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Diffstat (limited to 'compiler/GHC/Core/Opt/Exitify.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/Exitify.hs | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/compiler/GHC/Core/Opt/Exitify.hs b/compiler/GHC/Core/Opt/Exitify.hs index 89156418bc..b8ba685a5e 100644 --- a/compiler/GHC/Core/Opt/Exitify.hs +++ b/compiler/GHC/Core/Opt/Exitify.hs @@ -433,6 +433,7 @@ inlining. Exit join points, recognizable using `isExitJoinId` are join points with an occurrence in a recursive group, and can be recognized (after the occurrence analyzer ran!) using `isExitJoinId`. + This function detects joinpoints with `occ_in_lam (idOccinfo id) == True`, because the lambdas of a non-recursive join point are not considered for `occ_in_lam`. For example, in the following code, `j1` is /not/ marked @@ -446,6 +447,29 @@ To prevent inlining, we check for isExitJoinId * In `simplLetUnfolding` we simply give exit join points no unfolding, which prevents inlining in `postInlineUnconditionally` and call sites. +But see Note [Be selective about not-inlining exit join points] + +Note [Be selective about not-inlining exit join points] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +If we follow "do not inline exit join points" mantra throughout, +some bad things happen. + +* We can lose CPR information: see #21148 + +* We get useless clutter (#22084) that + - makes the program bigger (including duplicated code #20739), and + - adds extra jumps (and maybe stack saves) at runtime + +So instead we follow "do not inline exit join points" for a /single run/ +of the simplifier, right after Exitification. That should give a +sufficient chance for used-once things to inline, but subsequent runs +will inline them back in. (Annoyingly, as things stand, only with -O2 +is there a subsequent run, but that might change, and it's not a huge +deal anyway.) + +This is controlled by the Simplifier's sm_keep_exits flag; see +GHC.Core.Opt.Pipeline. + Note [Placement of the exitification pass] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I (Joachim) experimented with multiple positions for the Exitification pass in |