Implementing new primitive comparisons to allow branchless algorithms
This page presents motivation and technical details behind implementing new primitive comparison operators (this was originaly reported as Trac ticket #6135). See this page for instructions how to adjust your already existing code to work with new primops.
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). This definition is equivalent of:
(||) x y = case x of True -> True False -> y
During compilation definition of (||) gets inlined (assuming the optimizations are turned on) 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 assembly 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 $Main_g1_closure+1,%ebx jmp *0(%rbp)
There are five possible branches to take, although four of them have the same result. This is caused by code duplication introduced by case-of-case transform (see this blog post for a step by step derivation). According to Ben Lippmeier, who submitted the original bug report, mis-predicted branches are bad in object code because they stall the pipeline.
Note: this example was produced with GHC 7.6.3. At the moment of merging new primops into HEAD, there was no code duplication at the assembly level when using old primops. However, avoiding code duplication is not the main problem new primops are meant to solve. That problem are conditional branches.
Solution
This problem was solved by modifying comparison primops to return unboxed unlifted Int# instead of Bool, which is lifted and thus must be inspected with a case expression. Having Int# returned as a result of logical comparison allows to use branchless bitwise logical operators instead of branching logical operators defined by Haskell.
Eliminating branches using new primops
With the new primops we can rewrite the original expression that motivated the problem:
case (x <# 0#) || (x >=# width) || (y <# 0#) || (y >=# height) of True -> E1 False -> E2
as
case (x <$# 0#) `orI#` (x >=$# width) `orI#` (y <$# 0#) `orI#` (y >=$# height) of True -> E1 False -> E2
Let's analyze how that code gets compiled by the LLVM backend. For purposes of this analysis I will convert the above case expression to the following function:
f :: Int# -> Int# -> Int# -> Int# -> String f x y width height = case (x <$# 0#) `orI#` (x >=$# width) `orI#` (y <$# 0#) `orI#` (y >=$# height) of 1# -> "one" 0# -> "zero"
By dumping intermediate Cmm representation we can determine how variables are mapped to CPU registers. Arguments are passed to the function using a stack:
Main.f_slow() // [R1] { info_tbl: [] stack_info: arg_space: 0 updfr_space: Nothing } {offset cWY: R5 = I64[Sp + 24]; R4 = I64[Sp + 16]; R3 = I64[Sp + 8]; R2 = I64[Sp]; R1 = R1; Sp = Sp + 32; call Main.f_info(R5, R4, R3, R2, R1) args: 8, res: 0, upd: 8; } }
R2 contains the first argument x, R3 contains y, R4 contains width and R5 contains height. We can verify that by looking at the body of Main.f_info:
_sV9::I64 = R3; _sV3::I64 = R2; _sWx::I64 = %MO_S_Lt_W64(_sV3::I64, 0) | %MO_S_Ge_W64(_sV3::I64, R4) | %MO_S_Lt_W64(_sV9::I64, 0) | %MO_S_Ge_W64(_sV9::I64, R5);
Mappings between Cmm's R(2/3/4/5) registers and machine registers are defined in includes/stg/MachRegs.h:
#define REG_R2 r14 #define REG_R3 rsi #define REG_R4 rdi #define REG_R5 r8
Knowing that we can dump the assembly generated by LLVM backend:
Main_f_info: # BB#0: movq %rsi, %rax orq %r14, %rax shrq $63, %rax cmpq %rdi, %r14 setge %cl movzbl %cl, %ecx orq %rax, %rcx cmpq %r8, %rsi setge %al movzbl %al, %eax orq %rcx, %rax jne .LBB4_1 # BB#3: movq Main_f2_closure(%rip), %rax movl $Main_f2_closure, %ebx jmpq *%rax # TAILCALL .LBB4_1: cmpq $1, %rax jne .LBB4_2 # BB#4: movq Main_f1_closure(%rip), %rax movl $Main_f1_closure, %ebx jmpq *%rax # TAILCALL .LBB4_2: movq Main_f3_closure(%rip), %rax movl $Main_f3_closure, %ebx jmpq *%rax # TAILCALL
Let's analyze line by line the part responsible for evaluating the scrutinee:
movq %rsi, %rax # load y (stored in %rsi) into %rax register orq %r14, %rax # perform bitwise OR between y (now in %rax) and x (in %r14) # and store result in %rax shrq $63, %rax # shift %rax 63 bits to the right. If both x and y were # positive numbers, then their sign bits (MSB) were set to # 0 and so %rax is now 0. If at least one of them was # negative then its sign bit must have been 1 and so %rax # is now 1 cmpq %rdi, %r14 # compare width (in %rdi) with x (in %r14) and set the flags # in the flag register according to the result. setge %cl # if, based on flags set by cmpq, x was greater or equal to # width then we set %cl to 1. Otherwise it is set to 0. movzbl %cl, %ecx # zero the bits of %ecx register except the lowest 8 bits # containg the result of previous operation orq %rax, %rcx # perform logical OR on results of the previous test and # store the result in %rcx. At this point if %rcx is 1 # then either x was negative, or y was negative or # x was greater or equal to width cmpq %r8, %rsi # now we are checking whether y is greter or equal to # height. This is the same as previosly for x and width. setge %al # This time we set LSB of %al to 1 if y >= height movzbl %al, %eax # as previously, clear bits of %eax except lowest 8 bits orq %rcx, %rax # perform logical OR which combines result of previous # three comparisons with the last one jne .LBB2_1 # if ZF is not set it this means that either %rcx or %rax # was not zero, which means that at least one condition # in the scrutinee was true
The assembly does not contain comparisons and branches in the scrutinee of the case expression, but still uses jumps to select an appropriate branch of the case expression.
