Strictness in data declaration not matched in assembler?

[Tyson Whitehead]

On Thursday 16 October 2008 07:03:05 Roman Leshchinskiy wrote:
> On 16/10/2008, at 21:34, Simon Peyton-Jones wrote:
> > BUT people who care probably UNPACK their strict fields too, which
> > is even better.  The time you can't do that is for sum types
> >        data T = MkT ![Int]
>
> You also can't do it for polymorphic components. I've used code like:
>
>    data T a = MkT !a
>
>    foo :: T (a,b) -> a
>    foo (MkT (x,y)) = x
>
> Here, unpacking doesn't work but foo could still access the components
> of the pair directly.

This is actually the situation I was originally looking at.  I just simplified 
it for the sake of posting readable core and assembler.


Specifically, I was looking at some of the assembler GHC was generating for 
some array code to see if it could do a clean enough job to be used instead of 
C, and was finding this sort of thing because STUArrau is defined as

data STUArray s i a = STUArray !i !i !Int (MutableByteArray# s)

I also seem to recall seeing the same sort of thing in some of the state code 
I was also looking at because STRep is defined as

type STRep s a = State# s -> (# State# s, a #)

(the a being the issue here).


With regard to cost, it is probably not that representative, but a typical 
code path for the toy example I posted earlier goes from

        leaq -8(%rbp),%rax
        cmpq %r14,%rax
        -- not taken jump -- (stack overflow check passed)
        movq %rsi,%rbx
        movq $sni_info,-8(%rbp)
        addq $-8,%rbp
        testq $7,%rbx
        -- taken jump -- (x argument had already been forced)
        movq 7(%rbx),%rbx
        movq $snj_info,(%rbp)
        testq $7,%rbx
        -- taken jump -- (strict constructor argument is already forced)
        cmpq $1,7(%rbx)
        -- not taken jump -- (first of the case options)
        movl $Main_lvl_closure+1,%ebx
        addq $8,%rbp
        jmp *(%rbp)

to

        leaq -8(%rbp),%rax
        cmpq %r14,%rax
        -- not taken jump -- (stack overflow check passed)
        movq %rsi,%rbx
        movq $sni_info,-8(%rbp)
        addq $-8,%rbp
        testq $7,%rbx
        -- taken jump -- (x argument has already been forced)
        movq 7(%rbx),%rbx
        cmpq $1,7(%rbx)
        -- not taken jump -- (first of the case options)
        movl $Main_lvl_closure+1,%ebx
        addq $8,%rbp
        jmp *(%rbp)

which is a 22% reduction (18 to 14) in instructions executed in the entire 
function, or a 40% reduction (10 to 6) in instruction executed in the core of 
the function (i.e., after the function's argument is possibly forced).

Cheers!  -Tyson

PS:  Thanks everyone for the very informative and interesting discussion.