[llvm-dev] PPC64 Disassembler

Thu Nov 30 07:09:48 PST 2017

But where is the flat set? Maybe I can debug and check what is going on.

There is the possibility to be lldb usage, but it’s obtaining the disassembler as it is.
When lldb tries to instantiate the disassembler, it checks many information in
llvm/tools/lldb/source/Plugins/Disassembler/llvm/DisassemblerLLVMC.cpp:1014 =
“DisassemblerLLVMC::DisassemblerLLVMC(const ArchSpec &arch, const char *flavor_string)”,
which I couldn’t find if it’s doing correct. Is there a place that explains the “flavor” and
“features_str” variables used at DisassemblerLLVMC.cpp:1163
(“newLLVMCDisassembler(triple_str, cpu, features_str.c_str(), flavor, *this)”)?

Another question that I have is, how can I compare the data it gets with the real instructions?
I found the file “PPCGenDisassemblerTables.inc”, where the table is declared, and where the it
is decoded, so the current instruction bne cr7,0x2000092c (0x409e000c) is being processed by
the case 30 in the PPCGenDisassemblerTables.inc file:
      case 30:
        tmp = fieldFromInstruction(insn, 21, 5);
        if (decodeUImmOperand<5>(MI, tmp, Address, Decoder) == MCDisassembler::Fail) { return MCDisassembler::Fail; }
        tmp = fieldFromInstruction(insn, 16, 5);
        if (DecodeCRBITRCRegisterClass(MI, tmp, Address, Decoder) == MCDisassembler::Fail) { return MCDisassembler::Fail; }
        tmp = fieldFromInstruction(insn, 2, 14);
        MI.addOperand(MCOperand::createImm(tmp));
        return S;

Which is the correct reading for the BC instruction. What am I missing?

Some additional information:
MCInst opcode: 0x7cb
Decode Index: 0x1e

=================

Test case:

-          Build the file “lldb/packages/Python/lldbsuite/test/functionalities/stop-hook/main.cpp”

-          Run LLDB with the binary just generated

-          Add the breakpoint “b main.cpp:30”

-          Send the command “n”, stepping over to next line.
Result: The debugger will not stop and will execute the entire binary.

Remembering I’m using a little endian Power8 machine.

Thanks!

From: Nemanja Ivanovic [mailto:nemanja.i.ibm at gmail.com]
Sent: quinta-feira, 30 de novembro de 2017 07:54
To: Sean Fertile <sd.fertile at gmail.com>
Cc: Leonardo Bianconi <leonardo.bianconi at eldorado.org.br>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] PPC64 Disassembler

The `isBranch` flag is already set on the branch instructions. Furthermore, we do use the `isBranch()` query in a few places in the PPC back end, so this does work. Perhaps there's something specific about the lldb usage? Is it somehow possible that the `isBranch()` query is called on the wrong instruction?
Would you be able to provide a test case that reproduces the issue?

On Thu, Nov 30, 2017 at 2:30 AM, Sean Fertile via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hello Leonardo,

What is the opcode of the MCInstrDesc?

Sean

On Wed, Nov 29, 2017 at 1:48 PM, Leonardo Bianconi via llvm-dev
<llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi all,
>
>
>
> I’m working on lldb to make it available to ppc64le, but the “step over”
>
> is not working for some cases.
>
>
>
> When debugging, I can see that the disassembler analyze some instructions
>
> forward, looking for a branch instruction
>
> (llvm/tools/lldb/source/Plugins/Disassembler/llvm/DisassemblerLLVMC.cpp:87
>
> – “const bool can_branch = mc_disasm_ptr->CanBranch(inst);”), while
>
> trying to set the next breakpoint.
>
>
>
> On this case, the instruction is the “bne     cr7,0x2000092c”, which is a
> branch,
>
> but at llvm/lib/MC/MCInstrDesc.cpp:35 –
>
> “if (isBranch() || isCall() || isReturn() || isIndirectBranch())” it returns
> false,
>
> making lldb do not set the correct breakpoint, so the execution does not
> stop
>
> at next line, which should be the “step over” behavior.
>
>
>
> The variable “Flags” for the disassembled instruction does not have the
> branch
>
> flag.
>
>
>
> I have tried to change the file “/lib/Target/PowerPC/PPCInstrInfo.td”,
> adding
>
> “isBranch = 1” for the instruction "bc 4, $bi, $dst", but had not effect.
>
>
>
> Comparing with x86_64, building the same cpp file, the instruction
>
> “jne    0x4005eb” has the branch flag, which identifies it as a branch
> instruction.
>
>
>
> Where is the definition that an instruction is a branch? Is it a bug?
>
>
>
> Thanks!
>
>
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