Fix for undefined behavior while updating PC value on arm-linux

[omjavaid]

Summary

This patch provides fix for the cases where an expression or jump tries to update the value of PC in thumb mode.

As precaution for an undefined behavior encountered while setting PC we will clear thumb bit of new PC if we are already in thumb mode; that is CPSR thumb mode bit is set.

Also if intended behavior of setting new PC value is to change to ARM/Thumb mode then user must switch to arm or thumb mode by explicitly setting mode bit in CPSR. This update to CPSR should be done before writing new PC value.

[clayborg]

Looks good.

[tberghammer]

Looks good but please fix the indentation to match with the LLDB style (4 space)

lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm.cpp

NativeRegisterContextLinux_arm::DoWriteRegisterValue(uint32_t offset,
     assert(offset % 4 == 0 && "Try to write a register with unaligned offset");
     if (offset + sizeof(uint32_t) > sizeof(m_gpr_arm))
         return Error("Register isn't fit into the size of the GPR area");
 
     Error error = DoReadGPR(m_gpr_arm, sizeof(m_gpr_arm));
     if (error.Fail())
         return error;
 
-    m_gpr_arm[offset / sizeof(uint32_t)] = value.GetAsUInt32();
+    uint32_t reg_value = value.GetAsUInt32();
+    // As precaution for an undefined behavior encountered while setting PC we
+    // will clear thumb bit of new PC if we are already in thumb mode; that is
+    // CPSR thumb mode bit is set.
+    if (offset / sizeof(uint32_t) == gpr_pc_arm)
+    {
+        // Check if we are already in thumb mode and
+        // thumb bit of current PC is read out to be zero and
+        // thumb bit of next PC is read out to be one.
+        if ((m_gpr_arm[gpr_cpsr_arm] &  0x20) &&
+            !(m_gpr_arm[gpr_pc_arm] &  0x01) &&
+            (value.GetAsUInt32() & 0x01))
+        {
+            reg_value &= (~1ull);
+        }
+    }
+
+    m_gpr_arm[offset / sizeof(uint32_t)] = reg_value;
     return DoWriteGPR(m_gpr_arm, sizeof(m_gpr_arm));
 }
 
 Error
 NativeRegisterContextLinux_arm::DoReadFPR(void *buf, size_t buf_size)
 {
     return NativeProcessLinux::PtraceWrapper(PTRACE_GETVFPREGS,
                                              m_thread.GetID(),