Reduce result discount in conSize

Ticket #18282 showed that the result discount given by conSize
was massively too large.  This patch reduces that discount to
a constant 10, which just balances the cost of the constructor
application itself.

Note [Constructor size and result discount] elaborates, as
does the ticket #18282.

Reducing result discount reduces inlining, which affects perf.  I
found that I could increase the unfoldingUseThrehold from 80 to 90 in
compensation; in combination with the result discount change I get
these overall nofib numbers:

        Program           Size    Allocs   Runtime   Elapsed  TotalMem
--------------------------------------------------------------------------------
          boyer          -0.2%     +5.4%     -3.2%     -3.4%      0.0%
       cichelli          -0.1%     +5.9%    -11.2%    -11.7%      0.0%
      compress2          -0.2%     +9.6%     -6.0%     -6.8%      0.0%
   cryptarithm2          -0.1%     -3.9%     -6.0%     -5.7%      0.0%
         gamteb          -0.2%     +2.6%    -13.8%    -14.4%      0.0%
         genfft          -0.1%     -1.6%    -29.5%    -29.9%      0.0%
             gg          -0.0%     -2.2%    -17.2%    -17.8%    -20.0%
           life          -0.1%     -2.2%    -62.3%    -63.4%      0.0%
           mate          +0.0%     +1.4%     -5.1%     -5.1%    -14.3%
         parser          -0.2%     -2.1%     +7.4%     +6.7%      0.0%
      primetest          -0.2%    -12.8%    -14.3%    -14.2%      0.0%
         puzzle          -0.2%     +2.1%    -10.0%    -10.4%      0.0%
            rsa          -0.2%    -11.7%     -3.7%     -3.8%      0.0%
         simple          -0.2%     +2.8%    -36.7%    -38.3%     -2.2%
   wheel-sieve2          -0.1%    -19.2%    -48.8%    -49.2%    -42.9%
--------------------------------------------------------------------------------
            Min          -0.4%    -19.2%    -62.3%    -63.4%    -42.9%
            Max          +0.3%     +9.6%     +7.4%    +11.0%    +16.7%
 Geometric Mean          -0.1%     -0.3%    -17.6%    -18.0%     -0.7%

I'm ok with these numbers, remembering that this change removes
an *exponential* increase in code size in some in-the-wild cases.

I investigated compress2.  The difference is entirely caused by this
function no longer inlining

WriteRoutines.$woutputCodes
  = \ (w :: [CodeEvent]) ->
      let result_s1Sr
            = case WriteRoutines.outputCodes_$s$woutput w 0# 0# 8# 9# of
                (# ww1, ww2 #) -> (ww1, ww2)
      in (# case result_s1Sr of (x, _) ->
              map @Int @Char WriteRoutines.outputCodes1 x
         , case result_s1Sr of { (_, y) -> y } #)

It was right on the cusp before, driven by the excessive result
discount.  Too bad!

Happily, the compiler/perf tests show a number of improvements:
    T12227     compiler bytes-alloc  -6.6%
    T12545     compiler bytes-alloc  -4.7%
    T13056     compiler bytes-alloc  -3.3%
    T15263     runtime  bytes-alloc -13.1%
    T17499     runtime  bytes-alloc -14.3%
    T3294      compiler bytes-alloc  -1.1%
    T5030      compiler bytes-alloc -11.7%
    T9872a     compiler bytes-alloc  -2.0%
    T9872b     compiler bytes-alloc  -1.2%
    T9872c     compiler bytes-alloc  -1.5%

Metric Decrease:
    T12227
    T12545
    T13056
    T15263
    T17499
    T3294
    T5030
    T9872a
    T9872b
    T9872c

2 files changed, 46 insertions, 0 deletions

diff --git a/testsuite/tests/perf/compiler/T18282.hs b/testsuite/tests/perf/compiler/T18282.hs
new file mode 100644
index 0000000000..9039f0ad0f
--- /dev/null
+++ b/testsuite/tests/perf/compiler/T18282.hs
@@ -0,0 +1,40 @@
+module M
+  ( mkB2
+  ) where
+
+import Control.Monad.Reader
+import Data.Maybe
+
+data A1 = A1 (Maybe String) (Maybe String) (Maybe String) (Maybe String)
+data A2 = A2 A1 (Maybe String) (Maybe String) (Maybe String) (Maybe String)
+                (Maybe String) (Maybe String) (Maybe String) (Maybe String)
+
+data B1 = B1 !String !String !String !String
+data B2 = B2 !B1 !String !String !String !String !String !String !String !String
+
+type M a = ReaderT [(String, String)] (Either String) a
+
+resolve :: Maybe String -> String -> M (Maybe String)
+resolve (Just x) _ = pure (Just x)
+resolve Nothing  v = asks $ lookup v
+
+mkB1 :: A1 -> M B1
+mkB1 (A1 a b c d) = do
+  a' <- fromMaybe "" <$> resolve a "A"
+  b' <- fromMaybe "" <$> resolve b "B"
+  c' <- fromMaybe "" <$> resolve c "C"
+  d' <- fromMaybe "" <$> resolve d "D"
+  pure $ B1 a' b' c' d'
+
+mkB2 :: A2 -> M B2
+mkB2 (A2 a b c d e f g h i) = do
+  a' <- mkB1 a
+  b' <- fromMaybe "db" <$> resolve b "B"
+  c' <- fromMaybe "dc" <$> resolve c "C"
+  d' <- fromMaybe "dd" <$> resolve d "D"
+  e' <- fromMaybe "de" <$> resolve e "E"
+  f' <- fromMaybe "df" <$> resolve f "F"
+  g' <- fromMaybe "dg" <$> resolve g "G"
+  h' <- fromMaybe "dh" <$> resolve h "H"
+  i' <- fromMaybe "di" <$> resolve i "I"
+  pure $ B2 a' b' c' d' e' f' g' h' i'
diff --git a/testsuite/tests/perf/compiler/all.T b/testsuite/tests/perf/compiler/all.T
index e993d40587..2755024a9a 100644
--- a/testsuite/tests/perf/compiler/all.T
+++ b/testsuite/tests/perf/compiler/all.T
@@ -382,3 +382,9 @@ test ('T18304',
       ],
       compile,
       ['-v0 -O'])
+
+test ('T18282',
+      [ collect_compiler_stats('bytes allocated',2)
+      ],
+      compile,
+      ['-v0 -O'])