[Git][ghc/ghc][wip/loop-backtrace] Throw nontermination exceptions via `throw`

21 Jun 2026


      Ben Gamari pushed to branch wip/loop-backtrace at Glasgow Haskell Compiler / GHC


Commits:
b709d077 by Ben Gamari at 2026-06-21T17:01:08-04:00
Throw nontermination exceptions via `throw`

This ensures that the exception that results gets the usual backtrace
annotations.

This is a first step to #21878.

- - - - -


9 changed files:

- + changelog.d/builtin-exception-backtraces
- libraries/ghc-internal/include/RtsIfaceSymbols.h
- libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs
- rts/Prelude.h
- rts/RtsStartup.c
- rts/Schedule.c
- rts/include/rts/RtsToHsIface.h
- testsuite/tests/rts/LoopBacktrace.stderr
- testsuite/tests/rts/T2783.stderr


Changes:

=====================================
changelog.d/builtin-exception-backtraces
=====================================
@@ -0,0 +1,10 @@
+section: rts
+synopsis: Non-termination exceptions now have backtrace annotations
+issues: #21878
+mrs: !16158
+description: {
+  The `NonTermination` exception (manifesting in printed exception output as
+  `<<loop>>`) now include `Backtrace` `ExceptionAnnotations`, like exceptions
+  thrown from user-written Haskell.
+}
+


=====================================
libraries/ghc-internal/include/RtsIfaceSymbols.h
=====================================
@@ -20,7 +20,7 @@ CLOSURE(GHCziInternalziIOziException, blockedIndefinitelyOnSTM_closure)
 CLOSURE(GHCziInternalziIOziException, cannotCompactFunction_closure)
 CLOSURE(GHCziInternalziIOziException, cannotCompactPinned_closure)
 CLOSURE(GHCziInternalziIOziException, cannotCompactMutable_closure)
-CLOSURE(GHCziInternalziControlziExceptionziBase, nonTermination_closure)
+CLOSURE(GHCziInternalziControlziExceptionziBase, nonTerminationError_closure)
 CLOSURE(GHCziInternalziControlziExceptionziBase, nestedAtomically_closure)
 CLOSURE(GHCziInternalziControlziExceptionziBase, noMatchingContinuationPrompt_closure)
 #if defined(mingw32_HOST_OS)


=====================================
libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs
=====================================
@@ -108,7 +108,9 @@ module GHC.Internal.Control.Exception.Base (
         impossibleError, impossibleConstraintError,
         nonExhaustiveGuardsError, patError, noMethodBindingError,
         typeError,
-        nonTermination, nestedAtomically, noMatchingContinuationPrompt,
+        nonTermination, nonTerminationError,
+        nestedAtomically,
+        noMatchingContinuationPrompt,
   ) where
 
 import           GHC.Internal.Base (
@@ -448,6 +450,9 @@ impossibleConstraintError   s = errorWithoutStackTrace (unpackCStringUtf8# s)
 
 
 -- GHC's RTS calls this
+nonTerminationError :: IO ()
+nonTerminationError = throwIO NonTermination
+
 nonTermination :: SomeException
 nonTermination = toException NonTermination
 


=====================================
rts/Prelude.h
=====================================
@@ -58,7 +58,7 @@ extern StgClosure ZCMain_main_closure;
 #define cannotCompactFunction_closure ghc_hs_iface->cannotCompactFunction_closure
 #define cannotCompactPinned_closure ghc_hs_iface->cannotCompactPinned_closure
 #define cannotCompactMutable_closure ghc_hs_iface->cannotCompactMutable_closure
-#define nonTermination_closure    ghc_hs_iface->nonTermination_closure
+#define nonTerminationError_closure    ghc_hs_iface->nonTerminationError_closure
 #define nestedAtomically_closure  ghc_hs_iface->nestedAtomically_closure
 #define absentSumFieldError_closure ghc_hs_iface->absentSumFieldError_closure
 #define underflowException_closure ghc_hs_iface->underflowException_closure


=====================================
rts/RtsStartup.c
=====================================
@@ -193,7 +193,7 @@ static void initBuiltinGcRoots(void)
     getStablePtr((StgPtr)heapOverflow_closure);
     getStablePtr((StgPtr)unpackCString_closure);
     getStablePtr((StgPtr)blockedIndefinitelyOnMVar_closure);
-    getStablePtr((StgPtr)nonTermination_closure);
+    getStablePtr((StgPtr)nonTerminationError_closure);
     getStablePtr((StgPtr)blockedIndefinitelyOnSTM_closure);
     getStablePtr((StgPtr)allocationLimitExceeded_closure);
     getStablePtr((StgPtr)cannotCompactFunction_closure);


=====================================
rts/Schedule.c
=====================================
@@ -3276,6 +3276,17 @@ findAtomicallyFrameHelper (Capability *cap, StgTSO *tso)
   }
 }
 
