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Diffstat (limited to 'compiler/GHC/Core/Opt/OccurAnal.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/OccurAnal.hs | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/compiler/GHC/Core/Opt/OccurAnal.hs b/compiler/GHC/Core/Opt/OccurAnal.hs index fc374adb99..d463b66c50 100644 --- a/compiler/GHC/Core/Opt/OccurAnal.hs +++ b/compiler/GHC/Core/Opt/OccurAnal.hs @@ -2165,6 +2165,17 @@ So we have a fast-path that keeps the old tree if the occ_bs_env is empty. This just saves a bit of allocation and reconstruction; not a big deal. +This fast path exposes a tricky cornder, though (#22761). Supose we have + Unfolding = \x. let y = foo in x+1 +which includes a dead binding for `y`. In occAnalUnfolding we occ-anal +the unfolding and produce /no/ occurrences of `foo` (since `y` is +dead). But if we discard the occ-analysed syntax tree (which we do on +our fast path), and use the old one, we still /have/ an occurrence of +`foo` -- and that can lead to out-of-scope variables (#22761). + +Solution: always keep occ-analysed trees in unfoldings and rules, so they +have no dead code. See Note [OccInfo in unfoldings and rules] in GHC.Core. + Note [Cascading inlines] ~~~~~~~~~~~~~~~~~~~~~~~~ By default we use an rhsCtxt for the RHS of a binding. This tells the |