diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/Simplify.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/Simplify.hs | 27 |
1 files changed, 25 insertions, 2 deletions
diff --git a/compiler/GHC/Core/Opt/Simplify.hs b/compiler/GHC/Core/Opt/Simplify.hs index 1523394be9..2c1e0c21cb 100644 --- a/compiler/GHC/Core/Opt/Simplify.hs +++ b/compiler/GHC/Core/Opt/Simplify.hs @@ -2788,6 +2788,7 @@ rebuildCase env scrut case_bndr alts cont Nothing -> missingAlt env case_bndr alts cont Just (Alt _ bs rhs) -> simple_rhs env [] scrut bs rhs } + -- Case-of-known-constructor: scrutinee is a constructor application | Just (in_scope', wfloats, con, ty_args, other_args) <- exprIsConApp_maybe (getUnfoldingInRuleMatch env) scrut -- Works when the scrutinee is a variable with a known unfolding @@ -2803,8 +2804,16 @@ rebuildCase env scrut case_bndr alts cont ; case findAlt (DataAlt con) alts of Nothing -> missingAlt env0 case_bndr alts cont Just (Alt DEFAULT bs rhs) -> simple_rhs env0 scaled_wfloats case_bndr_rhs bs rhs - Just (Alt _ bs rhs) -> knownCon env0 scrut scaled_wfloats con ty_args - other_args case_bndr bs rhs cont + Just (Alt _ bs rhs) + -- See Note [Avoiding register pressure due to + -- case-of-known-constructor]. + | Var v <- scrut + , isLocalId v + , filter (not . isDeadBinder) bs `lengthAtLeast` 10 + -> reallyRebuildCase env scrut case_bndr alts cont + | otherwise + -> knownCon env0 scrut scaled_wfloats con ty_args + other_args case_bndr bs rhs cont } where simple_rhs env wfloats case_bndr_rhs bs rhs = @@ -3321,6 +3330,20 @@ and then f (h v) All this should happen in one sweep. + + +Note [Avoiding register pressure due to case-of-known-constructor] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In #17615 we noticed that case-of-known-constructor can produce quite extreme +register pressure with little benefit. For instance, consider the following +program scrutinizing a wide product constructor K: + + case x of { K _ _ x3 ... _ _ -> + <lots of stuff happens here, binding new_x3> + case x of { K x1 x2 x3 ... x99 x100 -> + K x1 x2 new_x3 ... x99 x100 } } + +The problem here is that the inner case on `x` can be eliminated. -} knownCon :: SimplEnv |