diff options
Diffstat (limited to 'compiler/codeGen')
| -rw-r--r-- | compiler/codeGen/StgCmmExpr.hs | 14 |
1 files changed, 13 insertions, 1 deletions
diff --git a/compiler/codeGen/StgCmmExpr.hs b/compiler/codeGen/StgCmmExpr.hs index ea64e456ce..30603ee904 100644 --- a/compiler/codeGen/StgCmmExpr.hs +++ b/compiler/codeGen/StgCmmExpr.hs @@ -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 |