Re: Proposal: change the Bits instance for Bool to align with other basic types and support branchless calculations

Hi all, I am not really convinced that the proposed implementation will be faster -- if the short-circuiting kicks in, I believe it will perform better, even if it uses conditional branching. So unless being convinced by benchmark results, I would prefer the lazier version of these operators. Cheers, Milan

-----Original message----- From: David Feuer Sent: 27 Sep 2014, 13:59

... 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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