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

1 files changed, 10 insertions, 1 deletions

diff --git a/compiler/llvmGen/LlvmMangler.hs b/compiler/llvmGen/LlvmMangler.hs
index acf344fe2d..eed13ba203 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