nativeGen/dwarf: Only produce DW_AT_source_note DIEs in -g3

Standard debugging tools don't know how to understand these so let's not
produce them unless asked.

4 files changed, 16 insertions, 8 deletions

diff --git a/compiler/GHC/CmmToAsm.hs b/compiler/GHC/CmmToAsm.hs
index 223eb33fb5..013ad24554 100644
--- a/compiler/GHC/CmmToAsm.hs
+++ b/compiler/GHC/CmmToAsm.hs
@@ -1187,9 +1187,10 @@ initNCGConfig dflags this_mod = NCGConfig
             ArchX86    -> v
             _          -> Nothing
 
-   , ncgDwarfEnabled        = debugLevel dflags > 0
+   , ncgDwarfEnabled        = debugLevel dflags >  0
    , ncgDwarfUnwindings     = debugLevel dflags >= 1
-   , ncgDwarfStripBlockInfo = debugLevel dflags < 2 -- We strip out block information when running with -g0 or -g1.
    , ncgExposeInternalSymbols = gopt Opt_ExposeInternalSymbols dflags
+   , ncgDwarfStripBlockInfo = debugLevel dflags <  2 -- We strip out block information when running with -g0 or -g1.
+   , ncgDwarfSourceNotes    = debugLevel dflags >= 3 -- We produce GHC-specific source-note DIEs only with -g3
    }
 
diff --git a/compiler/GHC/CmmToAsm/Config.hs b/compiler/GHC/CmmToAsm/Config.hs
index 141f1168be..8acd089757 100644
--- a/compiler/GHC/CmmToAsm/Config.hs
+++ b/compiler/GHC/CmmToAsm/Config.hs
@@ -40,6 +40,7 @@ data NCGConfig = NCGConfig
    , ncgDwarfUnwindings       :: !Bool            -- ^ Enable unwindings
    , ncgDwarfStripBlockInfo   :: !Bool            -- ^ Strip out block information from generated Dwarf
    , ncgExposeInternalSymbols :: !Bool            -- ^ Expose symbol table entries for internal symbols
+   , ncgDwarfSourceNotes      :: !Bool            -- ^ Enable GHC-specific source note DIEs
    }
 
 -- | Return Word size
diff --git a/compiler/GHC/CmmToAsm/Dwarf.hs b/compiler/GHC/CmmToAsm/Dwarf.hs
index d81bcd4f2e..7e03549b24 100644
--- a/compiler/GHC/CmmToAsm/Dwarf.hs
+++ b/compiler/GHC/CmmToAsm/Dwarf.hs
@@ -176,7 +176,7 @@ parent, B.
 -- | Generate DWARF info for a procedure debug block
 procToDwarf :: NCGConfig -> DebugBlock -> DwarfInfo
 procToDwarf config prc
-  = DwarfSubprogram { dwChildren = map blockToDwarf (dblBlocks prc)
+  = DwarfSubprogram { dwChildren = map (blockToDwarf config) (dblBlocks prc)
                     , dwName     = case dblSourceTick prc of
                          Just s@SourceNote{} -> sourceName s
                          _otherwise -> show (dblLabel prc)
@@ -195,14 +195,17 @@ procToDwarf config prc
   goodParent _ = True
 
 -- | Generate DWARF info for a block
-blockToDwarf :: DebugBlock -> DwarfInfo
-blockToDwarf blk
-  = DwarfBlock { dwChildren = concatMap tickToDwarf (dblTicks blk)
-                              ++ map blockToDwarf (dblBlocks blk)
+blockToDwarf :: NCGConfig -> DebugBlock -> DwarfInfo
+blockToDwarf config blk
+  = DwarfBlock { dwChildren = map (blockToDwarf config) (dblBlocks blk) ++ srcNotes
                , dwLabel    = dblCLabel blk
                , dwMarker   = marker
                }
   where
+    srcNotes
+      | ncgDwarfSourceNotes config = concatMap tickToDwarf (dblTicks blk)
+      | otherwise                  = []
+
     marker
       | Just _ <- dblPosition blk = Just $ mkAsmTempLabel $ dblLabel blk
       | otherwise                 = Nothing   -- block was optimized out
diff --git a/docs/users_guide/debug-info.rst b/docs/users_guide/debug-info.rst
index b1636bb81f..42afa9078f 100644
--- a/docs/users_guide/debug-info.rst
+++ b/docs/users_guide/debug-info.rst
@@ -24,7 +24,9 @@ useable by most UNIX debugging tools.
      * ``-g1``: produces stack unwinding records for top-level functions (sufficient for basic backtraces)
      * ``-g2``: produces stack unwinding records for top-level functions as well
        as inner blocks (allowing more precise backtraces than with ``-g1``).
-     * ``-g3``: same as ``-g2``.
+     * ``-g3``: produces GHC-specific DWARF information for use by more
+       sophisticated Haskell-aware debugging tools (see :ref:`dwarf-dies` for
+       details)
 
     If ⟨n⟩ is omitted, level 2 is assumed.
 
@@ -266,6 +268,7 @@ In particular GHC produces the following DWARF sections,
 ``.debug_arange``
   Address range information necessary for efficient lookup in debug information.
 
+.. _dwarf_dies:
 
 Debugging information entities
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~