#2420: Multi-method classes are inlined/specialized better than single-method
classes for strict types
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Reporter: sedillard | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.8.3
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Linux |
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Comment (by simonpj):
I know something about what is going on here.
First, there's nothing very bad happening. The code ''looks'' worse, but
at any call to `squarepd`, the whole definition will be inlined, and then
`$WPair` will be inlined, and it'll all turn out like the previous case.
So efficiency should be similar either way.
Why the difference between the two cases? Because the details of inlining
are regrettably delicate. The `__inline_me__` note makes the enclosed
expression look small, so that it'll be keen to inline. When it ''is''
inlined at an application, the `__inline_me__` wrapper is removed. But in
your example you have
{{{
squarepd = square
}}}
where `square` in this case is the `Pair` instance function, which is
marked `INLINE`. This isn't an application of `square`; rather it's more
like a renaming, so we end up with an `__inline_me__` around the
`squarepd` RHS.
Inside `__inline_me__` GHC is very reluctant to do any inlining at all; it
just blows up the thing to be inlined. Leave it until it ''is'' inlined.
If you instead wrote
{{{
squarepd x = square x
}}}
now it looks like a ''call'' of `square`, so the thing it inlined and the
`__inline_me__` is stripped off, and then `$WPair` can get inlined.
That's the general idea. For 6.10 I have two changes in the works that
will affect this: (a) a new way of treating INLINE pragmas, and (b) a new
way of treating instance declarations. So I'm not going to pursue this
further until I've done that. I'll leave it open to remind me to take
another look then.
Meanwhile: do you think the difference has a performance impact (contrary
to my claim)?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2420#comment:2>
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