diff options
Diffstat (limited to 'compiler/iface/ToIface.hs')
| -rw-r--r-- | compiler/iface/ToIface.hs | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/compiler/iface/ToIface.hs b/compiler/iface/ToIface.hs index f20fed214a..d32a0529af 100644 --- a/compiler/iface/ToIface.hs +++ b/compiler/iface/ToIface.hs @@ -1,4 +1,5 @@ {-# LANGUAGE CPP #-} +{-# LANGUAGE Strict #-} -- See Note [Avoiding space leaks in toIface*] -- | Functions for converting Core things to interface file things. module ToIface @@ -73,6 +74,32 @@ import Demand ( isTopSig ) import Data.Maybe ( catMaybes ) +{- Note [Avoiding space leaks in toIface*] + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Building a interface file depends on the output of the simplifier. +If we build these lazily this would mean keeping the Core AST alive +much longer than necessary causing a space "leak". + +This happens for example when we only write the interface file to disk +after code gen has run, in which case we might carry megabytes of core +AST in the heap which is no longer needed. + +We avoid this in two ways. +* First we use -XStrict in ToIface which avoids many thunks to begin with. +* Second we define NFData instance for IFaceSyn and use them to + force any remaining thunks. + +-XStrict is not sufficient as patterns of the form `f (g x)` would still +result in a thunk being allocated for `g x`. + +NFData is sufficient for the space leak, but using -XStrict reduces allocation +by ~0.1% when compiling with -O. (nofib/spectral/simple, T10370). +It's essentially free performance hence we use -XStrict on top of NFData. + +MR !1633 on gitlab, has more discussion on the topic. +-} + ---------------- toIfaceTvBndr :: TyVar -> IfaceTvBndr toIfaceTvBndr = toIfaceTvBndrX emptyVarSet |