Index: llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
===================================================================
--- llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
+++ llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
@@ -49,6 +49,11 @@
     : TargetFrameLowering(TargetFrameLowering::StackGrowsDown, Align(8),
                           -SystemZMC::CallFrameSize, Align(8),
                           false /* StackRealignable */) {
+  // Due to the SystemZ ABI, the DWARF CFA (Canonical Frame Address) is not
+  // equal to the incoming stack pointer, but to incoming stack pointer plus
+  // 160.  The getOffsetOfLocalArea() returned value is interpreted as "the
+  // offset of the local area from the CFA".
+
   // Create a mapping from register number to save slot offset.
   RegSpillOffsets.grow(SystemZ::NUM_TARGET_REGS);
   for (unsigned I = 0, E = array_lengthof(SpillOffsetTable); I != E; ++I)
@@ -57,6 +62,11 @@
 
 const TargetFrameLowering::SpillSlot *
 SystemZFrameLowering::getCalleeSavedSpillSlots(unsigned &NumEntries) const {
+  // The offsets in SpillOffsetTable are relative to incoming stack pointer
+  // and are therefore incorrect here since we have defined the Local Area
+  // Offset as -SystemZMC::CallFrameSize. This however does not matter since
+  // the created frame object offsets are not used as we build the STMG/LMG
+  // instructions.
   NumEntries = array_lengthof(SpillOffsetTable);
   return SpillOffsetTable;
 }