On 18-Aug-2015 19:35, Andrei Alexandrescu wrote:
On 8/18/15 12:31 PM, Dmitry Olshansky wrote:
On 18-Aug-2015 16:19, Andrei Alexandrescu wrote:
On 8/18/15 2:55 AM, Dmitry Olshansky wrote:
On 18-Aug-2015 01:33, Andrei Alexandrescu wrote:
On 8/17/15 2:47 PM, Dmitry Olshansky wrote:
Actually one can combine the two:
- use integer type tag for everything built-in
- use pointer tag for what is not
But a pointer tag can do everything that an integer tag does. --
Andrei
albeit quite a deal slooower.
I think there's a misunderstanding. Pointers _are_ 64-bit integers and
may be compared as such. You can use a pointer as an integer. -- Andrei
Integer in a small range is faster to switch on. Plus comparing to zero
is faster, so if the common type has tag == 0 it's a net gain.
Agreed. These are small gains though unless tight loops are concerned.
Strictly speaking pointer with vtbl is about as fast as switch but when
we have to switch on 2 types the vtbl dispatch needs to be based on 2
types instead of one. So ideally we need vtbl per pair of type to
support e.g. fast binary operators on TaggedAlgebraic.
But I'm talking about using pointers for indirect calls IN ADDITION to
using pointers for simple integral comparison. So the comparison is not
appropriate. It's better to have both options instead of just one.
If common type fast path with 0 is not relevant then the only gain of
integer is being able to fit it in a couple of bytes or even reuse some
vacant bits.
Another thing is that function addresses are rather sparse so switch
statement should do some special preprocessing to make it more dense:
- subtract start of the code segment (maybe, but this won't work with
DLLs though)
- shift right by 2(4?) as functions are usually aligned
--
Dmitry Olshansky
