- * - If the context_switch flag is set, indicating that there are more
- * threads waiting to run, we yield to the scheduler
+ * - 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