[llvm-dev] Quick question: How to BuildMI mov64mi32 arbitrary MMB address to memory

[K Jelesnianski via llvm-dev]

In the mean time I thought I could do the inverse and "sum" the size of
each instruction between the entry MBB and the target .LBB0_0 by doing a
double for loop
for each MBB
  for each MInstr
    currInstrBytes = getInstSizeInBytes(MInstr);
    totalAsmbytes += currInstrBytes;

Unfortunately X86-64 is missing a very convenient function called
"getInstSizeInBytes( MachineInstr* ) that I found in AAarch64InstrInfo. ;/
So I'm still unsure how to proceed.

On Mon, Sep 24, 2018 at 7:28 PM K Jelesnianski <kjski at vt.edu> wrote:

> Dear Dr. Northover,
>
> You've still got to access that symbol, and it's not obvious how a
>> block that's moving around in memory could do that. The same arguments
>> that it can't use %rip relative addressing for a local BB would seem
>> to apply to any other entity.
>>
>
> Agreed, for now I will probably perform need to perform load-time
> analysis, grab some info, and patch the binary to get around this. I have
> gone ahead and begun implementing your proposed work around.
>
> My question now is how would you create the BuildMI for the second
> assembly instr (addq (LBB0_0 - func), %rax) you proposed? I know I can get
> the global address of the MF we are currently in with
> .addGlobalAddress(M->getNamedValue(MF.getName())) but how do we take that
> and make an expression out of it. I am not sure the MBB object gives us any
> way to  get its address and perform the given subtraction expression of
> LBB0_0 - func.
>
> So far I have the first instruction working:
>
>     movq func at GOTPCREL(%rip), %rax
>>
>
> const Module *M = MF.getMMI().getModule();
> /* movq func at GOTPCREL(%rip), %rax */
> BuildMI(MBB, MBIt, DL, TII->get(X86::MOV64rm))
>                       .addReg(X86::RAX)         //dest
>                       .addReg(X86::RIP)         //base
>                       .addImm(0x1)              //scale
>                       .addReg(0x0)              //index
>                       .addGlobalAddress(M->getNamedValue(MF.getName()))
> //Disp
>                       .addReg(0x0);             //seg
>
>
>>     addq (.LBB0_0-func), %rax
>
>
>  /* addq (.LBB0_0-func), %rax ???? */
> BuildMI(MBB, MBIt, DL, TII->get(X86::ADD64ri32))
>                       .addReg(X86:RAX)          //destination
>                       .addReg(X86::RAX)         //base
>                       .addImm(0x1)              //scale
>                       .addReg(0x0)              //index
>                    .addImm(<<<< I assume expression is related to
> displacement and goes here >>>>>>>)
>                       .addReg(0x0);             //segment
>
> If I try to put a simple asm.s into llvm-mc -show-inst, it tells me to use
> MCExpr, but I am not sure that is correct (shown below). There does exist
> .addExpr but it only valid for MCInstBuilder, not
> MachineInstrBuilder::BuildMI.
> $ llvm-mc -show-inst asm.s
> foo:
> .LBB0_0:
>     movq    2099957(%rip), %rax     # <MCInst #1810 MOV64rm
>                                         #  <MCOperand Reg:35>
>                                         #  <MCOperand Reg:41>
>                                         #  <MCOperand Imm:1>
>                                         #  <MCOperand Reg:0>
>                                         #  <MCOperand Imm:2099957>
>                                         #  <MCOperand Reg:0>>
>     addq   (.LBB0_0  - foo) , %rax          # <MCInst #202 ADD64rm
>                                         #  <MCOperand Reg:35>
>                                         #  <MCOperand Reg:35>
>                                         #  <MCOperand Reg:0>
>                                         #  <MCOperand Imm:1>
>                                         #  <MCOperand Reg:0>
>                                         #  <MCOperand Expr:(.LBB0_0-foo)>
>                                         #  <MCOperand Reg:0>>
>     retq                            # <MCInst #2601 RETQ>
>
>
> > Do I need to make these symbols for the trampoline BBs as an IR opt
>> pass, can I get away with it using a MachineModule Pass to add the
>> trampolines per module (file) (so far I have only created BasicBlock,
>> MachineBasicBlock, and MachineFunction passes)??
>> If you go that route you can probably add entries to the
>> MachineConstantPool with a MachineModule pass. The same addressing
>> concerns seem to apply though.
>>
>
> I actually forgot about this constraint, we already have experienced some
> unintended side-effects when attempting to reference .rodata information
> (e.g. printf printing garbage). So that is something to look fix in the
> near future once this part is done. :)
> Thanks again for your reply!
>
> Sincerely,
>
> K Jelesnianski
>
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