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Partially fix whitespace parsing performance regression #1512

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Merged
merged 2 commits into from
Nov 4, 2019
Merged

Partially fix whitespace parsing performance regression #1512

merged 2 commits into from
Nov 4, 2019

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Gabriella439
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This undoes some of the performance regression introduced
in #1483

Before #1483:

benchmarked Line comment
time                 11.86 ms   (11.69 ms .. 11.98 ms)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 11.84 ms   (11.79 ms .. 11.89 ms)
std dev              129.4 μs   (107.2 μs .. 164.1 μs)

benchmarked Block comment
time                 13.20 ms   (13.00 ms .. 13.41 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 13.59 ms   (13.41 ms .. 13.94 ms)
std dev              600.0 μs   (142.2 μs .. 953.7 μs)

After #1483:

benchmarked Line comment
time                 288.7 ms   (282.8 ms .. 294.7 ms)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 292.3 ms   (290.8 ms .. 294.6 ms)
std dev              3.156 ms   (2.216 ms .. 4.546 ms)

benchmarked Block comment
time                 286.2 ms   (280.9 ms .. 292.6 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 290.6 ms   (288.3 ms .. 292.9 ms)
std dev              3.875 ms   (2.866 ms .. 5.500 ms)

After this change:

benchmarked Line comment
time                 61.44 ms   (60.37 ms .. 63.03 ms)
                     0.999 R²   (0.997 R² .. 1.000 R²)
mean                 61.41 ms   (60.74 ms .. 62.25 ms)
std dev              1.341 ms   (945.0 μs .. 1.901 ms)

benchmarked Block comment
time                 61.83 ms   (60.97 ms .. 63.14 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 61.16 ms   (60.33 ms .. 61.85 ms)
std dev              1.396 ms   (1.011 ms .. 1.907 ms)

@Gabriella439
Partially fix whitespace parsing performance regression
a3e9648 
This undoes some of the performance regression introduced
in #1483

Before #1483:

```
benchmarked Line comment
time                 11.86 ms   (11.69 ms .. 11.98 ms)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 11.84 ms   (11.79 ms .. 11.89 ms)
std dev              129.4 μs   (107.2 μs .. 164.1 μs)

benchmarked Block comment
time                 13.20 ms   (13.00 ms .. 13.41 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 13.59 ms   (13.41 ms .. 13.94 ms)
std dev              600.0 μs   (142.2 μs .. 953.7 μs)
```

After #1483:

```
benchmarked Line comment
time                 288.7 ms   (282.8 ms .. 294.7 ms)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 292.3 ms   (290.8 ms .. 294.6 ms)
std dev              3.156 ms   (2.216 ms .. 4.546 ms)

benchmarked Block comment
time                 286.2 ms   (280.9 ms .. 292.6 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 290.6 ms   (288.3 ms .. 292.9 ms)
std dev              3.875 ms   (2.866 ms .. 5.500 ms)
```

After this change:

```
benchmarked Line comment
time                 61.44 ms   (60.37 ms .. 63.03 ms)
                     0.999 R²   (0.997 R² .. 1.000 R²)
mean                 61.41 ms   (60.74 ms .. 62.25 ms)
std dev              1.341 ms   (945.0 μs .. 1.901 ms)

benchmarked Block comment
time                 61.83 ms   (60.97 ms .. 63.14 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 61.16 ms   (60.33 ms .. 61.85 ms)
std dev              1.396 ms   (1.011 ms .. 1.907 ms)
```
@Gabriella439
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This is about as fast as I can get with simple changes. Getting fully back to the original performance would require much more invasive fixes, but I think this is probably good enough.

sjakobi
sjakobi reviewed Nov 4, 2019
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Impressive!

dhall/src/Dhall/Parser/Expression.hs
@@ -262,14 +262,14 @@ parsers embedded = Parsers {..}
a <- operatorExpression

let alternative4A = do
try (whitespace *> _arrow)
_arrow
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Oh, that's smart! 👍

dhall/tests/Dhall/Test/Parser.hs
@@ -108,10 +109,14 @@ notesInLetInLet = do
(Just " ")
Nothing
(Just " ")
(Note "2" (NaturalLit 2))
(Note "2 " (Note "2" (NaturalLit 2)))
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Where do the additional Notes come from?

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The two Notes were actually always there, but they used to have the same source span so they would be flattened into one Note by this logic:

dhall-haskell/dhall/src/Dhall/Parser/Expression.hs

Lines 91 to 93 in 7eec31d

case e of
Note src₁ _ | laxSrcEq src₀ src₁ -> return e
_ -> return (Note src₀ e)

Now that they slightly differ they are no longer flattened in this way

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I guess we could change that rule to always return the inner Note – not sure whether that would be beneficial though…

@mergify mergify bot merged commit b3b6bb4 into master Nov 4, 2019
@mergify mergify bot deleted the gabriel/whitespace_parsing_performance branch November 4, 2019 02:12
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@Gabriella439 @sjakobi