Replace most occurences of foldl with foldl'.

This patch adds foldl' to GhcPrelude and changes must occurences
of foldl to foldl'. This leads to better performance especially
for quick builds where GHC does not perform strictness analysis.

It does change strictness behaviour when we use foldl' to turn
a argument list into function applications. But this is only a
drawback if code looks ONLY at the last argument but not at the first.
And as the benchmarks show leads to fewer allocations in practice
at O2.

Compiler performance for Nofib:

O2 Allocations:
        -1 s.d.                -----            -0.0%
        +1 s.d.                -----            -0.0%
        Average                -----            -0.0%

O2 Compile Time:
        -1 s.d.                -----            -2.8%
        +1 s.d.                -----            +1.3%
        Average                -----            -0.8%

O0 Allocations:
        -1 s.d.                -----            -0.2%
        +1 s.d.                -----            -0.1%
        Average                -----            -0.2%

Test Plan: ci

Reviewers: goldfire, bgamari, simonmar, tdammers, monoidal

Reviewed By: bgamari, monoidal

Subscribers: tdammers, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4929

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/specialise/Specialise.hs b/compiler/specialise/Specialise.hs
index 13a7cb7474..6f775dfdcb 100644
--- a/compiler/specialise/Specialise.hs
+++ b/compiler/specialise/Specialise.hs
@@ -2096,7 +2096,7 @@ mkDB bind = (bind, bind_fvs bind)
 -- | Identify the free variables of a 'CoreBind'
 bind_fvs :: CoreBind -> VarSet
 bind_fvs (NonRec bndr rhs) = pair_fvs (bndr,rhs)
-bind_fvs (Rec prs)         = foldl delVarSet rhs_fvs bndrs
+bind_fvs (Rec prs)         = foldl' delVarSet rhs_fvs bndrs
                            where
                              bndrs = map fst prs
                              rhs_fvs = unionVarSets (map pair_fvs prs)