Massive blowup of code size on trivial program
The test simplCore/should_compile/simpl015 was leading to massive compile times. This is weird: it looks like:
main = do { return ()
; return ()
... hundreds more times ...
; return () }It turns out that the cause was over-eager eta-expansion. Each return () gives a top-level PAP thus
lvl17 :: IO ()
lvl17 = returnIO ()But these definitions are then eta-expanded, thus:
lvl17 = ((\eta. returnIO () |> sym g) eta) |> g
where g :: State# RealWorld -> (# State# RealWorld, () #)
~
IO ()Now in general it makes sense to eta-expand through newtypes (as is done here), because it exposes more lambadas. But it really doesn't make sense to eta-expand a PAP like this.
Fortunately the fix is easy: use exprArity rather than manifestArity in SimplUtils.tryEtaExpandRhs.
The effect on simpl015 is dramatic: compiler allocation drops from 6.6G to 812M; and residency drops from 1.8G to 45M.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.8.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |