Roman Leshchinskiy wrote:
Simon Marlow wrote:
Roman Leshchinskiy wrote:
Hmm, I should have investigated further before posting. That's a very
Intel-specific reason, though. On the Sparc, for instance, it
wouldn't make much sense to implement divInt# in terms of quotInt#, I
think (but then again, we don't have a Sparc NCG).
Anyway, I don't think it's an inliner bug because divInt# and friends
aren't marked INLINE. If nobody objects, I'll go ahead and add the
INLINE pragmas at least. We lose the opportunity to have rewrite
rules for divInt# this way, though, which is a pity.
After inlining you'll get constant folding for quotInt#, so it should
be ok, no?
Yes, but I'd also like to have the rule divInt# x# 1# = x#. With the
current definition of divInt#, just inlining it and having the rule
quotInt# x# 1# = x# (which I'd like to add, too) wouldn't be quite
enough, I think.
It ought to be enough :) With an extra simplification (described below),
we could do it.
divInt# :: Int# -> Int# -> Int#
x# `divInt#` y#
| (x# ># 0#) && (y# <# 0#) = ((x# -# 1#) `quotInt#` y#) -# 1#
| (x# <# 0#) && (y# ># 0#) = ((x# +# 1#) `quotInt#` y#) -# 1#
| otherwise = x# `quotInt#` y#
so whey y# is 1#, we get
x# `divInt#` 1#
| (x# <# 0#) = ((x# +# 1#) `quotInt#` 1#) -# 1#
| otherwise = x# `quotInt#` 1#
and with quotInt# x# 1# == 1#, we get
x# `divInt#` 1#
| (x# <# 0#) = (x# +# 1#) -# 1#
| otherwise = x#
GHC doesn't simplify (x# +# 1#) -# 1#, but it should. If that happened,
we'd have
x# `divInt#` 1#
| (x# <# 0#) = x#
| otherwise = x#
which simplifies to just x#, and GHC does manage this, I just tried it.
(However if you change the conditional to (x# ==# 0#) then it doesn't
completely eliminate the case, this looks like a bug).
Cheers,
Simon
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