of clumsy string formatting and string
.    concatenation.
.   - These grammars could be written such that things like AST nodes
.       are seen as terminals.  This expands possibilities and allows
.       all of the pattern handlers to be coalesced into a grammar
.       that operates on ASTs and fires off semantic actions whenever
.       one of the recognizer patterns gets tickled by the right AST
.       fragment.
.   - Using strings as terminals is still cool; and necessary for
.       xdc's text/D-code parser.
.   - A simple parser-generator API example:

---------------------------------------
string makeParser()
{
    auto builder = new ParserBuilder!char;
    builder.pushSequence();
        builder.literal('x');
        builder.pushMaybe();
            builder.literal('y');
        builder.pop();
    builder.pop();
    return builder.toDCode("callMe");
}

const foo = makeParser();

pragma(msg, foo);
---------------------------------------
Current output:
http://pastebin.com/V3E0Ubbc
---------------------------------------

.   - Humans would probably never directly write grammars using this
.       API; it is intended for use by code that needs to write
.       grammars.  xdc would be such code: it's given a bunch of
.       pattern handlers and needs to turn them into a grammar.
.   - This API could also make it easier to write the parser
.       generators that humans /would/ use. For example, it could be
.       used as an experimental backend for a regular expression
.       engine that can handle limited recursion.
.   - The packrats usually generated from PEGs are nice and all, but
.       I'd really like to generate DFAs whenever possible, because
.       those seem to be regarded as being /very fast/.
.   - DFAs can't handle the recursive elements of PEGs, but they
.       should be able to handle everything non-recursive that
.       precedes or follows the recursive elements.
.   - The parser-generator API would be responsible for aggressively
.       converting PEG-like elements into regex/DFA elements whenever
.       possible.
.   - Regular expressions can be embedded in PEGs as long as you tell
.       them how much text to match.  You have to give them concrete
.       success/failure conditions that can be determined without
.       help from the rest of the PEG: things like "match as many
.       characters as possible" or "match as few characters as
.       possible".  Without that, the regex's backtracking (DFA'd
.       or otherwise) won't mesh with the PEG.  Give it a concrete
.       win/fail condition, however, and the embedded regex becomes
.       just another PEG building block that chews through some
.       source material and yields a yes/no result.  Such regular
.       expressions allow DFAs to be introduced into a recursive
.       descent or packrat parser.
.   - Many PEG elements can be converted into these well-behaved
.       regular expressions.
.      - PEG repetition is just regular expression repetition with
.          a wrapper around it that says "match as many characters
.          as possible".
.      - PEG ordered choice can be lowered into regular expression
.          unordered choice, which can then be converted into DFAs:
.          I suspect that this is true: (uv/xy)c == (uv|(^(uv)&xy))c
.          (or, by De Morgan's law: (uv/xy)c == (uv|(^(uv|^(xy))))c )
.          & is intersection.
.          ^ is negation.
.          Each letter (u,v,x,y,c) can be a subexpression
.            (non-recursive).
.      - PEG label matching can be inlined up to the point where
.          recursion occurs, thus allowing more elements to be
.          considered for DFA conversion.
.      - etc.

.- The parser would be defined using a PEG (most likely using Pegged
.    specifically).
.   - Although Pegged is an awesome achievement, I suspect its output
.       could be improved considerably.  The templated code it
.       generates is slow to compile and ALWAYS allocates parse
.       tree nodes at every symbol.
.   - I want to experiment with making Pegged (or a branch of it) emit
.       DFA/Packrat parser hybrids.  This could be done by making a
.       version of Pegged that uses the aforementioned
.       parser-generator API to create its parsers.
.   - Design principle:  avoid memory allocations like the plague.
.       The output should be a well-pruned AST, and not just a parse
.       tree that causes a bunch of allocations and needs massaging to
.       become useful.
.   - I really like Pegged and would contribute this stuff upward, if
.       accepted.

