Refactor iface file generation:wip/osa1/backend_refactoring

This commit refactors interface file generation to allow information
from the later passed (NCG, STG) to be stored in interface files.

We achieve this by splitting interface file generation into two parts:
* Partial interfaces, built based on the result of the core pipeline
* A fully instantiated interface, which also contains the final
fingerprints and can optionally contain information produced by the backend.

This change is required by !1304 and !1530.

-dynamic-too handling is refactored too: previously when generating code
we'd branch on -dynamic-too *before* code generation, but now we do it
after.

(Original code written by @AndreasK in !1530)

Performance
~~~~~~~~~~~

Before this patch interface files where created and immediately flushed
to disk which made space leaks impossible.
With this change we instead use NFData to force all iface related data
structures to avoid space leaks.

In the process of refactoring it was discovered that the code in the
ToIface Module allocated a lot of thunks which were immediately forced
when writing/forcing the interface file. So we made this module more
strict to avoid creating many of those thunks.

Bottom line is that allocations go down by about ~0.1% compared to
master.
Residency is not meaningfully different after this patch.
Runtime was not benchmarked.

Co-Authored-By: Andreas Klebinger <klebinger.andreas@gmx.at>
Co-Authored-By: Ömer Sinan Ağacan <omer@well-typed.com>

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