On 01-06-2013 09:59, bearophile wrote:
Alex Rønne Petersen:
I'm sure this has been brought up before, but I feel I need to bring
it up again (because I'm going to be writing a threaded-code
interpreter): http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
This is an incredibly important extension.
This was discussed more than once, and I think it's a valuable
improvement for certain kinds of low level D programming.
Walter says that he has not added this feature to D because it's useful
only to write interpreters and the like. I have found it useful in GCC
to also write very fast finite state machines to analyse genomic
strings. But even if Walter is right, writing interpreters (and
emulators) is an important purpose for a system language as D. You can't
write them efficient in scripting languages, and even Haskell/F# are not
good at all to write them. Languages like C/D/C++ (and few others, as
later probably Rust) are the only few natural languages to write them.
"Recently" the Python C interpreter was modified and speed up thanks to
this non-standard feature. CPython source code has two versions, one
with computed gotos and one without, to compile it even if your C
compiler doesn't support them or their GNU-C syntax.
I don't think there's any question as to the usefulness (and
essentialness) of this feature. I'm very close to just writing most of
the interpreter in C over a triviality like this.
Implementing it should be trivial as both LLVM and GCC support taking
the address of a block. I'm sure the DMD back end could be extended to
support it too.
Even if DMD back-end will never implement it, I think it's important to
define it formally and officially in the D language and add the relative
syntax to the front-end (plus a standard version identifier that allows
to write programs that have both a routine that uses computed gotos and
one that doesn't use them).
This has the big advantage that all future compilers that will want to
implement it will use the same nice standard syntax. If D doesn't define
this syntax, then maybe GDC will add it, and maybe later LDC will add
it, and later maybe other future compilers will add it, and we can't be
sure they will share the same syntax.
Another advantage of having this syntax in D, is that even if a compiler
back-end doesn't support computed gotos, the program compiles without
syntax errors when you use conditional compilation to disable the piece
of code that uses computed gotos.
Yes, good points.
Bye,
bearophile
--
Alex Rønne Petersen
a...@alexrp.com / a...@lycus.org
http://alexrp.com / http://lycus.org
