Allow list fusion for Text and Text.Lazy unpack #629
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meooow25
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Mar 25, 2025
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| foldrText :: (Char -> b -> b) -> b -> Text -> b | ||
| foldrText f z (Text arr off len) = go off | ||
| where | ||
| go !i | ||
| | i >= off + len = [] | ||
| | otherwise = let !(Iter c l) = iterArray arr i in c : go (i + l) | ||
| {-# INLINE [1] unpack #-} | ||
| | i >= off + len = z | ||
| | otherwise = let !(Iter c l) = iterArray arr i in f c (go (i + l)) | ||
| {-# INLINE foldrText #-} |
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Note: This can, and maybe should, use Data.Text.foldr. But it was already here, so I didn't change that.
Bodigrim
reviewed
Mar 25, 2025
src/Data/Text/Lazy.hs
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| -- * If it fuses: In phase 0, `foldrFB` inlines and `foldr` inlines. GHC | ||
| -- optimizes the fused code. | ||
| {-# RULES | ||
| "Text.Lazy.unpack" [~1] forall t. unpack t = Exts.build (\c n -> foldrFB c n t) |
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Suggested change
| "Text.Lazy.unpack" [~1] forall t. unpack t = Exts.build (\c n -> foldrFB c n t) | |
| "Text.Lazy.unpack" [~1] forall t. unpack t = Exts.build (\cons nil -> foldrFB cons nil t) |
and same for the strict Text. Fusion is cryptic enough, let's give an extra hint to readers.
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Done. I went with lcons and lnil because GHC warns about cons shadowing the top-level cons.
* Make Data.Text.unpack and Data.Text.Lazy.unpack good producers in list fusion. This allows them to fuse with good consumers of lists. Rewrite-back rules are included since the function bodies are large and we don't want to inline them if fusion doesn't occur. * For Data.Text.Lazy, this change means that `unpack`, which uses `unstreamList`, no longer fuses with `streamList` under Text's stream fusion framework. This scenario seems very unlikely, since nothing else must be done to the list in between the two functions. Even `pack . unpack` does not satisfy this rule. So we are not losing anything valuable here. * Add benchmarks for unpack, fusion and no fusion.
Bodigrim
approved these changes
Mar 26, 2025
Lysxia
approved these changes
Mar 26, 2025
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unpack, which usesunstreamList, no longer fuses withstreamListunder Text's stream fusion framework. This scenario seems very unlikely, since nothing else must be done to the list in between the two functions. Evenpack . unpackdoes not satisfy this rule. So we are not losing anything valuable here.Closes #628.
Benchmarks with GHC 9.10.1
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