.- When hacking on xdc, you don't need to be aware of WHEN your code
.    code gets executed in semantic analysis.  The dependency analysis
.    will guarantee that it always gets performed both
.    (a) when it's needed, and (b) when it has what it needs.
.   - This is what the "consumes" and "produces" variables are all
.       about in the above example.

.- Successfully lowering a D AST into the target backend's input will
.    almost certainly require multiple passes.  xdc's dependency
.    analyzer would automatically minimize the number of passes by
.    looking for patterns that are "siblings" in the dependency graph
.    (eg. neither depends on the other) and bunching as many such
.    patterns as possible into each pass.
.   - It really shouldn't generate very many more than the number of
.       passes that DMD has coded into it.  Ideally: no more than DMD,
.       if not fewer.
.   - I'd like to make the dependency analyzer output a graph that
.       can be used to track which patterns cause which passes to
.       exist, and show which patterns are in which passes.

.- Planned availability of backends.
.   - My first pick for a backend would be an ANSI C89 target.  I feel
.       that this would give it the most reach.
.   - The interpreter backend is along for the ride, as mentioned.
.   - Because the semantic analysis is composed of distinct and
.       loosely-coupled patterns, it is possible for xdc to generate
.       an analysis chain with the minimum number of lowerings needed
.       for a given backend.
.      - The interpreter backend would benefit from having the most
.          lowerings.  By requiring a lot of lowering, the interpreter
.          would only need to support a small number of constructs:
.         - if statements
.         - gotos
.         - function calls
.         - arithmetic expression evaluation
.         - builtin types (byte, short, int, long, float, double, etc)
.         - pointers
.         - Even structs are unnecessary: they can be seen as
.             typed dereferencing of untyped pointers.
.      - The C backend would benefit from slightly less lowering than
.         the interpreter backend.  It is useful for debugging if
.         you can mostly-sorta read the resulting C code, and your
.         C compiler will appreciate the extra optimization
.         opportunities.
.         - Looping constructs like while and for are welcome here.
.         - structs would be more readable.
.      - In the future, a Java or C# backend might use an entirely
.          different set of lowerings in later passes.
.         - Pointers are no longer considered "low".
.         - Classes should be kept as long as possible;
.             I'm pretty sure they bytecode (at least for Java)
.             has opcodes dedicated to classes.  Removing them
.             may cause pessimisation.
.      - The backend writer should not have to worry about rewriting
.          the semantic analysis to suit their needs.  They just define
.          some features and say which ones they need available in the
.          AST, and xdc's semantic-analysis-generator will handle the
.          rest.
.   - Notably, a backend should just be more lowerings, with the
.       result being text or binary code instead of AST nodes.
.      - Backends are essentially defined by the set of AST/language
.          features that they consume and any special lowerings needed
.          to convert generic AST/language features into
.          backend-specific AST/language features.


== Closing Thoughts ==

I am realizing that there are multiple reasons that compel me to write this document:
- To share my ideas with others, on the off-chance that someone else might see this vision too and be better equipped to deliver.
- To suggest capabilities that any community-endorsed compiler tool (ex: compiler-as-a-ctfe-library) should have.
- To see if I might be able to get the help I need to make it a reality.

I just can't decide which reasons are more important.  But there is a common thread: I want this vision to become reality and do really cool things while filling a bunch of missing links in D's ecosystem.

I have to ask:

Would you pay for this?
If so, then I might be able to do a kickstarter at some point.
I am not independently wealthy or retired (or both?) like Walter, nor am I able to survive on zero hours of sleep each night like Andrei, and this would be a big project.  I think it would need full-time attention or it would never become useful in a reasonable timeframe.

Also, assuming you understand the design, are there any gaping holes in this?
This is my first attempt to share these ideas with a larger group, and thus an opportunity to anticipate troubles.

...

Well, I'm anxious to see how well the venerable D community receives this bundle of ideas.  Be chatty.  I'll try to keep up.

Thank you for reading.