GoingNative 6: The D Episode with Walter Bright and Andrei Alexandrescu

[Alex Rønne Petersen]

On 23-02-2012 07:51, Walter Bright wrote:
> On 2/22/2012 7:51 PM, bearophile wrote:
>> Andrei says that some new languages suffer because they have a poor
>> implementation, because creating the base for a language is a lot of
>> work.
>> Today this is issue is much less of a problem, new languages are
>> implemented
>> on the JavaVM,
>
> Using the JVM forces your program into Java semantics. For example,
> there are no structs in the JVM bytecode. No pointers, either. Nor
> unsigned types. Your new language is fairly boxed in to being a rehash
> of Java semantics.

I still cannot fathom how the Scala guys thought using the JVM was a 
good idea.

>
>
>> DotNetVM.
>
> Cristi's D compiler on .NET had large problems because array slicing was
> not expressible in the .NET intermediate code. It's not nearly as bad as
> the JVM in that regard, but it's still limited.

I assume the problem here was the .ptr property? I can't think of 
anything else about array slices that would be problematic in CIL.

>
>
>  > System languages are implemented with LLVM.
>
> That works if your language is expressible as C, because LLVM is a C/C++
> back end. If your language has different semantics (like how Go does
> stacks), using LLVM can be a large problem.

I don't think that's true. D, Rust, C# (CIL in general), Cray, 
ActionScript, Python, Java, Lua, Haskell, and many others have been 
compiled with LLVM successfully.

LLVM is very much engineered for C and C++, but it has many other 
features that those languages don't make use of at all (see for example 
the precise GC support; and this is not even something Apple uses, as 
their Obj-C GC is conservative). There is also the language-specific 
support for OCaml's precise stack maps. Also, LLVM has segmented stacks 
on x86 these days. That said, LLVM is definitely not as easy to use for 
managed languages as it is for systems/native languages.

So, while the design of LLVM certainly is driven by Clang primarily, 
it's not as if they don't welcome non-C family features.

>
> Note that early C++ compilers suffered badly when they were forced into
> using C back ends, because C++ wanted new features (like COMDATs) which
> are not expressible in C and so not supported by C back ends. Ditto for
> any language feature that needs something in the back end that C/C++
> doesn't need.
>
>
>> Regarding the comparison between dynamic languages like Python or Ruby
>> and D:
>> what Andrei has said is not fully fair. A simple common scripting
>> task: read
>> the lines of a text file and put them in a hash. This is probably
>> faster in
>> Python compared to D. I am willing to write a benchmark too, if asked.
>
> That would be comparing library code, not language code. Much of
> Python's implementation is in C, not Python.
>
>
>> Regarding what Walter has said, that most of the code of D
>> applications will
>> be @safe: if safe code gets (or has to get) so common as he says, then it
>> will be good for the D compiler to learn some tricks to avoid
>> (optimize away)
>> array bound tests in some cases.
>
> I looked into this years ago. Very little of array bounds checking can
> be optimized away. I've been working on optimizers for 25 years now,
> including a native code generating Java compiler, and I do know a few
> things about how to do arrays.

In Mono, we found that ABC removal was actually beneficial in some code; 
consider for example allocating small static arrays locally and indexing 
them with constants (or very simple expressions).

>
> Clang has some pretty good ideas, like the spell checker on undefined
> identifiers. But others talked about in the spiel at GoingNative have
> been in compilers for 30 years.
>

-- 
- Alex