LlvmMangler: Be more selective when mangling object types

Summary:
We previously did a wholesale replace of `%function` to `%object` to
mangle object `.type` annotations. This is bad as it can end up
replacing appearances of `"%function"` in the user's code. We now look
for a proper `.type` keyword before performing the replacement.

Thanks to @rwbarton for pointing out the bug.

Test Plan:
Previously,

    $ echo 'main = putStrLn "@function"' > test.hs
    $ ghc -fllvm test.hs
    $ ./test
    @object

Now,
    $ echo 'main = putStrLn "@function"' > test.hs
    $ ghc -fllvm test.hs
    $ ./test
    @function

Reviewers: rwbarton, austin

Reviewed By: rwbarton, austin

Subscribers: phaskell, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D150

GHC Trac Issues: #9439

1 files changed, 14 insertions, 4 deletions

diff --git a/compiler/llvmGen/LlvmMangler.hs b/compiler/llvmGen/LlvmMangler.hs
index 7084a2e727..8652a890cf 100644
--- a/compiler/llvmGen/LlvmMangler.hs
+++ b/compiler/llvmGen/LlvmMangler.hs
@@ -58,13 +58,23 @@ llvmFixupAsm dflags f1 f2 = {-# SCC "llvm_mangler" #-} do
     hClose w
     return ()
 
+-- | This rewrites @.type@ annotations of function symbols to @%object@.
+-- This is done as the linker can relocate @%functions@ through the
+-- Procedure Linking Table (PLT). This is bad since we expect that the
+-- info table will appear directly before the symbol's location. In the
+-- case that the PLT is used, this will be not an info table but instead
+-- some random PLT garbage.
 rewriteSymType :: B.ByteString -> B.ByteString
 rewriteSymType s =
-    foldl (\s' (typeFunc,typeObj)->replace typeFunc typeObj s') s types
+    B.unlines $ map (rewrite '@' . rewrite '%') $ B.lines s
   where
-    types = [ (B.pack "@function", B.pack "@object")
-            , (B.pack "%function", B.pack "%object")
-            ]
+    rewrite :: Char -> B.ByteString -> B.ByteString
+    rewrite prefix x
+        | isType x = replace funcType objType x
+        | otherwise = x
+      where
+        funcType = prefix `B.cons` B.pack "function"
+        objType  = prefix `B.cons` B.pack "object"
 
 -- | Splits the file contents into its sections
 readSections :: Handle -> Handle -> IO [Section]