Bug 916106 - Avoid unwind loop when erroneously in leaf. r=BenWa

tools/profiler/EHABIStackWalk.cpp

@@ -342,8 +342,19 @@ bool EHInterp::unwind() {
 
     // 1011000: Finish
     if (insn == I_FINISH) {
-      if (mState[R_PC] == 0)
+      if (mState[R_PC] == 0) {
         mState[R_PC] = mState[R_LR];
+        // Non-standard change (bug 916106): Prevent the caller from
+        // re-using LR.  Since the caller is by definition not a leaf
+        // routine, it will have to restore LR from somewhere to
+        // return to its own caller, so we can safely zero it here.
+        // This makes a difference only if an error in unwinding
+        // (e.g., caused by starting from within a prologue/epilogue)
+        // causes us to load a pointer to a leaf routine as LR; if we
+        // don't do something, we'll go into an infinite loop of
+        // "returning" to that same function.
+        mState[R_LR] = 0;
+      }
       return true;
     }