diff options
Diffstat (limited to 'compiler/simplCore/CSE.hs')
| -rw-r--r-- | compiler/simplCore/CSE.hs | 56 |
1 files changed, 49 insertions, 7 deletions
diff --git a/compiler/simplCore/CSE.hs b/compiler/simplCore/CSE.hs index 0758ce930a..35862aeabe 100644 --- a/compiler/simplCore/CSE.hs +++ b/compiler/simplCore/CSE.hs @@ -15,7 +15,7 @@ import GhcPrelude import CoreSubst import Var ( Var ) import VarEnv ( elemInScopeSet, mkInScopeSet ) -import Id ( Id, idType, isDeadBinder +import Id ( Id, idType, isDeadBinder, idHasRules , idInlineActivation, setInlineActivation , zapIdOccInfo, zapIdUsageInfo, idInlinePragma , isJoinId, isJoinId_maybe ) @@ -392,9 +392,15 @@ cse_bind toplevel env (in_id, in_rhs) out_id delayInlining :: TopLevelFlag -> Id -> Id -- Add a NOINLINE[2] if the Id doesn't have an INLNE pragma already +-- See Note [Delay inlining after CSE] delayInlining top_lvl bndr | isTopLevel top_lvl , isAlwaysActive (idInlineActivation bndr) + , idHasRules bndr -- Only if the Id has some RULES, + -- which might otherwise get lost + -- These rules are probably auto-generated specialisations, + -- since Ids with manual rules usually have manually-inserted + -- delayed inlining anyway = bndr `setInlineActivation` activeAfterInitial | otherwise = bndr @@ -494,13 +500,49 @@ a SPECIALISE pragma. Then CSE kicks in and notices that the RHSs of Now there is terrible danger that, in an importing module, we'll inline 'g' before we have a chance to run its specialisation! -Solution: during CSE, when adding a top-level - g = f -binding after a "hit" in the CSE cache, add a NOINLINE[2] activation -to it, to ensure it's not inlined right away. +Solution: during CSE, afer a "hit" in the CSE cache + * when adding a binding + g = f + * for a top-level function g + * and g has specialisation RULES +add a NOINLINE[2] activation to it, to ensure it's not inlined +right away. + +Notes: +* Why top level only? Because for nested bindings we are already past + phase 2 and will never return there. + +* Why "only if g has RULES"? Because there is no point in + doing this if there are no RULES; and other things being + equal it delays optimisation to delay inlining (#17409) + + +---- Historical note --- + +This patch is simpler and more direct than an earlier +version: + + commit 2110738b280543698407924a16ac92b6d804dc36 + Author: Simon Peyton Jones <simonpj@microsoft.com> + Date: Mon Jul 30 13:43:56 2018 +0100 + + Don't inline functions with RULES too early + +We had to revert this patch because it made GHC itself slower. + +Why? It delayed inlining of /all/ functions with RULES, and that was +very bad in TcFlatten.flatten_ty_con_app + +* It delayed inlining of liftM +* That delayed the unravelling of the recursion in some dictionary + bindings. +* That delayed some eta expansion, leaving + flatten_ty_con_app = \x y. let <stuff> in \z. blah +* That allowed the float-out pass to put sguff between + the \y and \z. +* And that permanently stopped eta expasion of the function, + even once <stuff> was simplified. -Why top level only? Because for nested bindings we are already past -phase 2 and will never return there. -} tryForCSE :: CSEnv -> InExpr -> OutExpr |