Walter Bright:
dmd currently doesn't do that, but that's not the language's
fault, it's a quality of implementation issue.
I understand the difference between what compilers are able to do
(today or in future), and what the language specs allow compiler
writers to do. So in theory I agree.
In practice there is also the well known fallacy of the
"sufficiently smart compiler":
http://c2.com/cgi/wiki?SufficientlySmartCompiler
This fallacy implies that if you want to actually see a compiler
able to perform a certain optimization, such optimization must be
rather "easy", this means it must be easy for the compiler to
infer as true all the conditions necessary to apply that
optimization (and then you need someone to actually implement it,
in a community as small as the D one optimizations can't be top
priority).
The other problem with optimizations is that often if you can't
rely on them, that means you can't be certain they are used in
the code you are writing, then it's like they don't exist. A good
example of this is the Scheme standard requiring all Scheme
compilers to implement the tail call optimization.
Since the final switch does not allow a 'default' case, the
check can be omitted, and the generated code is a simple
index-jump, just like the computed goto example.
You have seen the asm I have shown in the next post. You see
those "jmp *%ecx" at the end of each case. Computed gotos in this
case are not just a single index-jump, there is an index-jump at
the end of each case. Is your future hypothetical D compiler able
to do that?
Bye,
bearophile
