Re: expression

[Michael Lucy]

On Thu, Jun 24, 2010 at 1:55 AM, Ken Raeburn <address@hidden> wrote:
> On Jun 23, 2010, at 17:09, Michael Lucy wrote:
>> Is there any scheme expression that will just get ignored when the
>> scheme code is compiled?
>>
>> I'm generating some code with a function like:
>>
>> (define (gen-update-ab updatea updateb)
>>  `(begin
>>     ,(if updatea `(set! a (+ a 1)) `(donothing))
>>     ,(if updateb `(set! b (+ b 1)) `(donothing))))
>>
>> And ideally I could replace the donothing function with something that
>> will get discarded during compilation.
>
> A simple constant like #f or '() or 42 shouldn't cause any evaluation to 
> happen, unless it's the last expression and thus the value to be returned (in 
> which case you'd just be returning a simple constant).

Ah, I see.  I tried that with variables, and just assumed that if they
weren't optimized out constants wouldn't be either (in retrospect not
such a great assumption):

scheme@(guile-user)> ,c (begin a b 1)
Disassembly of #<objcode d123d28>:

   0    (assert-nargs-ee/locals 0)
   2    (load-symbol "a")               ;; a
   7    (link-now)
   8    (variable-ref)
   9    (drop)
  10    (load-symbol "b")               ;; b
  15    (link-now)
  16    (variable-ref)
  17    (drop)
  18    (make-int8:1)                   ;; 1
  19    (return)

Out of curiosity, why are variables left in?  Are there situations
where evaluating a variable will have side-effects?

Anyway, thanks!  That makes life a lot simpler.

>
> I don't know if there's a canonical efficient way to generate it, but it 
> looks like "(if #f #f)" will be optimized by the current compiler into just 
> pushing the magic "undefined value" onto the stack.  Getting rid of the push 
> altogether *IF* the result of the expression is unused is up to the 
> optimizer; you shouldn't be jumping through hoops in the code you generate to 
> make that happen.  But it appears that it does happen at least in simple 
> cases:
>
> scheme@(guile-user)> ,c (begin (if #f #f) (if #f #f) 42)
> Disassembly of #<objcode 101678308>:
>
>   0    (assert-nargs-ee/locals 0)
>   2    (make-int8 42)                  ;; 42
>   4    (return)
>
> Hmm... here's another way, though I've no idea if RnRS lets you not have any 
> expressions in here:
>
> scheme@(guile-user)> ,c (begin)
> Disassembly of #<objcode 10163f848>:
>
>   0    (assert-nargs-ee/locals 0)
>   2    (void)
>   3    (return)
>
> scheme@(guile-user)> ,c (begin (begin) (begin) 42)
> Disassembly of #<objcode 1016782a8>:
>
>   0    (assert-nargs-ee/locals 0)
>   2    (make-int8 42)                  ;; 42
>   4    (return)
>
>
>> There are alternatives, but they're pretty ugly (significantly moreso
>> in my actual code than they look below):
>
> Depends how you define ugliness. :-)
>
> Your approach expands each possible update into one S-expression.  So you 
> need *something* there for each one, even if it's just a constant, or an 
> empty "begin".
>
> The alternative, I think, is rearranging your code so that no expression gets 
> added to the list when there's nothing to do; one way to do that, off the top 
> of my head, would be to generate a possibly-empty list of expressions rather 
> than exactly one for each possible update, and then merge the lists together. 
>  Or start with an empty list, and tack S-expressions on the front for updates 
> that are needed, then add the "begin" and return the result.
>
> But in terms of the generated byte code, the constants should just go away if 
> they're not actually used, and nested "begin" lists should get flattened, so 
> I don't think it should matter much either way.
>
> If you want the macro expansion to be human-readable for debugging purposes, 
> you could also consider putting in some kind of no-op annotation that 
> shouldn't generate code, for example a let loop with a meaningful label name 
> and no useful body, or wrapping code that actually has effects but doesn't 
> use the label:
>
> scheme@(guile-user)> ,c (begin (let no-update-needed-for-a () #f) 42)
> Disassembly of #<objcode 1030dc468>:
>
>   0    (assert-nargs-ee/locals 0)
>   2    (br :L171)                      ;; -> 10
>   6    (br :L172)                      ;; -> 14
>  10    (br :L173)                      ;; -> 6
>  14    (make-int8 42)                  ;; 42
>  16    (return)
>
> Ehh... okay, maybe there's a little work to be done for that one. :-)
>
> Ken