[lldb] Add isConstant mode for FA locations

This is similar to 9fe455f, but for FA locations instead of register
locations.
This is useful for unwind plans that cannot create abstract unwind rules, but
instead must inspect the state of the program to determine the current CFA.

Author: felipepiovezan

lldb/include/lldb/Symbol/UnwindPlan.h

@@ -215,6 +215,7 @@ class UnwindPlan {
         isRegisterDereferenced, // FA = [reg]
         isDWARFExpression,      // FA = eval(dwarf_expr)
         isRaSearch,             // FA = SP + offset + ???
+        isConstant,             // FA = constant
       };
 
       FAValue() : m_value() {}
@@ -259,6 +260,15 @@ class UnwindPlan {
         m_value.expr.length = len;
       }
 
+      bool IsConstant() const { return m_type == isConstant; }
+
+      void SetIsConstant(uint64_t constant) {
+        m_type = isConstant;
+        m_value.constant = constant;
+      }
+
+      uint64_t GetConstant() const { return m_value.constant; }
+
       uint32_t GetRegisterNumber() const {
         if (m_type == isRegisterDereferenced || m_type == isRegisterPlusOffset)
           return m_value.reg.reg_num;
@@ -329,6 +339,8 @@ class UnwindPlan {
         } expr;
         // For m_type == isRaSearch
         int32_t ra_search_offset;
+        // For m_type = isConstant
+        uint64_t constant;
       } m_value;
     }; // class FAValue
 

lldb/source/Symbol/UnwindPlan.cpp

@@ -187,6 +187,8 @@ operator==(const UnwindPlan::Row::FAValue &rhs) const {
         return !memcmp(m_value.expr.opcodes, rhs.m_value.expr.opcodes,
                        m_value.expr.length);
       break;
+    case isConstant:
+      return m_value.constant == rhs.m_value.constant;
     }
   }
   return false;
@@ -214,6 +216,8 @@ void UnwindPlan::Row::FAValue::Dump(Stream &s, const UnwindPlan *unwind_plan,
   case isRaSearch:
     s.Printf("RaSearch@SP%+d", m_value.ra_search_offset);
     break;
+  case isConstant:
+    s.Printf("0x%" PRIx64, m_value.constant);
   }
 }
 

lldb/source/Target/RegisterContextUnwind.cpp

@@ -2088,6 +2088,12 @@ bool RegisterContextUnwind::ReadFrameAddress(
     UnwindLogMsg("No suitable CFA found");
     break;
   }
+  case UnwindPlan::Row::FAValue::isConstant: {
+    address = fa.GetConstant();
+    address = abi_sp->FixCodeAddress(address);
+    UnwindLogMsg("CFA value set by constant is 0x%" PRIx64, address);
+    return true;
+  }
   default:
     return false;
   }