Stack unwinding emulation: handle adjustment of FP

[tberghammer]

Summary

Stack unwinding emulation: handle adjustment of FP

This change is improving the instruction emulation based unwinding to
handle when the frame pointer is adjusted (increment/decrement) after
it has been initialized. The situation can occur in the prologue of
some function where FP is adjusted before it is copied back to SP.

Example code (thumb, generated by gcc 4.8):
< +0>: push {r4, r7, lr}
< +2>: sub sp, #0x14
< +4>: add r7, sp, #0x0
...
<+50>: adds r7, #0x14 ; The CL fixes the handling of this instruction
<+52>: mov sp, r7 ; Previously unwinding from here was broken
<+54>: pop {r4, r7, pc}

[ovyalov]

LGTM

[clayborg]

Looks good to me.

lldb/trunk/source/Plugins/UnwindAssembly/InstEmulation/UnwindAssemblyInstEmulation.cpp

switch (context.type)
         case EmulateInstruction::eContextAdjustBaseRegister:
         case EmulateInstruction::eContextRegisterPlusOffset:
         case EmulateInstruction::eContextAdjustPC:
         case EmulateInstruction::eContextRegisterStore:
         case EmulateInstruction::eContextSupervisorCall:
         case EmulateInstruction::eContextTableBranchReadMemory:
         case EmulateInstruction::eContextWriteRegisterRandomBits:
         case EmulateInstruction::eContextWriteMemoryRandomBits:
-        case EmulateInstruction::eContextArithmetic:
         case EmulateInstruction::eContextAdvancePC:    
         case EmulateInstruction::eContextReturnFromException:
         case EmulateInstruction::eContextPushRegisterOnStack:
         case EmulateInstruction::eContextRegisterLoad:
 //            {
 //                const uint32_t reg_num = reg_info->kinds[m_unwind_plan_ptr->GetRegisterKind()];
 //                if (reg_num != LLDB_INVALID_REGNUM)
 //                {
 //                    const bool can_replace_only_if_unspecified = true;
 //
 //                    m_curr_row.SetRegisterLocationToUndefined (reg_num, 
 //                                                               can_replace_only_if_unspecified, 
 //                                                               can_replace_only_if_unspecified);
 //                    m_curr_row_modified = true;
 //                }
 //            }
             break;
 
+        case EmulateInstruction::eContextArithmetic:
+            {
+                // If we adjusted the current frame pointer by a constant then adjust the CFA offset
+                // with the same amount.
+                lldb::RegisterKind kind = m_unwind_plan_ptr->GetRegisterKind();
+                if (m_fp_is_cfa && reg_info->kinds[kind] == m_cfa_reg_info.kinds[kind] &&
+                    context.info_type == EmulateInstruction::eInfoTypeRegisterPlusOffset &&
+                    context.info.RegisterPlusOffset.reg.kinds[kind] == m_cfa_reg_info.kinds[kind])
+                {
+                    const int64_t offset = context.info.RegisterPlusOffset.signed_offset;
+                    m_curr_row->GetCFAValue().IncOffset(-1 * offset);
+                    m_curr_row_modified = true;
+                }
+            }
+            break;
+
         case EmulateInstruction::eContextAbsoluteBranchRegister:
         case EmulateInstruction::eContextRelativeBranchImmediate:
             {
                 if (context.info_type == EmulateInstruction::eInfoTypeISAAndImmediate &&
                     context.info.ISAAndImmediate.unsigned_data32 > 0)
                 {
                     m_forward_branch_offset = context.info.ISAAndImmediateSigned.signed_data32;
                 }