7.3 Instances in Which Unwinding May Fail

A successful stack trace may not be possible in the following situations:

Procedures that have multiple (secondary) entry points.
Code sequences in which DP (gr27) is modified. Note that DP should never be altered by user code, only by system code as is absolutely necessary.

7.2 Unwinding From Millicode

7.4 Callee-Saves Register Spill