| 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.
You're right. As Simon says, GHC's Core language has no type distinction between an Int that is evaluated and an Int that might be a thunk, so we generate conservative code.
However, even for strict functions we do not *guarantee* to pass evaluated arguments. Consider
f x y = if y==0 then g x else 0
g x = x+1
Here g is strict, but f is not (in x). The code that GHC generates for 'f' simply passes 'x' along to 'g'. It does not evaluate 'x' and then pass it. So the fact that 'g' is strict is used to *allow* the caller to use call by value; it does not *require* the caller to do so. So that means that 'g' cannot rely on getting an evaluated argument. One could change that, but that's the way it is right now.
For strict *constructors*, on the other hand, we *do* guarantee to evaluate the argument before building the constructor. We generate a wrapper thus
wC = \ab. case a of { a' -> C a' b }
(Remember 'case' always evaluates in Core.) So for strict constructors we could take advantage of the known evaluated-ness of the result to avoid the test.
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]
Whether this case is important enough to be worth the complexity cost, I'm not sure.
If someone felt like creating a ticket for this thread, and pasting in the payload of the messages, that'd help us not to forget it. As of now, it doesn't look like a terribly big win to me. But I could be wrong -- intuition is not always a good guide.
Simon
| -----Original Message-----
| From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-
| bounces@haskell.org] On Behalf Of Lennart Augustsson
| Sent: 15 October 2008 16:09
| To: Tyson Whitehead
| Cc: GHC users
| Subject: Re: Strictness in data declaration not matched in assembler?
|
| 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.
|
| -- Lennart
|
| On Wed, Oct 15, 2008 at 3:58 PM, Tyson Whitehead