Commit 4c8bf63b authored by Sebastian Graf's avatar Sebastian Graf

Add Note [Dead case binders in -O0]

After reverting Phab:D5358, Simon (Peyton Jones) asked for a Note
summarising why we want to keep the dead case binder check in `cgCase`.

Summary from mail conversation:

* Phab:D5324 means that we no longer /recompute/ dead-ness of case-binders in
  STG-land

* But TidyPgm preserves dead-ness info (see CoreTidy.tidyIdBndr)

* And so we can take advantage of it to avoid a redundant load. This load
  would be eliminated by CmmSink, but that only happens with -O
parent 8f52ab92
......@@ -305,7 +305,7 @@ cgCase (StgOpApp (StgPrimOp op) args _) bndr (AlgAlt tycon) alts
= do { tag_expr <- do_enum_primop op args
-- If the binder is not dead, convert the tag to a constructor
-- and assign it.
-- and assign it. See Note [Dead case binders in -O0]
; unless (isDeadBinder bndr) $ do
{ dflags <- getDynFlags
; tmp_reg <- bindArgToReg (NonVoid bndr)
......@@ -385,6 +385,18 @@ Now the trouble is that 's' has VoidRep, and we do not bind void
arguments in the environment; they don't live anywhere. See the
calls to nonVoidIds in various places. So we must not look up
's' in the environment. Instead, just evaluate the RHS! Simple.
Note [Dead case binders in -O0]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Before CmmSink came to eliminate dead assignments, omitting assignment of dead
case binders was a cheap and worthwhile optimisation. This probably also was the
reason for occurrence hacks such as in Phab:D5339 to exist, because the
occurrence information preserved by 'CoreTidy.tidyIdBndr' was insufficient.
Nowadays, with CmmSink there's little reason to complicate the code by checking
for dead case binders, except that CmmSink won't run with -O0. Since the
majority of case binders are dead, this optimisation probably still has a great
benefit-cost ratio and we want to keep it for -O0. See also Phab:D5358.
-}
cgCase (StgApp v []) _ (PrimAlt _) alts
......
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