Disable the SRT offset optimisation on MachO platforms

Unfortunately, this optimisation is infeasible on MachO platforms (e.g.
Darwin) due to an object format limitation. Specifically, linking fails
with errors of the form:

     error: unsupported relocation with subtraction expression, symbol
     '_integerzmgmp_GHCziIntegerziType_quotInteger_closure' can not be
     undefined in a subtraction expression

Apparently MachO does not permit relocations' subtraction expressions to
refer to undefined symbols. As far as I can tell this means that it is
essentially impossible to express an offset between symbols living in
different compilation units. This means that we lively can't use this
optimisation on MachO platforms.

Test Plan: Validate on Darwin

Reviewers: simonmar, erikd

Subscribers: rwbarton, thomie, carter, angerman

GHC Trac Issues: #15169

Differential Revision: https://phabricator.haskell.org/D4715

1 files changed, 20 insertions, 2 deletions

diff --git a/compiler/cmm/CLabel.hs b/compiler/cmm/CLabel.hs
index 3dfd7a7d1d..8f614ab34f 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,
@@ -975,13 +977,29 @@ 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