[lldb-dev] Source-level stepping with emulated instructions

[Pavel Labath via lldb-dev]

Hi Kjell,

if you say these instructions are similar to function calls, then it 
sounds to me like the best option would be to get lldb to treat them 
like function calls. I think (Jim can correct me if I'm wrong) this 
consists of two things:
- make sure lldb recognizes that these instructions can alter control 
flow (Disassembler::GetIndexOfNextBranchInstruction). You may have done 
this already.
- make sure lldb can unwind out of these "functions" when it winds up 
inside them. This will ensure the user does not stop in these functions 
when he does a "step over". This means providing it the correct unwind 
info so it knows where the functions will return. (As the functions know 
how to return to the original instructions, this information has to be 
somewhere, and is hopefully accessible to the debugger.) Probably the 
cleanest way to do that would be to create a new Module object, which 
would contain the implementations of all these functions, and all of 
their debug info. Then you could provide the unwind info through the 
usual channels (e.g. .debug_frame), and it has the advantage that you 
can also include any other information about these functions (names, 
line numbers, whatever...)

pl

On 15/01/2022 07:49, Kjell Winblad via lldb-dev wrote:
> Hi!
> 
> I'm implementing LLDB support for a new processor architecture that
> the company I'm working for has created. The processor architecture
> has a few emulated instructions. An emulated instruction works by
> jumping to a specific address that contains the start of a block of
> instructions that emulates the emulated instructions. The emulated
> instructions execute with interrupts turned off to be treated as
> atomic by the programmer. So an emulated instruction is similar to a
> function call. However, the address that the instruction jumps to is
> implicit and not specified by the programmer.
> 
> I'm facing a problem with the emulated instructions when implementing
> source-level stepping (the LLDB next and step commands) for C code in
> LLDB. LLDB uses hardware stepping to step through the address range
> that makes up a source-level statement. This algorithm works fine
> until the PC jumps to the start of the block that implements an
> emulated instruction. Then LLDB stops because the PC exited the
> address range for the source-level statement. This behavior is not
> what we want. Instead, LLDB should ideally step through the emulation
> instructions and continue until the current source-level statement has
> been completed.
> 
> My questions are:
> 
> 1. Is there currently any LLDB plugin functionality or special DWARF
> debug information to handle the kind of emulated instructions that I
> have described? All the code for the emulated instructions is within
> the same address range that does not contain any other code.
> 2. If the answer to question 1 is no, do you have suggestions for
> extending LLVM to support this kind of emulated instructions?
> 
> Best regards,
> Kjell Winblad
> _______________________________________________
> lldb-dev mailing list
> lldb-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev