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<h3>
Sebastian Graf pushed to branch wip/dmdanal-precise-exn
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/f5344aa5534a33125ddf6a18499701c948745898">f5344aa5</a></strong>
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<span>by Sebastian Graf</span>
<i>at 2020-04-01T20:07:42+02:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">DmdAnal: Reflect precise exceptions in demand types
In #13380 and #17676 we saw that we didn't preserve precise exception
semantics in demand analysis. We fixed that with minimal changes in
!2956, but that is terribly unprincipled.
That unprincipledness results in a loss of precision, which is tracked
by these new test cases:
- `T13380b`: Regression in dead code elimination, because !2956 was too
syntactic about `raiseIO#`
- `T13380c`: No need to apply the "IO hack" when the IO action may not
throw a precise exception
- `T13380d`: Demonstrating unsoundness of the "IO hack" when resorting
to manual state token threading and direct use of primops.
More details below.
The "IO hack" (which is a fallback to preserve precise exceptions
semantics and thus soundness, rather than some smart thing that
increases precision) is quite a misnomer and is called
`mayThrowPreciseException` now.
Also there is a slight chance that the IO hack was unsound before,
because it assumes that precise exceptions can only be thrown from a
case scrutinee of type `(# State# RealWorld, _ #)`.
I couldn't come up with a program using the `IO` abstraction that
violates this assumption. But it's easy to do so via manual state token
threading and direct use of primops, see `T13380d`. Also similar code
might be generated by Nested CPR in the (hopefully not too) distant
future.
Hence, we now have a more careful test in `forcesRealWorld` that passes
`T13380d`.
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compiler/GHC.hs
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compiler/GHC/Core/Arity.hs
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compiler/GHC/Core/Lint.hs
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compiler/GHC/Core/Op/CallArity.hs
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compiler/GHC/Core/Op/DmdAnal.hs
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