Computed gotos on Reddit

[bearophile]

deadalnix:

> The presented example is insanely unsafe. By giving invalid 
> input, you can basically branch randomly.
>
> The check added by the switch is what is required to make it 
> safe, so it isn't faster at the end.
>
> The case in the article isn't very strong.

Thank you for your answer. We have not yet designed how D 
computed gotos are, both in syntax and semantics. This means that 
maybe there are ways to make them a little safer, in non-release 
mode.

Part of the GCC-C code in the article was:

static void* dispatch_table[] = {
         &&do_halt, &&do_inc, &&do_dec, &&do_mul2,
         &&do_div2, &&do_add7, &&do_neg};
#define DISPATCH() goto *dispatch_table[code[pc++]]

In D there are no preprocessor macros and the array bounds are 
enforced at run-time, this avoids some possible bugs of similar 
code.

Also, instead of void* maybe a different type can be introduced, 
so only label addresses of the current function can be put in 
dispatch_table and pointer variables given to goto. This 
statically avoids some other possible bugs.

It's true that in the end computed gotos are unsafe, you 
generally need to validate the bytecode/state transition before 
feeding them to the computed goto, and in D (unlike GCC-C) you 
are able to forbid the usage of computed gotos in @safe 
functions/methods. This doesn't avoid bugs, but helps contain 
them in delimited places. We have the @safe/@trusted/@system for 
a purpose.

In D modules like std.reflection suggested by Andrei are useful, 
because D is usable to write lot of high-level code too, I use a 
highly functional D style in some little D "scripts". But D is 
also a system language, and sometimes I desire to use it for 
tasks where C is used. GCC has computed gotos since many years 
(and Clang supports them) and as you see in the linked article 
some very common language interpreters you see around use 
computed gotos if the C compiler supports them. This is not a 
must-have feature for D, but dismissing it just because it's not 
safe is a bad idea.

Bye,
bearophile