Hello Don,
bearophile wrote:
Don:
Based on everyone's comments, this is what I have come up with:
Looks good.
* If y == 0, x ^^ y is 1.
* If both x and y are integers, and y > 0, x^^y is equivalent to
{ auto u = x; foreach(i; 1..y) { u *= x; } return u; }
You can rewrite both of those as:
{ typeof(x) u = 1; foreach (i; 0 .. y) { u *= x; } return u; }
(1) Although the following special cases could be defined...
* If x == 1, x ^^ y is 1
* If x == -1 and y is even, x^^y == 1
* If x == -1 and y is odd, x^^y == -1
... they are not sufficiently useful to justify the major increase
in
complexity which they introduce.
This is not essential:
(-1)**n is a common enough shortcut to produce an alternating +1 -1,
you can see it used often enough in Python code (and in mathematics).
This search gives 433 results:
http://www.google.com/codesearch?q=\%28-1\%29\s*\*\*\s*[0-9a-zA-Z%28]
+lang%3Apython
When used for this purpose (-1) is always compile time constant, so
the compiler can grow a simple rule the rewrites:
(-1) ^^ n
as
(n & 1) ? -1 : 1
That's an interesting one.
With this proposal, that optimisation could still be made when it is
known that n>=0. We *could* make a special rule for compile-time
constant -1 ^^ n, to allow the optimisation even when n<0. But then
you
have to explain why: x = -1; y = x^^-2; is illegal, but y = -1^^-2 is
legal. Can that be justified?
Maybe. I think it's worth looking into.
