Finish stable split

Long ago, the stable name table and stable pointer tables were one.
Now, they are separate, and have significantly different
implementations. I believe the time has come to finish the split
that began in #7674.

* Divide `rts/Stable` into `rts/StableName` and `rts/StablePtr`.

* Give each table its own mutex.

* Add FFI functions `hs_lock_stable_ptr_table` and
`hs_unlock_stable_ptr_table` and document them.
  These are intended to replace the previously undocumented
`hs_lock_stable_tables` and `hs_lock_stable_tables`,
  which are now documented as deprecated synonyms.

* Make `eqStableName#` use pointer equality instead of unnecessarily
comparing stable name table indices.

Reviewers: simonmar, bgamari, erikd

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15555

Differential Revision: https://phabricator.haskell.org/D5084

1 files changed, 13 insertions, 8 deletions

diff --git a/compiler/codeGen/StgCmmPrim.hs b/compiler/codeGen/StgCmmPrim.hs
index a6e13086aa..266ab3a0f6 100644
--- a/compiler/codeGen/StgCmmPrim.hs
+++ b/compiler/codeGen/StgCmmPrim.hs
@@ -352,14 +352,6 @@ emitPrimOp dflags [res] ByteArrayContents_Char [arg]
 emitPrimOp dflags [res] StableNameToIntOp [arg]
    = emitAssign (CmmLocal res) (cmmLoadIndexW dflags arg (fixedHdrSizeW dflags) (bWord dflags))
 
---  #define eqStableNamezh(r,sn1,sn2)                                   \
---    (r = (((StgStableName *)sn1)->sn == ((StgStableName *)sn2)->sn))
-emitPrimOp dflags [res] EqStableNameOp [arg1,arg2]
-   = emitAssign (CmmLocal res) (CmmMachOp (mo_wordEq dflags) [
-                                   cmmLoadIndexW dflags arg1 (fixedHdrSizeW dflags) (bWord dflags),
-                                   cmmLoadIndexW dflags arg2 (fixedHdrSizeW dflags) (bWord dflags)
-                         ])
-
 emitPrimOp dflags [res] ReallyUnsafePtrEqualityOp [arg1,arg2]
    = emitAssign (CmmLocal res) (CmmMachOp (mo_wordEq dflags) [arg1,arg2])
 
@@ -1405,9 +1397,22 @@ translateOp dflags SameMutableArrayArrayOp= Just (mo_wordEq dflags)
 translateOp dflags SameSmallMutableArrayOp= Just (mo_wordEq dflags)
 translateOp dflags SameTVarOp             = Just (mo_wordEq dflags)
 translateOp dflags EqStablePtrOp          = Just (mo_wordEq dflags)
+-- See Note [Comparing stable names]
+translateOp dflags EqStableNameOp         = Just (mo_wordEq dflags)
 
 translateOp _      _ = Nothing
 
+-- Note [Comparing stable names]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+--
+-- A StableName# is actually a pointer to a stable name object (SNO)
+-- containing an index into the stable name table (SNT). We
+-- used to compare StableName#s by following the pointers to the
+-- SNOs and checking whether they held the same SNT indices. However,
+-- this is not necessary: there is a one-to-one correspondence
+-- between SNOs and entries in the SNT, so simple pointer equality
+-- does the trick.
+
 -- These primops are implemented by CallishMachOps, because they sometimes
 -- turn into foreign calls depending on the backend.