[source]

compiler/iface/ToIface.hs

Note [Inlining and hs-boot files]

[note link]

Consider this example (#10083, #12789):

———- RSR.hs-boot ———— module RSR where

data RSR eqRSR :: RSR -> RSR -> Bool

———- SR.hs ———— module SR where

import {-# SOURCE #-} RSR data SR = MkSR RSR eqSR (MkSR r1) (MkSR r2) = eqRSR r1 r2

———- RSR.hs ———— module RSR where

import SR data RSR = MkRSR SR – deriving( Eq ) eqRSR (MkRSR s1) (MkRSR s2) = (eqSR s1 s2) foo x y = not (eqRSR x y)

When compiling RSR we get this code

RSR.eqRSR :: RSR -> RSR -> Bool
RSR.eqRSR = \ (ds1 :: RSR.RSR) (ds2 :: RSR.RSR) ->
            case ds1 of _ { RSR.MkRSR s1 ->
            case ds2 of _ { RSR.MkRSR s2 ->
            SR.eqSR s1 s2 }}

RSR.foo :: RSR -> RSR -> Bool
RSR.foo = \ (x :: RSR) (y :: RSR) -> not (RSR.eqRSR x y)
Now, when optimising foo:
Inline eqRSR (small, non-rec) Inline eqSR (small, non-rec)

but the result of inlining eqSR from SR is another call to eqRSR, so everything repeats. Neither eqSR nor eqRSR are (apparently) loop breakers.

Solution: in the unfolding of eqSR in SR.hi, replace eqRSR in SR with noinline eqRSR, so that eqRSR doesn’t get inlined. This means that when GHC inlines eqSR, it will not also inline eqRSR, exactly as would have been the case if foo had been defined in SR.hs (and marked as a loop-breaker).

But how do we arrange for this to happen? There are two ingredients:

1. When we serialize out unfoldings to IfaceExprs (toIfaceVar), for every variable reference we see if we are referring to an ‘Id’ that came from an hs-boot file. If so, we add a noinline to the reference.

2. But how do we know if a reference came from an hs-boot file or not? We could record this directly in the ‘IdInfo’, but actually we deduce this by looking at the unfolding: ‘Id’s that come from boot files are given a special unfolding (upon typechecking) ‘BootUnfolding’ which say that there is no unfolding, and the reason is because the ‘Id’ came from a boot file.

Here is a solution that doesn’t work: when compiling RSR, add a NOINLINE pragma to every function exported by the boot-file for RSR (if it exists). Doing so makes the bootstrapped GHC itself slower by 8% overall (on #9872a-d, and T1969: the reason is that these NOINLINE’d functions now can’t be profitably inlined outside of the hs-boot loop.