On 14-05-2012 06:18, Jonathan M Davis wrote:
On Monday, May 14, 2012 05:47:54 Alex Rønne Petersen wrote:
It's kinda funny that something which on dlang.org's FAQ is described as
a compiler optimization hint (http://dlang.org/const-faq.html#const)
isn't useful at all for this purpose...
Oh, it's definitely useful for optimizations. It just doesn't work with a
couple of idioms that some programmers like to use in order to optimize their
code.
- Jonathan M Davis
I have yet to see any compiler make sensible use of the information
provided by both C++'s const and D's const.
const in particular is completely useless to an optimizer because it
does not give it any information that it can use for anything. The kind
of information that an optimization pass, in general, wants to see is
whether something is guaranteed to *never* change. const does not
provide this information. const simply guarantees that the code working
on the const data cannot alter it (but at the same time allows *other*
code to alter it), which, as said, is useless to the optimizer.
immutable is a different story. immutable actually opens the door to
many optimization opportunities exactly because the optimizer knows that
the data will not be altered, ever. This allows it to (almost)
arbitrarily reorder code, fold many computations at compile time, do
conditional constant propagation, dead code elimination, ...
This seems reasonable. But now consider that the majority of functions
*are written for const, not immutable*. Thereby, you're throwing away
the immutable guarantee, which is what the *compiler* (not the
*programmer*) cares about. immutable is an excellent idea in theory, but
in practice, it doesn't help the compiler because you'd have to either
a) templatize all functions operating on const/immutable data so the
compiler can retain the immutable guarantee when the input is such, or
b) explicitly duplicate code for the const and the immutable case.
Both approaches clearly suck. Templates don't play nice with
polymorphism, and code duplication is...well...duplication. So, most of
druntime and phobos is written for const because const is the bridge
between the mutable and immutable world, and writing code against that
rather than explicitly against mutable/immutable data is just simpler.
But this completely ruins any opportunity the compiler has to optimize!
(An interesting fact is that even the compiler engineers working on
compilers for strictly pure functional languages have yet to take full
advantage of the potential that a pure, immutable world offers. If
*they* haven't done it yet, I don't think we're going to do it for a
long time to come.)
Now, you might argue that the compiler could simply say "okay, this data
is const, which means it cannot be changed in this particular piece of
code and thus nowhere else, since it is not explicitly shared, and
therefore not touched by any other threads". This would be great if
shared wasn't a complete design fallacy. Unfortunately, in most real
world code, shared just doesn't cut it, and data is often shared between
threads without using the shared qualifier (__gshared is one example).
shared is another can of worms entirely. I can list a few initial
reasons why it's unrealistic and impractical:
1) It is extremely x86-biased; implementing it on other architectures is
going to be...interesting (read: on many architectures, impossible at
ISA level).
2) There is no bridge between shared and unshared like there is for
mutable and immutable. This means that all code operating on shared data
has to be templatized (no, casts will not suffice; the compiler can't
insert memory barriers then) or code has to be explicitly duplicated for
the shared and unshared case. Funnily, the exact same issue mentioned
above for const and immutable!
3) It only provides documentation value. The low-level atomicity that it
is supposed to provide (but doesn't yet...) is of extremely questionable
value. In my experience, I never actually access shared data from
multiple threads simultaneously, but rather, transfer the data from one
thread to another and use it exclusively in the other thread (i.e.
handing over the ownership). In such scenarios, shared just adds
overhead (memory barriers are Bad (TM) for performance).
/rant
--
- Alex
