[commit: ghc] master: Introduce -fcatch-bottoms (8e51bfc)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/8e51bfc33c17aef41677a2b6189e3d4f31454cbc/ghc

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

commit 8e51bfc33c17aef41677a2b6189e3d4f31454cbc
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Wed Jul 19 22:31:16 2017 -0400

    Introduce -fcatch-bottoms
    
    This flag instructs the simplifier to emit ``error`` expressions in the
    continutation of empty case analyses (which should bottom and
    consequently not return). This is helpful when debugging demand analysis
    bugs which can sometimes manifest as segmentation faults.
    
    Test Plan: Validate
    
    Reviewers: simonpj, austin
    
    Subscribers: niteria, rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3736


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

8e51bfc33c17aef41677a2b6189e3d4f31454cbc
 compiler/coreSyn/CorePrep.hs                       | 17 +++++++++++++++--
 compiler/coreSyn/CoreUtils.hs                      |  1 +
 compiler/main/DynFlags.hs                          |  2 ++
 docs/users_guide/debugging.rst                     |  7 +++++++
 utils/mkUserGuidePart/Options/CompilerDebugging.hs |  6 ++++++
 5 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/compiler/coreSyn/CorePrep.hs b/compiler/coreSyn/CorePrep.hs
index 5327acd..4f7a0da 100644
--- a/compiler/coreSyn/CorePrep.hs
+++ b/compiler/coreSyn/CorePrep.hs
@@ -645,8 +645,21 @@ cpeRhsE env (Case scrut bndr ty alts)
        ; let bndr1 = bndr `setIdUnfolding` evaldUnfolding
             -- Record that the case binder is evaluated in the alternatives
        ; (env', bndr2) <- cpCloneBndr env bndr1
-       ; alts' <- mapM (sat_alt env') alts
-       ; return (floats, Case scrut' bndr2 ty alts') }
+       ; let alts'
+                 -- This flag is intended to aid in debugging strictness
+                 -- analysis bugs. These are particularly nasty to chase down as
+                 -- they may manifest as segmentation faults. When this flag is
+                 -- enabled we instead produce an 'error' expression to catch
+                 -- the case where a function we think should bottom
+                 -- unexpectedly returns.
+               | gopt Opt_CatchBottoms (cpe_dynFlags env)
+               , not (altsAreExhaustive alts)
+               = addDefault alts (Just err)
+               | otherwise = alts
+               where err = mkRuntimeErrorApp rUNTIME_ERROR_ID ty
+                                             "Bottoming expression returned"
+       ; alts'' <- mapM (sat_alt env') alts'
+       ; return (floats, Case scrut' bndr2 ty alts'') }
   where
     sat_alt env (con, bs, rhs)
        = do { (env2, bs') <- cpCloneBndrs env bs
diff --git a/compiler/coreSyn/CoreUtils.hs b/compiler/coreSyn/CoreUtils.hs
index eec524f..540a36e 100644
--- a/compiler/coreSyn/CoreUtils.hs
+++ b/compiler/coreSyn/CoreUtils.hs
@@ -30,6 +30,7 @@ module CoreUtils (
         exprIsBig, exprIsConLike,
         rhsIsStatic, isCheapApp, isExpandableApp,
         exprIsLiteralString, exprIsTopLevelBindable,
+        altsAreExhaustive,
 
         -- * Equality
         cheapEqExpr, cheapEqExpr', eqExpr,
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 2be121e..5e33c2e 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -470,6 +470,7 @@ data GeneralFlag
    | Opt_CprAnal
    | Opt_WorkerWrapper
    | Opt_SolveConstantDicts
+   | Opt_CatchBottoms
 
    -- Interface files
    | Opt_IgnoreInterfacePragmas
@@ -3778,6 +3779,7 @@ fFlagsDeps = [
   flagSpec "version-macros"                   Opt_VersionMacros,
   flagSpec "worker-wrapper"                   Opt_WorkerWrapper,
   flagSpec "solve-constant-dicts"             Opt_SolveConstantDicts,
+  flagSpec "catch-bottoms"                    Opt_CatchBottoms,
   flagSpec "show-warning-groups"              Opt_ShowWarnGroups,
   flagSpec "hide-source-paths"                Opt_HideSourcePaths,
   flagSpec "show-hole-constraints"            Opt_ShowHoleConstraints,
diff --git a/docs/users_guide/debugging.rst b/docs/users_guide/debugging.rst
index fd4adc7..af937ae 100644
--- a/docs/users_guide/debugging.rst
+++ b/docs/users_guide/debugging.rst
@@ -389,6 +389,13 @@ Checking for consistency
     instead of ``undef`` in calls. This makes it easier to catch subtle
     code generator and runtime system bugs (e.g. see :ghc-ticket:`11487`).
 
+.. ghc-flag:: -fcatch-bottoms
+
+    Instructs the simplifier to emit ``error`` expressions in the continuation
+    of empty case analyses (which should bottom and consequently not return).
+    This is helpful when debugging demand analysis bugs which can sometimes
+    manifest as segmentation faults.
+
 .. _checking-determinism:
 
 Checking for determinism
diff --git a/utils/mkUserGuidePart/Options/CompilerDebugging.hs b/utils/mkUserGuidePart/Options/CompilerDebugging.hs
index 9704020..e68216b 100644
--- a/utils/mkUserGuidePart/Options/CompilerDebugging.hs
+++ b/utils/mkUserGuidePart/Options/CompilerDebugging.hs
@@ -278,4 +278,10 @@ compilerDebuggingOptions =
            "Takes a string argument."
          , flagType = DynamicFlag
          }
+  , flag { flagName = "-fcatch-bottoms"
+         , flagDescription =
+           "Insert ``error`` expressions after bottoming expressions; useful "++
+           "when debugging the compiler."
+         , flagType = DynamicFlag
+         }
   ]