Strictness in data declaration not matched in assembler?

[Jan-Willem Maessen]

On Oct 15, 2008, at 11:08 AM, Lennart Augustsson wrote:

> I totally agree.  Getting the value of the field should just evaluate
> x and then use a pointer indirection; there should be no conditional
> jumps involved in getting the value.
> GHC is just doing the wrong thing.

Can indirection nodes occur in this context?  [I'd think not, but it  
depends on what pointer we're storing when we force the thunk in the  
constructor.]  I could see the need for the test if indirection  
handling is required.

-Jan

>  -- Lennart
>
> On Wed, Oct 15, 2008 at 3:58 PM, Tyson Whitehead  
> <twhitehead at gmail.com> wrote:
>> On Wednesday 15 October 2008 10:48:26 you wrote:
>>> Strictness does not imply unboxing.
>>>
>>> To see why not, think about the fact that unboxing breaks sharing.  
>>> By
>>> keeping the pointer-indirection in place, we can share even strict
>>> fields between related values.
>>
>> I believe I realize that.  What I was wondering about was the fact  
>> that it
>> seemed to think the pointer might be to a thunk (instead of  
>> constructor
>> closure).  Doesn't the strictness flag mean the following assembler  
>> would work
>>
>> sni_info:
>>       movq 7(%rbx),%rbx
>>       movq $snj_info,(%rbp)
>>       jmp snj_info
>>
>> (which could be cleaned up further by combining it with snj_info)  
>> instead of
>>
>> sni_info:
>>       movq 7(%rbx),%rbx
>>       movq $snj_info,(%rbp)
>>       testq $7,%rbx
>>       jne snj_info
>>       jmp *(%rbx)
>>
>> (i.e., the whole test if it is a thunk and conditionally evaluate  
>> it bit is
>> unnecessary due to constructor the strictness flag).
>>
>> Cheers!  -Tyson
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