[commit: ghc] master: Add a comment about why we use nextWrapperNum rather than a unique (1a0af0e)

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/1a0af0ece368caf0b6752f05e5e571cd4137ef36

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

commit 1a0af0ece368caf0b6752f05e5e571cd4137ef36
Author: Ian Lynagh <[email protected]>
Date:   Thu Nov 8 12:27:03 2012 +0000

    Add a comment about why we use nextWrapperNum rather than a unique

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

 compiler/typecheck/TcEnv.lhs |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index aa39673..934a9fb 100644
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -772,13 +772,25 @@ mkWrapperName :: (MonadIO m, HasDynFlags m, HasModule m)
 mkWrapperName what nameBase
     = do dflags <- getDynFlags
          thisMod <- getModule
-         let wrapperRef = nextWrapperNum dflags
+         let -- Note [Generating fresh names for ccall wrapper]
+             wrapperRef = nextWrapperNum dflags
              pkg = packageIdString  (modulePackageId thisMod)
              mod = moduleNameString (moduleName      thisMod)
          wrapperNum <- liftIO $ readIORef wrapperRef
          liftIO $ writeIORef wrapperRef (wrapperNum + 1)
          let components = [what, show wrapperNum, pkg, mod, nameBase]
          return $ mkFastString $ zEncodeString $ intercalate ":" components
+
+{-
+Note [Generating fresh names for FFI wrappers]
+
+We used to use a unique, rather than nextWrapperNum, to distinguish
+between FFI wrapper functions. However, the wrapper names that we
+generate are external names. This means that if a call to them ends up
+in an unfolding, then we can't alpha-rename them, and thus if the
+unique randomly changes from one compile to another then we get a
+spurious ABI change (#4012).
+-}
 \end{code}
 
 %************************************************************************