[MLton-user] option ref optimized to a null pointer?
Wesley W. Terpstra
terpstra at dvs1.informatik.tu-darmstadt.de
Sat Nov 24 07:15:23 PST 2007
I was profiling and optimizing a message heap when I noticed that
MLton.size suggests MLton is being especially clever.
It seems MLton represented the type '(int * int ref) option array' as
I would have done it in C. It used an array of pointers to objects
where each object is a pair of integers. So I pay 4+4+4 bytes per
object (2 ints + header tag) and 12+4x bytes in the array (12 for the
usual array overhead and 4 for each pointer).
What surprised me is that it seems MLton can turn the NONE case of
option + pointer into a "null pointer". Is it my imagination, or is
this really what's going on? Very cool. I guess this also applies to
I was also happy to see that MLton had flattened the reference in the
What rules must I obey to make sure MLton can continue to apply these
optimizations? For the flattened tuple, I gather I shouldn't try
ripping the 'int ref' out of the tuple and passing it through a
function barrier. But when does this 'option ref' -> 'null pointer'
optimization happen? Always?
It might be nice if there were MLton representation guidelines
somewhere. While it's nice that MLton just did the right thing, that's
partly because I already knew not to break split the tuple with a ref. I
imagine there are other guidelines which I don't know.
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