Simon Marlow wrote:
Roman Leshchinskiy wrote:
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,
Right. In general, we should teach GHC a bit more about arithmetic.
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.
Hmm, I hadn't realised that GHC does this. Is it only for simple rhss?
Roman
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