Implementation details
Below is a summary of implementation details and decisions:
- The new comparison primops return a value of type Int#: 1# represents True and 0# represents False. The Int# type was chosen because in Haskell it is more common to use signed Int type insetad of unsigned Word. By using Int# the users can easily convert unboxed result into a boxed value, without need to use word2Int# and int2word# primops.
- Unlike C, 2# or -3# don't represent a Boolean value. More concretely, you can use tagToEnum# to convert one of these Int# values to a Bool, but tagToEnum# does no error checking, so it would be Very Very Bad to call it on 2#. Our plan is to provide safe isTrue# and isFalse# functions, which will check whether its Int# parameter is a valid representation of True (i.e. it is 1#) or False (i.e. it is 0#). This is not possible at the moment due to deficiency in the code generator (see #8326), but we do provide isTrue# function for you to use (defined in GHC.Types, re-exported by GHC.Base and GHC.Exts). Currently it is an alias to tagToEnum# and is therefore unsafe, but once we solve #8326 we will turn it into a safe function.
- As a small side-task, four new logical bitwise primops have been implemented: andI#, orI#, xorI# and negI# (#7689). These operate on values of type Int#. Earlier we only had bitwise logical primops operating on values of type Word#.
- Functions for comparing values of Integer type are not primops from technical point of view, because they are implemented in Haskell (in case of integer-gmp also with FFI), but they pretend to be ones. There are six primops for comparing Integer values:
eqInteger# :: Integer -> Integer -> Int# neqInteger# :: Integer -> Integer -> Int# leInteger# :: Integer -> Integer -> Int# ltInteger# :: Integer -> Integer -> Int# gtInteger# :: Integer -> Integer -> Int# geInteger# :: Integer -> Integer -> Int#
Each of these functions has a wrapper that calls isTrue# and returns a Bool. These wrappers are: eqInteger, neqInteger, leInteger, ltInteger, gtInteger and geInteger.
- Other libraries that were modified to work with the new primops are: array, base, dph, ghc-prim, primitive and template-haskell.
- GHC received an internal module compiler/utils/ExtsCompat46 that allows to bootstrap with GHC versions that have old primops (i.e. GHC 7.6 and GHC 7.4). This module is meant to be temporary - see #8330.
Benchmarks
Below is a benchmark for the proof-of-concept branchless filter function that demonstrates performance gains possible with the new primops:
{-# LANGUAGE BangPatterns, MagicHash #-} module Main ( main ) where import Control.Monad.ST (runST) import Criterion.Config (Config, cfgPerformGC, defaultConfig, ljust) import Criterion.Main import Data.Vector.Unboxed.Mutable (unsafeNew, unsafeSlice, unsafeWrite) import Data.Vector.Unboxed as U (Vector, filter, foldM', fromList, length, unsafeFreeze) import GHC.Exts (Int (I#), (>=$#)) import System.Random (RandomGen, mkStdGen, randoms) import Prelude hiding (filter, length) filterN :: U.Vector Int -> U.Vector Int filterN vec = runST $ do let !size = length vec fVec <- unsafeNew size let put i x = do let !(I# v) = x inc = I# (v >=$# 0#) unsafeWrite fVec i x return $ i + inc fSize <- foldM' put 0 vec unsafeFreeze $ unsafeSlice 0 fSize fVec main :: IO () main = return (mkStdGen 1232134332) >>= defaultMainWith benchConfig (return ()) . benchmarks benchmarks :: RandomGen g => g -> [Benchmark] benchmarks gen = let dataSize = 10 ^ (7 :: Int) inputList = take dataSize . randoms $ gen :: [Int] inputVec = fromList inputList isPositive = (> 0) in [ bgroup "Filter" [ bench "New" $ whnf (filterN) inputVec , bench "Vector" $ whnf (filter isPositive) inputVec ] ] benchConfig :: Config benchConfig = defaultConfig { cfgPerformGC = ljust True }
Compile and run with:
ghc -O2 -fllvm -optlo-O3 Main.hs ./Main -o report.html
Benchmarking shows that filterN function is about 55-65% faster than the filter function based on stream fusion (tested for unboxed vectors containing 10 thousand and 10 million elements). Below is an example benchmarking report from criterion:
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