[commit: ghc] master: Mangle .subsections_via_symbols away. (1686f30)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/1686f30951292e94bf3076ce8b3eafb0bcbba91d/ghc

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

commit 1686f30951292e94bf3076ce8b3eafb0bcbba91d
Author: Moritz Angermann <moritz.angermann at gmail.com>
Date:   Mon Mar 6 17:31:04 2017 -0500

    Mangle .subsections_via_symbols away.
    
    This is a rather stupid mangler hack. However, when using prefix data
    with llvm, on systems that support -dead_strip (macOS, iOS), the prefix
    data is stipped.
    
    llvm generiously adds .subsections_via_symbols for macho in any case.
    
    Thus we use our trusted mangler to drop the .subsections_via_symbols
    line from the assembly.
    
    This ultimately means that for (macOS, llvm), and (iOS, llvm) will not
    benefit from -dead_strip. Yet, this patch will allow building ghc on
    macOS again.
    
    Test Plan: build ghc with llvm on macOS
    
    Reviewers: rwbarton, bgamari, austin
    
    Reviewed By: rwbarton, bgamari
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D3287


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

1686f30951292e94bf3076ce8b3eafb0bcbba91d
 compiler/llvmGen/LlvmMangler.hs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/compiler/llvmGen/LlvmMangler.hs b/compiler/llvmGen/LlvmMangler.hs
index acf344f..eed13ba 100644
--- a/compiler/llvmGen/LlvmMangler.hs
+++ b/compiler/llvmGen/LlvmMangler.hs
@@ -47,11 +47,20 @@ type Rewrite = DynFlags -> B.ByteString -> Maybe B.ByteString
 -- | Rewrite a line of assembly source with the given rewrites,
 -- taking the first rewrite that applies.
 rewriteLine :: DynFlags -> [Rewrite] -> B.ByteString -> B.ByteString
-rewriteLine dflags rewrites l =
+rewriteLine dflags rewrites l
+  -- We disable .subsections_via_symbols on darwin and ios, as the llvm code
+  -- gen uses prefix data for the info table.  This however does not prevent
+  -- llvm from generating .subsections_via_symbols, which in turn with
+  -- -dead_strip, strips the info tables, and therefore breaks ghc.
+  | isSubsectionsViaSymbols l =
+    (B.pack "## no .subsection_via_symbols for ghc. We need our info tables!")
+  | otherwise =
     case firstJust $ map (\rewrite -> rewrite dflags rest) rewrites of
       Nothing        -> l
       Just rewritten -> B.concat $ [symbol, B.pack "\t", rewritten]
   where
+    isSubsectionsViaSymbols = B.isPrefixOf (B.pack ".subsections_via_symbols")
+
     (symbol, rest) = splitLine l
 
     firstJust :: [Maybe a] -> Maybe a