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previously, GHC had the "let/app-invariant" which said that the RHS of a let or the argument of an application must be of lifted type or ok for speculation. We want this on let to freely float them around, and we wanted that on app to freely convert between the two (e.g. in beta-reduction or inlining). However, the app invariant meant that simple code didn't stay simple and this got in the way of rules matching. By removing the app invariant, this thus fixes #20554. The new invariant is now called "let-can-float invariant", which is hopefully easier to guess its meaning correctly. Dropping the app invariant means that everywhere where we effectively do beta-reduction (in the two simplifiers, but also in `exprIsConApp_maybe` and other innocent looking places) we now have to check if the argument must be evaluated (unlifted and side-effecting), and analyses have to be adjusted to the new semantics of `App`. Also, `LetFloats` in the simplifier can now also carry such non-floating bindings. The fix for DmdAnal, refine by Sebastian, makes functions with unlifted arguments strict in these arguments, which changes some signatures. This causes some extra calls to `exprType` and `exprOkForSpeculation`, so some perf benchmarks regress a bit (while others improve). Metric Decrease: T9020 Metric Increase: LargeRecord T12545 T15164 T16577 T18223 T5642 T9961 Co-authored-by:Sebastian Graf <sebastian.graf@kit.edu>
previously, GHC had the "let/app-invariant" which said that the RHS of a let or the argument of an application must be of lifted type or ok for speculation. We want this on let to freely float them around, and we wanted that on app to freely convert between the two (e.g. in beta-reduction or inlining). However, the app invariant meant that simple code didn't stay simple and this got in the way of rules matching. By removing the app invariant, this thus fixes #20554. The new invariant is now called "let-can-float invariant", which is hopefully easier to guess its meaning correctly. Dropping the app invariant means that everywhere where we effectively do beta-reduction (in the two simplifiers, but also in `exprIsConApp_maybe` and other innocent looking places) we now have to check if the argument must be evaluated (unlifted and side-effecting), and analyses have to be adjusted to the new semantics of `App`. Also, `LetFloats` in the simplifier can now also carry such non-floating bindings. The fix for DmdAnal, refine by Sebastian, makes functions with unlifted arguments strict in these arguments, which changes some signatures. This causes some extra calls to `exprType` and `exprOkForSpeculation`, so some perf benchmarks regress a bit (while others improve). Metric Decrease: T9020 Metric Increase: LargeRecord T12545 T15164 T16577 T18223 T5642 T9961 Co-authored-by:
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