Fixes to "Retract Hp *before* checking for HpLim==0"

[ghc-hetmet.git] / rts / HeapStackCheck.cmm

diff --git a/rts/HeapStackCheck.cmm b/rts/HeapStackCheck.cmm
index 1533ae0..a1b6d65 100644 (file)
--- a/rts/HeapStackCheck.cmm
+++ b/rts/HeapStackCheck.cmm
@@ -26,6 +26,17 @@ import LeaveCriticalSection;
  *    - If HpLim==0, indicating that we should context-switch, we yield
  *      to the scheduler (return ThreadYielding).
  *
+ * Note that we must leave no slop in the heap (this is a requirement
+ * for LDV profiling, at least), so if we just had a heap-check
+ * failure, then we must retract Hp by HpAlloc.  How do we know
+ * whether there was a heap-check failure?  HpLim might be zero, and
+ * yet we got here as a result of a stack-check failure.  Hence, we
+ * require that HpAlloc is only non-zero if there was a heap-check
+ * failure, otherwise it is zero, so we can always safely subtract
+ * HpAlloc from Hp.
+ *
+ * Hence, HpAlloc is zeroed in LOAD_THREAD_STATE().
+ *
  *    - If the context_switch flag is set (the backup plan if setting HpLim
  *      to 0 didn't trigger a context switch), we yield to the scheduler
  *     (return ThreadYielding).
@@ -70,6 +81,7 @@ import LeaveCriticalSection;
         }                                              \
         if (HpAlloc <= BLOCK_SIZE                              \
             && bdescr_link(CurrentNursery) != NULL) {          \
+            HpAlloc = 0;                                        \
             CLOSE_NURSERY();                                   \
             CurrentNursery = bdescr_link(CurrentNursery);      \
             OPEN_NURSERY();                                    \