Implement cast worker/wrapper properly
Implement cast worker/wrapper properly
The cast worker/wrapper transformation transforms x = e |> co into y = e x = y |> co
This is done by the simplifier, but we were being careless about transferring IdInfo from x to y, and about what to do if x is a NOINLNE function. This resulted in a series of bugs: #17673 (closed), #18093, #18078 (closed).
This patch fixes all that:
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Main change is in GHC.Core.Opt.Simplify, and the new prepareBinding function, which does this cast worker/wrapper transform. See Note [Cast worker/wrappers].
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There is quite a bit of refactoring around prepareRhs, makeTrivial etc. It's nicer now.
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Some wrappers from strictness and cast w/w, notably those for a function with a NOINLINE, should inline very late. There wasn't really a mechanism for that, which was an existing bug really; so I invented a new finalPhase = Phase (-1). It's used for all simplifier runs after the user-visible phase 2,1,0 have run. (No new runs of the simplifier are introduced thereby.)
See new Note [Compiler phases] in GHC.Types.Basic; the main changes are in GHC.Core.Opt.Driver
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Doing this made me trip over two places where the AnonArgFlag on a FunTy was being lost so we could end up with (Num a -> ty) rather than (Num a => ty)
- In coercionLKind/coercionRKind
- In contHoleType in the Simplifier
I fixed the former by defining mkFunctionType and using it in coercionLKind/RKind.
I could have done the same for the latter, but the information is almost to hand. So I fixed the latter by
- adding sc_hole_ty to ApplyToVal (like ApplyToTy),
- adding as_hole_ty to ValArg (like TyArg)
- adding sc_fun_ty to StrictArg Turned out I could then remove ai_type from ArgInfo. This is just moving the deck chairs around, but it worked out nicely.
See the new Note [AnonArgFlag] in GHC.Types.Var
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When looking at the 'arity decrease' thing (#18093) I discovered that stable unfoldings had a much lower arity than the actual optimised function. That's what led to the arity-decrease message. Simple solution: eta-expand.
It's described in Note [Eta-expand stable unfoldings] in GHC.Core.Opt.Simplify
I made a test case for #18078 (closed), and a very similar one for #17673 (closed).
The net effect of all this on nofib is very modest, but positive:
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Program Size Allocs Runtime Elapsed TotalMem
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anna -0.4% -0.1% -3.1% -3.1% 0.0%
fannkuch-redux -0.4% -0.3% -0.1% -0.1% 0.0%
maillist -0.4% -0.1% -7.8% -1.0% -14.3%
primetest -0.4% -15.6% -7.1% -6.6% 0.0%
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Min -0.9% -15.6% -13.3% -14.2% -14.3%
Max -0.3% 0.0% +12.1% +12.4% 0.0%
Geometric Mean -0.4% -0.2% -2.3% -2.2% -0.1%