> On Mar 9, 2015, at 4:18 AM, Bhushan Attarde <bhushan.atta...@imgtec.com>
> wrote:
>
> I am working on assembly profiler for MIPS using EmulateInstruction.
>
> I have set the ContextType to eContextPopRegisterOffStack in the function
> which emulates “ld ra,40(sp)”
> (i.e. load/restore ra from stack) instruction.
>
> 0xffffffff802009a4 <bar>:
> 0xffffffff802009a4: 67bdffd0 daddiu sp,sp,-48
> 0xffffffff802009a8: ffbf0028 sd ra,40(sp) -----> store return address
> 0xffffffff802009ac: ffbe0020 sd s8,32(sp)
> 0xffffffff802009b0: 03a0f02d move s8,sp
> <..end of prologue...>
> <..some other instructions...>
> <..start of epilogue...>
> 0xffffffff802009f4: 03c0e82d move sp,s8
> 0xffffffff802009f8: dfbf0028 ld ra,40(sp) ------> restore return
> address
> 0xffffffff802009fc: dfbe0020 ld s8,32(sp)
> 0xffffffff80200a00: 67bd0030 daddiu sp,sp,48
> 0xffffffff80200a04: 03e00008 jr ra
> 0xffffffff80200a08: 00000000 nop
>
> With this setting, the unwinder fails when we try to unwind from
> 0xffffffff802009fc.
> The call sequence is main()->foo()->bar() however, backtrace displays only
> current frame.
> ----------------------------------------
> (lldb) bt
> * thread #1: tid = 0x0001, 0xffffffff802009fc a.out_64`bar(p=4) + 88 at
> a.c:30, stop reason = breakpoint 1.1
> * frame #0: 0xffffffff802009fc a.out_64`bar(p=4) + 88 at a.c:30
> ---------------------------------------
>
> Below is a part of generated log:
> --------------------------------------------------------------------------------------
> th1/fr0 0xffffffff802009fc: CFA=sp+48 => fp=[CFA-16] ra= <same> pc=[CFA-8]
>
> th1/fr0 CFA is 0xffffffff800fff60: Register sp (29) contents are
> 0xffffffff800fff30, offset is 48
> th1/fr0 initialized frame current pc is 0xffffffff802009fc cfa is
> 0xffffffff800fff60 using EmulateInstructionMIPS64 UnwindPlan
> th1/fr0 requested caller's saved PC but this UnwindPlan uses a RA reg;
> getting ra (31) instead
> th1/fr0 supplying caller's saved ra (31)'s location using
> EmulateInstructionMIPS64 UnwindPlan
> th1/fr0 could not supply caller's ra (31) location, IsSame
> th1/fr1 could not get pc value
> Frame 1 invalid RegisterContext for this frame, stopping stack walk
> th1 Unwind of this thread is complete.
> --------------------------------------------------------------------------------------
>
> Here we just need to tell the unwinder that the register has been restored
> and take this register from live register context.
> Which ContextType to use in such case?
>
It seems like this is what is being done in
UnwindAssemblyInstEmulation::WriteRegister (...):
case EmulateInstruction::eContextPopRegisterOffStack:
{
const uint32_t reg_num =
reg_info->kinds[m_unwind_plan_ptr->GetRegisterKind()];
if (reg_num != LLDB_INVALID_REGNUM)
{
m_curr_row->SetRegisterLocationToSame (reg_num,
must_replace);
m_curr_row_modified = true;
m_curr_insn_restored_a_register = true;
}
}
break;
> 1. There exists similar ContextType - eContextRegisterLoad, but it is
> unimplemented (just breaks out and does nothing) in
> UnwindAssemblyInstEmulation::WriteRegister().
> So will it be a good option to implement eContextRegisterLoad and set the
> register location to itself using SetRegisterLocationToRegister (reg_num,
> reg_num, true)
> OR
> 2. Add new ContextType say eContextRegisterRestore to EmulateInstruction
> class and implement it in UnwindAssemblyInstEmulation::WriteRegister().
>
> Also, consider location of pc if we are restoring the return address register.
> This is similar to how eContextPushRegisterOnStack handles return address
> register in UnwindAssemblyInstEmulation::WriteMemory().
If all goes well it seems like he code in
UnwindAssemblyInstEmulation::WriteRegister() should be setting the register to
be in the register itself. Can you step through this code and make sure we are
happy with all of the register numbers?
Greg
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