Support for signed def_cfa_offset instruction

[20ashah]

Summary

Support for signed def_cfa_offset instruction

When writing dwarf information for the debug frame section, the signed version of the def_cfa_offset instruction is required since our stack grows up (adding to the SP to allocate more space). Currently, only the unsigned version is being emitted, and it encodes the offset as a ULEB instead of an SLEB.

[MaskRay]

We need tests. Is this the only place that needs to be patched to support stacks going upward?

llvm/lib/MC/MCDwarf.cpp

     const bool IsRelative =
       Instr.getOperation() == MCCFIInstruction::OpAdjustCfaOffset;
 
-    Streamer.emitInt8(dwarf::DW_CFA_def_cfa_offset);
+    int Offset = Instr.getOffset();
 
     if (IsRelative)
-      CFAOffset += Instr.getOffset();
+      CFAOffset += Offset;
     else
-      CFAOffset = Instr.getOffset();
+      CFAOffset = Offset;
 
-    Streamer.emitULEB128IntValue(CFAOffset);
+    if (Offset < 0) {
+      Streamer.emitInt8(dwarf::DW_CFA_def_cfa_offset_sf);
+      Streamer.emitSLEB128IntValue(CFAOffset / dataAlignmentFactor);
+    } else {
+      Streamer.emitInt8(dwarf::DW_CFA_def_cfa_offset);
+      Streamer.emitULEB128IntValue(CFAOffset);
+    }
 
     return;
   }