[project @ 2002-06-28 14:06:52 by simonpj]

-----------------------------------
	Fix the CAF info field of error Ids
	-----------------------------------

A bizarre bug.   In MkId, we build the Id for various error-y
Ids (like pAT_ERROR_ID) that we grab out of thin air in various
places (like the desugarer).  They were marked as not referring
to any CAFs, but this was a lie!  In fact, they refer to 'untangle'
(see GHC.Err) and thence to a CAF.

Result: GC crash under very obscure circumstances.  (Rob's optimistic
evaluator tickled it.)

Solution: give them more conservative IdInfo.

Two other better solutions to think about:

* Don't grab them out of thin air; instead get them from
  an interface file.

* Treat them as always-live (requires mod to garbage collector)
  so they don't need to be mentioned in SRTs at all

diff --git a/ghc/compiler/basicTypes/IdInfo.lhs b/ghc/compiler/basicTypes/IdInfo.lhs
index 18980bc..a251c7e 100644 (file)
--- a/ghc/compiler/basicTypes/IdInfo.lhs
+++ b/ghc/compiler/basicTypes/IdInfo.lhs
@@ -11,7 +11,7 @@ module IdInfo (
        GlobalIdDetails(..), notGlobalId,       -- Not abstract
 
        IdInfo,         -- Abstract
-       vanillaIdInfo, noCafIdInfo,
+       vanillaIdInfo, noCafIdInfo, hasCafIdInfo,
        seqIdInfo, megaSeqIdInfo,
 
        -- Zapping
@@ -396,10 +396,11 @@ vanillaIdInfo
            newStrictnessInfo   = Nothing
           }
 
-noCafIdInfo = vanillaIdInfo `setCgInfo`    CgInfo NoCafRefs
+hasCafIdInfo = vanillaIdInfo `setCgInfo`    CgInfo MayHaveCafRefs
+noCafIdInfo  = vanillaIdInfo `setCgInfo`    CgInfo NoCafRefs
        -- Used for built-in type Ids in MkId.
-       -- Many built-in things have fixed types, so we shouldn't
-       -- run around generalising them
+       -- These must have a valid CgInfo set, so you can't
+       --      use vanillaIdInfo!
 \end{code}
 
 

diff --git a/ghc/compiler/basicTypes/MkId.lhs b/ghc/compiler/basicTypes/MkId.lhs
index d8fab3c..954ada9 100644 (file)
--- a/ghc/compiler/basicTypes/MkId.lhs
+++ b/ghc/compiler/basicTypes/MkId.lhs
@@ -74,7 +74,7 @@ import Id             ( idType, mkGlobalId, mkVanillaGlobal, mkSysLocal,
                          mkTemplateLocals, mkTemplateLocalsNum,
                          mkTemplateLocal, idNewStrictness, idName
                        )
-import IdInfo          ( IdInfo, noCafIdInfo,
+import IdInfo          ( IdInfo, noCafIdInfo, hasCafIdInfo,
                          setUnfoldingInfo, 
                          setArityInfo, setSpecInfo, setCafInfo,
                          setAllStrictnessInfo,
@@ -483,7 +483,7 @@ mkRecordSelId tycon field_label
     default_alt | no_default = []
                | otherwise  = [(DEFAULT, [], error_expr)]
 
-       -- the default branch may have CAF refs, because it calls recSelError etc.
+       -- The default branch may have CAF refs, because it calls recSelError etc.
     caf_info    | no_default = NoCafRefs
                | otherwise  = MayHaveCafRefs
 
@@ -985,9 +985,18 @@ pcMiscPrelId key mod str ty info
 pc_bottoming_Id key mod name ty
  = pcMiscPrelId key mod name ty bottoming_info
  where
+    bottoming_info = hasCafIdInfo `setAllStrictnessInfo` Just strict_sig
+       -- Do *not* mark them as NoCafRefs, because they can indeed have
+       -- CAF refs.  For example, pAT_ERROR_ID calls GHC.Err.untangle,
+       -- which has some CAFs
+       -- In due course we may arrange that these error-y things are
+       -- regarded by the GC as permanently live, in which case we
+       -- can give them NoCaf info.  As it is, any function that calls
+       -- any pc_bottoming_Id will itself have CafRefs, which bloats
+       -- SRTs.
+
     strict_sig    = mkStrictSig (mkTopDmdType [evalDmd] BotRes)
-    bottoming_info = noCafIdInfo `setAllStrictnessInfo` Just strict_sig
-       -- these "bottom" out, no matter what their arguments
+       -- These "bottom" out, no matter what their arguments
 
 (openAlphaTyVar:openBetaTyVar:_) = openAlphaTyVars
 openAlphaTy  = mkTyVarTy openAlphaTyVar