Proposal: change the Bits instance for Bool to align with other basic types and support branchless calculations
David Feuer
david.feuer at gmail.com
Sat Sep 27 17:59:04 UTC 2014
Currently, the (.&.) and (.|.) methods for Bool are short-circuiting,
defined like this:
instance Bits Bool where
(.&.) = (&&)
(.|.) = (||)
Unlike the instances for Int, Word, etc., this gives short-circuiting
behavior (conditionally lazy in the second operand). Unfortunately, this
requires a conditional branch to implement, which can sometimes be bad.
Since (&&) and (||) are readily available to anyone who wants
short-circuiting, I propose that we use the following instead. Note that
the Bits class does not specify anything about this aspect of instance
behavior.
x .&. y = tagToEnum# (dataToTag# x `andI#` dataToTag# y)
x .|. y = tagToEnum# (dataToTag# x `orI#` dataToTag# y)
The rest of the operations look like this:
x `xor` y = tagToEnum# (dataToTag# x `xorI#` dataToTag# y)
complement x = tagToEnum# (dataToTag# x `xorI#` 1#)
shift x s = testBit x s
rotate x _ = x
-- I don't think we gain anything changing this one.
bit 0 = True
bit _ = False
testBit x b = tagToEnum# (dataToTag# x `andI#` (dataToTag# b ==# 0#))
bitSizeMaybe _ = Just 1
bitSize _ = 1
isSigned _ = False
popCount x = I# (dataToTag# x)
instance FiniteBits Bool where
finiteBitSize _ = 1
countTrailingZeros x = I# (dataToTag# x `xorI#` 1#)
countLeadingZeros x = I# (dataToTag# x `xorI#` 1#)
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