[lldb-dev] Symbolic information in disassembler output

[Sean Callanan via lldb-dev]

Greg is right that this was a libedis feature and has no equivalent in 
LLDB today.

MCInst, however, doesn't have enough information by itself to do this.  
The reason is that for many things that are considered "operands," the 
MCInst has several underlying operands.  For example, an operand that 
was expressed as a register + an offset would be represented in MCInst 
as a register operand and in immediate operand, and only correlating the 
opcode with the LLVM instruction tables (and possibly some special 
knowledge) would tell you that the two belong together.

Additionally, libedis could express the semantics of the instruction 
operands (e.g., "this is a source operand and represents the result of 
dereferncing rbp - 4") , and inform the client what ranges of characters 
in the generated string represented each high level operand.  Both of 
these features are not exposed anywhere at the moment, and in fact the 
underlying knowledge was lost when the edis TableGen backend was deprecated.

There are a few LLDB features that reads instructions and attempt to 
interpret them:

  * The fast unwinder looks for specific bit patterns (see
    UnwindAssembly_x86::GetFastUnwindPlan in UnwindAssembly-x86.cpp);
  * The ARM instruction emulator has its own home-grown instruction
    table (see EmulateInstructionARM64.cpp); and
  * The crash diagnose functionality actually parses the output strings
    from the disassembler (see DoGuessValueAt in StackFrame.cpp).

Sean

On 8/31/17 11:09 AM, Greg Clayton via lldb-dev wrote:
> I believe libedis was deprecated many years ago and hasn't returned. 
> We use the standard LLVM disassembler, so any features need to be 
> built into llvm::MCInst.
>
>> On Aug 31, 2017, at 10:42 AM, Tatyana Krasnukha via lldb-dev 
>> <lldb-dev at lists.llvm.org <mailto:lldb-dev at lists.llvm.org>> wrote:
>>
>> Hello,
>> As I understand it, old disassembler (based on libedis) could print 
>> symbolic information instead/beside address operand of an 
>> instruction. And it looks like there is not such ability in 
>> disassembler now. Is thisresponsibility shifted on some other 
>> component of lldb? Or it was considered as useless and was removed at 
>> all?
>> Thanks,
>> Tatyana
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