On Friday, 6 December 2013 at 22:52:46 UTC, Walter Bright wrote:
On 12/6/2013 2:40 PM, bearophile wrote:
I think in your list you have missed the point 8, that is
templates allow for
data specialization, or for specialization based on
compile-time values.
The common example of the first is the C sort() function
compared to the type
specialized one.
That's a good example.
2. D knows when functions are pure. C has to make worst case
assumptions.
Perhaps D purity were designed for usefulness, code
correctness, etc. but not to
help compilers. I remember some recent discussions in this
newsgroup by
developers of GDC that explained why the guarantees D offers
over C can't lead
to true improvements in the generated code. If this is true
then perhaps D has
some features that weren't designed in hindsight of what
back-ends really need
to optimize better.
dmd can and does remove multiple calls to strongly pure
functions with the same arguments.
and what about holes in immutable, pure and rest type system?
There are also situations where D is slower than D: when D
can't prove that an
array will be accessed in bounds [*].
In the cases where D cannot, can C? Nope. C doesn't even know
what an array is. Can any other language? Nope.
And when a D compiler because of separate compilation can't
de-virtualize a virtual class method call.
Can C devirtualize function calls? Nope.
C doesn't have virtual functions. By the way, does D devirtualize
them? AFAIK it doesn't either, but I do remember spec page was
talking about it (this is so Dish - advertize optimization trick
in spec and do not implement it).