+static void throwNontermination(Capability *cap, StgTSO *tso) {
+  StgStack *stack = tso->stackobj;
+  stack->sp -= 3;
+  stack->sp[0] = (W_)&stg_enter_info;
+  stack->sp[1] = (W_)nonTerminationError_closure;
+  stack->sp[2] = (W_)&stg_ap_v_info;
+  tso->why_blocked = NotBlocked;
+  appendToRunQueue(cap,tso);
+}
+
+
 /* -----------------------------------------------------------------------------
    resurrectThreads is called after garbage collection on the list of
    threads found to be garbage.  Each of these threads will be woken
@@ -3313,8 +3324,7 @@ resurrectThreads (StgTSO *threads)
                                   (StgClosure *)blockedIndefinitelyOnMVar_closure);
             break;
         case BlockedOnBlackHole:
-            throwToSingleThreaded(cap, tso,
-                                  (StgClosure *)nonTermination_closure);
+            throwNontermination(cap, tso);
             break;
         case BlockedOnSTM:
             throwToSingleThreaded(cap, tso,


=====================================
rts/include/rts/RtsToHsIface.h
=====================================
@@ -25,7 +25,7 @@ typedef struct {
     StgClosure *cannotCompactFunction_closure;  // GHC.Internal.IO.Exception.cannotCompactFunction_closure
     StgClosure *cannotCompactPinned_closure;  // GHC.Internal.IO.Exception.cannotCompactPinned_closure
     StgClosure *cannotCompactMutable_closure;  // GHC.Internal.IO.Exception.cannotCompactMutable_closure
-    StgClosure *nonTermination_closure;  // GHC.Internal.Control.Exception.Base.nonTermination_closure
+    StgClosure *nonTerminationError_closure;  // GHC.Internal.Control.Exception.Base.nonTerminationError_closure
     StgClosure *nestedAtomically_closure;  // GHC.Internal.Control.Exception.Base.nestedAtomically_closure
     StgClosure *noMatchingContinuationPrompt_closure;  // GHC.Internal.Control.Exception.Base.noMatchingContinuationPrompt_closure
     StgClosure *blockedOnBadFD_closure;  // GHC.Internal.Event.Thread.blockedOnBadFD_closure


=====================================
testsuite/tests/rts/LoopBacktrace.stderr
=====================================
@@ -3,19 +3,19 @@ LoopBacktrace: Uncaught exception ghc-internal:GHC.Internal.Control.Exception.Ba
 <<loop>>
 
 IPE backtrace:
-    Cmm$rts/StgStartup.cmm. (:)
-    Cmm$rts/Exception.cmm. (:)
-    GHC.Internal.Base.thenIO (libraries/ghc-internal/src/GHC/Internal/Base.hs:2336:1-72)
-    GHC.Internal.IO.Handle.Text. (:)
-    GHC.Internal.Show. (:)
-    Main.x (LoopBacktrace.hs:6:1-9)
-    Cmm$rts/Updates.cmm. (:)
-    GHC.Num.Integer. (:)
-    Cmm$rts/HeapStackCheck.cmm. (:)
-    GHC.Internal.IO.throwIO (libraries/ghc-internal/src/GHC/Internal/IO.hs:285:36)
-    GHC.Internal.Exception.bindIO (libraries/ghc-internal/src/GHC/Internal/Base.hs:2333:1-76)
-    GHC.Internal.Exception.Backtrace.bindIO (libraries/ghc-internal/src/GHC/Internal/Base.hs:2333:1-76)
-    GHC.Internal.Exception.Backtrace.$ (libraries/ghc-internal/src/GHC/Internal/Base.hs:2267:1-9)
+  GHC.Internal.Exception.Backtrace.collectBacktraces' (libraries/ghc-internal/src/GHC/Internal/Exception/Backtrace.hs:(179,1)-(202,25))
+  GHC.Internal.Exception.Backtrace.collectBacktraces (libraries/ghc-internal/src/GHC/Internal/Exception/Backtrace.hs:174:39-56)
+  GHC.Internal.Exception.toExceptionWithBacktrace (libraries/ghc-internal/src/GHC/Internal/Exception.hs:(179,26)-(181,53))
+  GHC.Internal.IO.throwIO (libraries/ghc-internal/src/GHC/Internal/IO.hs:293:36)
+  Cmm$rts/HeapStackCheck.cmm. (:)
+  GHC.Internal.Bignum.Integer.integerAdd (libraries/ghc-internal/src/GHC/Internal/Bignum/Integer.hs:(547,1)-(571,52))
+  Cmm$rts/Updates.cmm. (:)
+  Main.x (LoopBacktrace.hs:6:1-9)
+  GHC.Internal.Show.show (libraries/ghc-internal/src/GHC/Internal/Show.hs:497:10-21)
+  GHC.Internal.IO.Handle.Text.hPutStr' (libraries/ghc-internal/src/GHC/Internal/IO/Handle/Text.hs:667:29-37)
+  GHC.Internal.Base.thenIO (libraries/ghc-internal/src/GHC/Internal/Base.hs:2337:1-72)
+  Cmm$rts/Exception.cmm. (:)
+  Cmm$rts/StgStartup.cmm. (:)
 HasCallStack backtrace:
-  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:446:23 in ghc-internal:GHC.Internal.Control.Exception.Base
+  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:452:23 in ghc-internal:GHC.Internal.Control.Exception.Base
 


=====================================
testsuite/tests/rts/T2783.stderr
=====================================
@@ -1,3 +1,7 @@
 T2783: Uncaught exception ghc-internal:GHC.Internal.Control.Exception.Base.NonTermination:
 
 <<loop>>
+
+HasCallStack backtrace:
+  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:452:23 in ghc-internal:GHC.Internal.Control.Exception.Base
+



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b709d07758de0315cfa9022662305b67...

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Ben Gamari (＠bgamari)

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