Version 9 (modified by jstolarek, 3 years ago) (diff) |
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Implementing primitive Bool#
This page gathers the notes about implementing primitive logical operations and thus resolving ticket #6135.
The problem
Consider following fragment of code:
case (x <# 0#) || (x >=# width) || (y <# 0#) || (y >=# height) of True -> E1 False -> E2
This kind of code is common in image processing (and array programming in general) where one needs to check whether the (x,y) coordinates are within the image. Primitive comparison operators <# and >=# have type Int# -> Int# -> Bool. Logical OR operator (||) is defined as:
(||) :: Bool -> Bool -> Bool True || _ = True False || x = x
in GHC.Classes (ghc-prim library) which is equivalent of:
(||) x y = case x of True -> True False -> y
During the compilation process (assuming the optimizations are turned on) the definition of (||) gets inlined and then case-of-case transform is performed successively. This results in following Core (cleaned up for clarity):
case <# x 0 of _ { False -> case >=# x width of _ { False -> case <# y 0 of _ { False -> case >=# y height of _ { False -> E2 True -> E1 }; True -> E1 }; True -> E1 }; True -> E1 };
and in following assembler code:
.Lc1rf: testq %r14,%r14 jl .Lc1rk cmpq %rdi,%r14 jge .Lc1rp testq %rsi,%rsi jl .Lc1ru cmpq %r8,%rsi jge .Lc1rz movl $Main_g2_closure+1,%ebx jmp *0(%rbp) .Lc1rk: movl $Main_g1_closure+1,%ebx jmp *0(%rbp) .Lc1rp: movl $Main_g1_closure+1,%ebx jmp *0(%rbp) .Lc1ru: movl $Main_g1_closure+1,%ebx jmp *0(%rbp) .Lc1rz: movl $Main_g1_closure+1,%ebx jmp *0(%rbp)
There are five possible branches to take, although four of them have the same result. This is caused by code duplication introduced by case-of-case transform (see this blog post for a step by step derivation). According to Ben Lippmeier, who submitted the original bug report, mis-predicted branches are bad in object code because they stall the pipeline.
Workarounds
It is possible to work around the issue of code duplication by using GHC primops tagToEnum# and dataToTag#. These allow to distinguish between True and False by means of accessing the tag of a data type constructor. This means that dataToTag# can convert True to 1# and False to 0#, while tagToEnum# does the opposite (see paper Faster Laziness Using Dynamic Pointer Tagging for more details):
ghci> import GHC.Exts ghci> import GHC.Prim ghci> :set -XMagicHash ghci> I# (dataToTag# True) 1 ghci> I# (dataToTag# False) 0 ghci> (tagToEnum# 0#) :: Bool False ghci> (tagToEnum# 1#) :: Bool True ghci>
Having the possibility of converting Bool to an unboxed Int# allows us to compute results of logical expression by means of logical bitwise operations. The result can be converted back to a Bool so this is transparent on the Haskell source level, except for the fact that defined logical binary operators will be strict in both their arguments.
NOTE: Validity of this solution is based on assumption that True will always have a tag of 1#, while False will have a tag of 0#. Changing this invariant in the future would make these primitive logical operators invalid.
First workaround
First workaround assumes converting each result of comparison into an unboxed Int and replacing || with +#:
case (dataToTag# (x <# 0#)) +# (dataToTag# (x >=# width)) +# (dataToTag# (y <# 0#)) +# (dataToTag# (y >=# height)) of 0# -> E2 -- note that branch order is reversed _ -> E1
This compiles to:
case +# (+# (+# (dataToTag# (<# x 0)) (dataToTag# (>=# x width))) (dataToTag# (<# y 0))) (dataToTag# (>=# y height)) of _ { __DEFAULT -> E1; 0 -> E2 }
Similarly we can convert logical && into multiplication.
Second workaround
The above workaround is a bit clumsy: dataToTag#s make the code verbose and it may not be very obvious what the code is doing. Hence the second workaround, that defines an alternative logical or operator:
(||#) :: Bool -> Bool -> Bool (||#) x y = let xW = int2Word# (dataToTag# x) yW = int2Word# (dataToTag# y) zI = word2Int# (yW `or#` xW) in tagToEnum# zI
This operator is defined in terms of primops dataToTag#, tagToEnum# and a bitwise or primop or#. Since the last one operates only on Words we need to use int2Word# and word2Int# for conversion between these data types. Luckily, GHC does a good job of removing unnecessary conversions between data types. This means that:
case (x <# 0#) ||# (x >=# width) ||# (y <# 0#) ||# (y >=# height) of True -> E1 False -> E2
compiles to:
case tagToEnum# (word2Int# (or# (int2Word# (dataToTag# (>=# y height))) (or# (int2Word# (dataToTag# (<# y 0))) (or# (int2Word# (dataToTag# (>=# x width))) (int2Word# (dataToTag# (<# x 0))))))) of _ { False -> E2; True -> E1 }
Primitive logical operators &&# and not# can be defined in a similar matter.
NOTE: Neither of this two workarounds produces good object code. The reason is that comparison operators return a Bool as a thunk that needs to be evaluated. The real solution requires that no thunk is created.
Solutions
A good beginning would be to implementing second of the above workarounds as a primop. Then we need to create primops that return unboxed values instead of a thunk. The big question is should an unboxed version of Bool be introduced into the language?
First approach
Treat Bool as a boxed version of primitive Bool#. True would be equivalent of B# True#, False of B# False#:
data Bool = B# True# | B# False# -- B# :: Bool# -> Bool
Not sure if this can be considered equivalent to what the Haskell Report says about Bool. We need to ensure that Bool# is populated only by True# and False# and that these two are translated to 1# and 0# in the Core. It should be impossible to write such a function at Haskell level:
g :: Bool -> Int -> Int g (B# b) (I# i) = I# (b + i)
This approach might require one additional case expression to inspect the value of Bool at the Core level. For example:
f :: Int -> Int -> Int f x y = if x > y then x else y
would compile to:
case x of _ { I# xP -> case y of _ { I# yP -> case ># xP yP of _ { B# bP -> case bP of _ { 1# -> e1; 0# -> e2 } } } }
This would complicate Core a bit but it should be possible to compile such Core to exactly the same result as with normal Bool. This code assumes that ># has type Int# -> Int# -> Bool, but to truly avoid branching in the Core we need .># :: Int# -> Int# -> Bool# so that we get a primitive value that doesn't need to be inspected using case expression but can be directly used by primitive logical operators.
Second approach
Second approach assumes creating type Bool# that is independent of type Bool. Boxing and unboxing would have to be done explicitly via additional functions:
data Bool = True | False -- no changes here bBox :: Bool# -> Bool bBox 1# = True bBox 0# = False bUnbox :: Bool -> Bool# bUnbox True = 1# bUnbox False = 0#
Bool# could not be implemented as an ADT because it is unlifted and unboxed, while ADT value constructors need to be boxed and lifted (see comments in compiler/types/TyCon.lhs). There would need to be some magical way of ensuring that Bool# is populated only by #0 and 1# and that these values cannot be mixed with unboxed integers. Perhaps this could be done by preventing programmer from explicitly creating values of that type (can this be done?) and allow her only to use values returned from functions.
Another problem with this approach is that it would introduce primitive logical operations ||# and &&# with type Int# -> Int# -> Int# - it is questionable whether anyone would want such operations available to the programmer. I think it is desirable to have primitive logical operators of type Bool# -> Bool# -> Bool#.
Places of interest in the source code
The file prelude/primops.txt.pp defines PrimOps and their type signatures.
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