Type family reduction irregularity (change from 7.10.3 to 8.0.1)

I was recently made aware that vinyl's performance dramatically deteriorated with GHC 8.0.1 when compared to GHC 7.x. Vinyl is an extensible records library that's been around for about four years; the aspect of its design relevant here is the basic HList-style indexed GADT. Care was taken in the past so that working with, say, a Storable Vector of records would suffer no overhead from vinyl when compared with standard records, and we have a benchmark suite to spot check this.

In the move to GHC-8.0.1, it turns out that we do introduce overhead. Inspecting the Core reveals that the benchmark inner loop includes a case match on an HEq_sc value whose presence I would not expect, and that is not present when compiling with GHC-7.10.3:

case HEq_sc
         @ Nat
         @ Nat
         @ ('S 'Z)
         @ (RIndex
              '("normal", V3 Float)
              '['("tex", V2 Float), '("normal", V3 Float)])
         ($s$fRElemar:S_$s$fRElemar:S_$cp1RElem
          `cast` (N:~[0] <Nat>_N <'S 'Z>_N <RIndex
                                              '("normal",
                                                V3 Float)
                                              '['("tex",
                                                  V2 Float),
                                                '("normal",
                                                  V3 Float)]>_N
                  :: (('S 'Z :: Nat)
                      ~
                      (RIndex
                         '("normal", V3 Float)
                         '['("tex", V2 Float),
                           '("normal",
                             V3 Float)] :: Nat) :: Constraint)
                     ~R#
                     (('S 'Z :: Nat)
                      ~~
                      (RIndex
                         '("normal", V3 Float)
                         '['("tex", V2 Float),
                           '("normal",
                             V3 Float)] :: Nat) :: Constraint)))
  of cobox0 { __DEFAULT ->

I have since made an effort to reproduce the issue, and discovered more fragility than I expected. I am attaching two modules: Rec.hs defines a kind of record type, Main.hs is a test program that I will reproduce here,

{-# LANGUAGE DataKinds #-}
module Main where
import Rec

type MyRec = Rec '[ '("A",Int), '("B",Int), '("C",Int) ]

getC :: MyRec -> Int
getC = getField (Proxy::Proxy '("C",Int))

doubleC :: MyRec -> MyRec
doubleC r = setC (2 * (getC r)) r
  where setC = set . (Field :: Int -> Field '("C",Int))

main :: IO ()
main = print (getC (Field 1 :& Field 2 :& Field 3 :& Nil :: MyRec))

If the doubleC definition is present, the Core emitted (with -O2) includes an HEq_sc case in the RHS of getC. If doubleC is commented out, that case HEq_sc ... is no longer present. In this example, the offending piece of Core is,

case HEq_sc
       @ Nat
       @ Nat
       @ (Index '("C", Int) '['("B", Int), '("C", Int)])
       @ ('S 'Z)
       ($s$fHasFieldkr:S_$s$fHasFieldkr:S_$cp1HasField
        `cast` (N:~[0] <Nat>_N <Index
                                  '("C", Int) '['("B", Int), '("C", Int)]>_N <'S 'Z>_N
                :: ((Index '("C", Int) '['("B", Int), '("C", Int)] :: Nat)
                    ~
                    ('S 'Z :: Nat) :: Constraint)
                   ~R#
                   ((Index '("C", Int) '['("B", Int), '("C", Int)] :: Nat)
                    ~~
                    ('S 'Z :: Nat) :: Constraint)))
of cobox1 { __DEFAULT ->

If the contents of Rec.hs are included in Main.hs, the case HEq_sc ... is not present in the resulting Core.

The result of what looks like a failure to normalize the Index type family (or RIndex in vinyl) manifests as a 2x slowdown in the benchmark available in the vinyl repository.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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