Remove a 'unsafeCoerce' from 'Unsafe.coerce' #330
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Previously, the linear `Unsafe.coerce` was implemented using two `unsafeCoerce`'s, one to get a function `a -> b`, and another one to cast that function to be a linear one. After the unsafe equality proofs change[1], we have a `unsafeEqualityProof` function to cast two types to be the same without looking at the value, so we can linearly return it, removing the need for the outer `unsafeCoerce`. This removes one allocation from the generated STG code for 'Data.Array.Unlifted.get', improving the performance slightly. [1]: https://gitlab.haskell.org/ghc/ghc/-/commit/74ad75e87317196c600dfabc61aee1b87d95c214
utdemir
commented
Jun 11, 2021
| coerce a = | ||
| case NonLinear.unsafeEqualityProof @a @b of | ||
| NonLinear.UnsafeRefl -> a | ||
| {-# INLINE coerce #-} |
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This INLINE actually does not have an effect on my test case, since GHC already was inlining it even without. But I thought it's better to be explicit.
aspiwack
approved these changes
Jun 11, 2021
I didn't give too much detail :). It does help the benchmarks but only a little bit. I saw that there ends up being one less allocation on |
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Previously, the linear
Unsafe.coercewas implemented using twounsafeCoerce's, one to get a functiona -> b, and another one to cast that function to be a linear one.After the unsafe equality proofs change, we have a
unsafeEqualityProoffunction to cast two types to be the same without looking at the value, so we can linearly return it, removing the need for the outerunsafeCoerce.This removes one allocation from the generated STG code for
Data.Array.Unlifted.get, improving the performance slightly.