[commit: ghc] master: Improve exprIsBottom (0579fe9)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/0579fe99b933384172d19beb6a00dc8a1238101a/ghc

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

commit 0579fe99b933384172d19beb6a00dc8a1238101a
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Dec 31 09:07:33 2015 +0000

    Improve exprIsBottom
    
    This fixes Trac #11290, by being sligthtly cleverer about finding
    what expressions are bottom.  Actually this might have minor
    other side benefits.


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

0579fe99b933384172d19beb6a00dc8a1238101a
 compiler/coreSyn/CoreUtils.hs                 |  2 ++
 compiler/simplCore/SetLevels.hs               | 13 ++++++++++++-
 testsuite/tests/deriving/should_compile/all.T |  3 ++-
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/compiler/coreSyn/CoreUtils.hs b/compiler/coreSyn/CoreUtils.hs
index f31eac6..d89612a 100644
--- a/compiler/coreSyn/CoreUtils.hs
+++ b/compiler/coreSyn/CoreUtils.hs
@@ -810,6 +810,8 @@ exprIsBottom e
     go n (Cast e _)              = go n e
     go n (Let _ e)               = go n e
     go n (Lam v e) | isTyVar v   = go n e
+    go _ (Case _ _ _ alts)       = null alts
+       -- See Note [Empty case alternatives] in CoreSyn
     go _ _                       = False
 
 {- Note [Bottoming expressions]
diff --git a/compiler/simplCore/SetLevels.hs b/compiler/simplCore/SetLevels.hs
index b742a29..b84d67b 100644
--- a/compiler/simplCore/SetLevels.hs
+++ b/compiler/simplCore/SetLevels.hs
@@ -542,7 +542,7 @@ See Maessen's paper 1999 "Bottom extraction: factoring error handling out
 of functional programs" (unpublished I think).
 
 When we do this, we set the strictness and arity of the new bottoming
-Id, *immediately*, for two reasons:
+Id, *immediately*, for three reasons:
 
   * To prevent the abstracted thing being immediately inlined back in again
     via preInlineUnconditionally.  The latter has a test for bottoming Ids
@@ -551,6 +551,17 @@ Id, *immediately*, for two reasons:
   * So that it's properly exposed as such in the interface file, even if
     this is all happening after strictness analysis.
 
+  * In case we do CSE with the same expression that *is* marked bottom
+        lvl          = error "urk"
+          x{str=bot) = error "urk"
+    Here we don't want to replace 'x' with 'lvl', else we may get Lint
+    errors, e.g. via a case with empty alternatives:  (case x of {})
+    Lint complains unless the scrutinee of such a case is clearly bottom.
+
+    This was reported in Trac #11290.   But since the whole bottoming-float
+    thing is based on the cheap-and-cheerful exprIsBottom, I'm not sure
+    that it'll nail all such cases.
+
 Note [Bottoming floats: eta expansion] c.f Note [Bottoming floats]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Tiresomely, though, the simplifier has an invariant that the manifest
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 36d49f5..ff26d37 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -31,7 +31,8 @@ test('T4966', normal, compile, [''])
 test('drv-functor1', normal, compile, [''])
 test('drv-functor2', normal, compile, [''])
 test('drv-foldable-traversable1', normal, compile, [''])
-test('T6031', extra_clean(['T6031a.o', 'T6031a.hi']), multimod_compile, ['T6031', '-v0'])
+test('T6031', extra_clean(['T6031a.o', 'T6031a.hi']), multimod_compile, ['T6031', '-v0 -O'])
+# Adding -O on T6031 to expose Trac #11245 regardless of way
 test('T1133',
      extra_clean(['T1133.o-boot', 'T1133.hi-boot', 'T1133a.o', 'T1133a.hi']),
      run_command,