[llvm-dev] How to create a proper MachineMemOperand?

[Will Lester via llvm-dev]

Hi all,

I met a new problem in the process of my project.

I have got an address by mmap. And I want to build a MachineInstr of MOV64mr to move a value to the address. But it seems that the MachinePointerInfo needs a pointer to a Value.

So can I create a MachineMemOperand directly with the address (is a long*) ? Or I need to try some other methods?

Thanks a lot,

Will

[陳韋任 via llvm-dev]

Usually I will search the target directory, X86 in your case, to see
how MOV64mr is used with BuildMI to generate the instruction I want.

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Wei-Ren Chen (陳韋任)
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[Will Lester via llvm-dev]

Thanks for your advice. But I have searched X86 and find no more information than I have known.

The problem is not to create a MachineMemOperand, but to create a MachineMemOperand with certain address. This scene does not exist in the original code.

Since I have to do this in the process of register allocating, inserting IR instruction seems to be infeasible. I have thought of storing the address in a global variable in the IR, and loading it afterwards. But it seems to be improper as mentioned before.

May I do not create a MachineMemOperand, just store the address in a register, and use some MachineInstr to store the value to the address?

[Tim Northover via llvm-dev]

On 17 April 2018 at 03:49, Will Lester via llvm-dev

<llvm...@lists.llvm.org> wrote:
> The problem is not to create a MachineMemOperand, but to create a
> MachineMemOperand with certain address. This scene does not exist in the
> original code.

MachineMemOperands are used to tell compiler optimizations where a
memory instruction will access. It's not the actual representation
used to tell the CPU where to load or store. So you can conservatively
create a dummy one that essentially tells LLVM "this may write
anywhere" and your program will compile correctly.

To do that you'd write something like
"MF->getMachineMemOperand(MachinePointerInfo(),
MachineMemOperand::MOStore, 0, 1)". I've not tested this in code, so
you may have to tweak it.

Cheers.

Tim.