Don't refer to blocks in debug info when -g1

-g1 removes block information, but it turns out that procs can
refer to block information through parents.
Note [Splitting DebugBlocks] explains the parentage relationship.

Test Plan:
* ./validate
* added a new test

Reviewers: bgamari, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14894

Differential Revision: https://phabricator.haskell.org/D4496

(cherry picked from commit 0cbb13b3)

compiler/nativeGen/Dwarf.hs

@@ -182,10 +182,17 @@ procToDwarf df prc
                          _otherwise -> showSDocDump df $ ppr $ dblLabel prc
                     , dwLabel    = dblCLabel prc
                     , dwParent   = fmap mkAsmTempDieLabel
-                                   $ mfilter (/= dblCLabel prc)
+                                   $ mfilter goodParent
                                    $ fmap dblCLabel (dblParent prc)
-                      -- Omit parent if it would be self-referential
                     }
+  where
+  goodParent a | a == dblCLabel prc = False
+               -- Omit parent if it would be self-referential
+  goodParent a | not (externallyVisibleCLabel a)
+               , debugLevel df < 2 = False
+               -- We strip block information when running -g0 or -g1, don't
+               -- refer to blocks in that case. Fixes #14894.
+  goodParent _ = True
 
 -- | Generate DWARF info for a block
 blockToDwarf :: DynFlags -> DebugBlock -> DwarfInfo

testsuite/tests/simplCore/should_run/T14894.hs

+{-# OPTIONS_GHC -g1 -O #-}
+import System.Environment
+summap :: (Int -> Int) -> (Int -> Int)
+summap f n = f 10
+{-# NOINLINE summap #-}
+
+main = do
+  n <- length `fmap` getArgs
+  print $ summap (+ n) n

testsuite/tests/simplCore/should_run/T14894.stdout

+10

testsuite/tests/simplCore/should_run/all.T

@@ -82,3 +82,4 @@ test('T14768', reqlib('vector'), compile_and_run, [''])
 test('T14868',
      [when((arch('powerpc64') or arch('powerpc64le')), expect_broken(11261))],
      compile_and_run, [''])
+test('T14894', normal, compile_and_run, [''])