David Herman wrote at 10/09/2010 07:46 PM:
I thought about the "am I falling through?" approach you've been taking, but the problem
is that it keeps having to recompute the same test. In C-like languages, one of the benefits of
`switch' [1] is that fall-through is expected to either be a literal "execute the next
instruction in the PC" or at least a jump to a fixed address. So I prefer an approach that
sets up a basic-block-like structure, like so:
I like this way of thinking.
Here's an expansion of Shriram's example:
(define (cas1 v)
(let ([disc-v v])
(let/ec
k
(syntax-parameterize
([break (syntax-rules () [(_ v) (k v)])])
(define (temp2) (case disc-v [(1) (temp4)] [else (temp3)]))
(define (temp3) (case disc-v [(2) (temp5)] [else (temp6)]))
(define (temp4) (display "1") (temp5))
(define (temp5) (display "2") (break 2) (temp7))
(define (temp6) (case disc-v [(3) (temp7)]))
(define (temp7) 3)
(temp2)))))
That seems a little unfamiliar to me because of the linear search with
multiple tests. I instead used a single "case", on the perhaps naive
assumption that that's easiest for a compiler to optimize:
(define (cas1 v)
(let ((temp3 (lambda () 3)))
(let ((temp2 (lambda () (display "2") 2)))
(let ((temp1 (lambda () (display "1") (temp2))))
(case v
((1) (temp1))
((2) (temp2))
((3) (temp3)))))))
Couldn't a compiler could optimize a "case" at least as well as any
syntax transformer I wrote, unless I had special knowledge about the
actual runtime inputs that a static optimizer doesn't have (which I
don't)? (Examples: binary search, jump tables, branching on tags/types,
dynamic optimizations.)
I was guessing then that, if I let the compiler do what it wants with a
single "case" and then went with tail calls to chain fallthrough between
thunks, it doesn't get much better than that for a compiler.
But I'm just making that up, since I don't know how smart the compiler,
and modern CPUs and JITs mean that we can't just count instructions in
disassembly dumps to have an easy idea what'll go on.
--
http://www.neilvandyke.org/
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