[commit: ghc] ghc-8.6: Revert "Revert "Disable the SRT offset optimisation on MachO platforms"" (f442bc6)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-8.6
Link       : http://ghc.haskell.org/trac/ghc/changeset/f442bc6c966e85f845a8167ab97ab71d73cd8520/ghc

>---------------------------------------------------------------

commit f442bc6c966e85f845a8167ab97ab71d73cd8520
Author: Ben Gamari <ben at smart-cactus.org>
Date:   Tue Sep 18 11:45:27 2018 -0400

    Revert "Revert "Disable the SRT offset optimisation on MachO platforms""
    
    This reverts commit c15d44f8b3f00bfe152c2f9d3c6f60efd204fb23.


>---------------------------------------------------------------

f442bc6c966e85f845a8167ab97ab71d73cd8520
 compiler/cmm/CLabel.hs             | 22 ++++++++++++++++++++--
 compiler/cmm/CmmBuildInfoTables.hs | 18 ++++++++++++++++--
 compiler/cmm/CmmInfo.hs            |  5 ++++-
 includes/rts/storage/InfoTables.h  |  3 +++
 4 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/compiler/cmm/CLabel.hs b/compiler/cmm/CLabel.hs
index 472bd3c..1a9bc73 100644
--- a/compiler/cmm/CLabel.hs
+++ b/compiler/cmm/CLabel.hs
@@ -94,10 +94,12 @@ module CLabel (
 
         mkHpcTicksLabel,
 
+        -- * Predicates
         hasCAF,
         needsCDecl, maybeLocalBlockLabel, externallyVisibleCLabel,
         isMathFun,
         isCFunctionLabel, isGcPtrLabel, labelDynamic,
+        isLocalCLabel,
 
         -- * Conversions
         toClosureLbl, toSlowEntryLbl, toEntryLbl, toInfoLbl, hasHaskellName,
@@ -974,13 +976,29 @@ idInfoLabelType info =
 
 
 -- -----------------------------------------------------------------------------
--- Does a CLabel need dynamic linkage?
 
+-- | Is a 'CLabel' defined in the current module being compiled?
+--
+-- Sometimes we can optimise references within a compilation unit in ways that
+-- we couldn't for inter-module references. This provides a conservative
+-- estimate of whether a 'CLabel' lives in the current module.
+isLocalCLabel :: Module -> CLabel -> Bool
+isLocalCLabel this_mod lbl =
+  case lbl of
+    IdLabel name _ _
+      | isInternalName name -> True
+      | otherwise           -> nameModule name == this_mod
+    LocalBlockLabel _       -> True
+    _                       -> False
+
+-- -----------------------------------------------------------------------------
+
+-- | Does a 'CLabel' need dynamic linkage?
+--
 -- When referring to data in code, we need to know whether
 -- that data resides in a DLL or not. [Win32 only.]
 -- @labelDynamic@ returns @True@ if the label is located
 -- in a DLL, be it a data reference or not.
-
 labelDynamic :: DynFlags -> Module -> CLabel -> Bool
 labelDynamic dflags this_mod lbl =
   case lbl of
diff --git a/compiler/cmm/CmmBuildInfoTables.hs b/compiler/cmm/CmmBuildInfoTables.hs
index ecbe89d..3d13fc7 100644
--- a/compiler/cmm/CmmBuildInfoTables.hs
+++ b/compiler/cmm/CmmBuildInfoTables.hs
@@ -16,6 +16,7 @@ import Hoopl.Label
 import Hoopl.Collections
 import Hoopl.Dataflow
 import Module
+import Platform
 import Digraph
 import CLabel
 import PprCmmDecl ()
@@ -120,7 +121,7 @@ offset to the SRT can be stored in 32 bits (all code lives within a
 the info table by storing the srt_offset in the srt field, which is
 half a word.
 
-On x86_64 with TABLES_NEXT_TO_CODE:
+On x86_64 with TABLES_NEXT_TO_CODE (except on MachO, due to #15169):
 
 - info->srt is zero if there's no SRT, otherwise:
 - info->srt is an offset from the info pointer to the SRT object
@@ -636,14 +637,27 @@ oneSRT dflags staticFuns blockids lbls isCAF cafs = do
         let newSRTMap = Map.fromList [(cafLbl, srtEntry) | cafLbl <- lbls]
         put (Map.union newSRTMap srtMap)
 
+    this_mod = thisModule topSRT
+
   case Set.toList filtered of
     [] -> do
       srtTrace "oneSRT: empty" (ppr lbls) $ return ()
       updateSRTMap Nothing
       return ([], [], [])
 
+    -- When we have only one entry there is no need to build a new SRT at all.
     [one@(SRTEntry lbl)]
-      | not (labelDynamic dflags (thisModule topSRT) lbl) -> do
+      | -- Info tables refer to SRTs by offset (as noted in the section
+        -- "Referring to an SRT from the info table" of Note [SRTs]). However,
+        -- when dynamic linking is used we cannot guarantee that the offset
+        -- between the SRT and the info table will fit in the offset field.
+        -- Consequently we build a singleton SRT in in this case.
+        not (labelDynamic dflags this_mod lbl)
+
+        -- MachO relocations can't express offsets between compilation units at
+        -- all, so we are always forced to build a singleton SRT in this case.
+          && (not (osMachOTarget $ platformOS $ targetPlatform dflags)
+             || isLocalCLabel this_mod lbl) -> do
         updateSRTMap (Just one)
         return ([], map (,lbl) blockids, [])
 
diff --git a/compiler/cmm/CmmInfo.hs b/compiler/cmm/CmmInfo.hs
index 3b2eea1..43cba25 100644
--- a/compiler/cmm/CmmInfo.hs
+++ b/compiler/cmm/CmmInfo.hs
@@ -271,7 +271,10 @@ mkSRTLit dflags _ Nothing    = ([], CmmInt 0 (halfWordWidth dflags))
 mkSRTLit dflags _ (Just lbl) = ([CmmLabel lbl], CmmInt 1 (halfWordWidth dflags))
 
 
--- | is the SRT offset field inline in the info table on this platform?
+-- | Is the SRT offset field inline in the info table on this platform?
+--
+-- See the section "Referring to an SRT from the info table" in
+-- Note [SRTs] in CmmBuildInfoTables.hs
 inlineSRT :: DynFlags -> Bool
 inlineSRT dflags = platformArch (targetPlatform dflags) == ArchX86_64
   && tablesNextToCode dflags
diff --git a/includes/rts/storage/InfoTables.h b/includes/rts/storage/InfoTables.h
index 137cfe2..db50d16 100644
--- a/includes/rts/storage/InfoTables.h
+++ b/includes/rts/storage/InfoTables.h
@@ -156,6 +156,9 @@ typedef union {
 #if defined(x86_64_TARGET_ARCH) && defined(TABLES_NEXT_TO_CODE)
 // On x86_64 we can fit a pointer offset in half a word, so put the SRT offset
 // in the info->srt field directly.
+//
+// See the section "Referring to an SRT from the info table" in
+// Note [SRTs] in CmmBuildInfoTables.hs
 #define USE_INLINE_SRT_FIELD
 #endif