Reduce code bloat for literal strings by avoiding RULES and inlining

[Bodigrim]

Closes #464.

What I don't really understand is why in spite of {-# NOINLINE unpackCStringAscii# #-} and {-# NOINLINE unpackCString# #-} GHC still applies worker-wrapper transformation:

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
emojis1857 :: Addr#
emojis1857
  = "\\240\\159\\143\\180\\243\\160\\129\\167\\243\\160\\129\\162\\243\\160\\129\\183\\243\\160\\129\\172\\243\\160\\129\\179\\243\\160\\129\\191"#

-- RHS size: {terms: 8, types: 10, coercions: 0, joins: 0/0}
emojis1856 :: Text
emojis1856
  = case $wunpackCString# emojis1857 of
    { (# ww1_ahS0, ww2_ahS1, ww3_ahS2 #) ->
    Text ww1_ahS0 ww2_ahS1 ww3_ahS2
    }

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
emojis1859 :: Addr#
emojis1859 = "wales"#

-- RHS size: {terms: 8, types: 10, coercions: 0, joins: 0/0}
emojis1858 :: Text
emojis1858
  = case $wunpackCStringAscii# emojis1859 of
    { (# ww1_ahS6, ww2_ahS7, ww3_ahS8 #) ->
    Text ww1_ahS6 ww2_ahS7 ww3_ahS8
    }

This does not happen with text-1.2.5.0:

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
emojis1857 :: Addr#
emojis1857
  = "\\240\\159\\143\\180\\243\\160\\129\\167\\243\\160\\129\\162\\243\\160\\129\\183\\243\\160\\129\\172\\243\\160\\129\\179\\243\\160\\129\\191"#

-- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
emojis1856 :: Text
emojis1856 = unpackCString# emojis1857

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
emojis1859 :: Addr#
emojis1859 = "wales"#

-- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
emojis1858 :: Text
emojis1858 = unpackCString# emojis1859

@AndreasPK any ideas?

@sjakobi does it help to reduce compile times?

CC @jgm @mpickering

[AndreasPK]

GHC generates workers for NOINLINE bindings. Which is also known to cause issues in some cases, see https://gitlab.haskell.org/ghc/ghc/-/issues/20364

It seems the old unpack version was just unstream (S.streamCString# addr#) which might have been opaque to GHC. GHC does a better job understanding the non-streaming version so does W/W to avoid allocating the Text constructor in certain situations.

I imagine this is actually beneficial for runtime performance when it helps. If that's often enough to be worth it I can not say.

[Bodigrim]

@AndreasPK thanks, I did not know it. I guess it's not a big deal here: I imagine additional Core nodes should be reasonably fast to compile.

[AndreasPK]

@AndreasPK thanks, I did not know it. I guess it's not a big deal here: I imagine additional Core nodes should be reasonably fast to compile.

Yeah it should just be something like unpackC= case $wunpackC of (# x1, x2, x3 #) -> Text x1 x2 x3 which get's inlined.

[mpickering]

I compiled the emojis package with this patch and it makes a massive difference.

before:

 102,867,674,224 bytes allocated in the heap
  33,195,407,952 bytes copied during GC
     587,046,336 bytes maximum residency (65 sample(s))
       2,699,840 bytes maximum slop
            1628 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      1010 colls,     0 par   15.477s  15.510s     0.0154s    0.2033s
  Gen  1        65 colls,     0 par   24.926s  24.972s     0.3842s    0.7519s

  TASKS: 6 (1 bound, 5 peak workers (5 total), using -N1)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.193s  (  0.193s elapsed)
  MUT     time   46.399s  ( 47.811s elapsed)
  GC      time   40.404s  ( 40.482s elapsed)
  EXIT    time    0.001s  (  0.008s elapsed)
  Total   time   86.996s  ( 88.493s elapsed)

after:

** Deleting temp dirs:
  20,667,160,256 bytes allocated in the heap
   4,245,384,896 bytes copied during GC
     145,651,272 bytes maximum residency (34 sample(s))
       1,995,288 bytes maximum slop
             372 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       603 colls,     0 par    2.678s   2.701s     0.0045s    0.0339s
  Gen  1        34 colls,     0 par    2.390s   2.405s     0.0707s    0.1364s

  TASKS: 6 (1 bound, 5 peak workers (5 total), using -N1)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.212s  (  0.222s elapsed)
  MUT     time   11.108s  ( 11.983s elapsed)
  GC      time    5.068s  (  5.107s elapsed)
  EXIT    time    0.001s  (  0.009s elapsed)
  Total   time   16.389s  ( 17.321s elapsed)