Allocating a fixed size object in a stack frame at a fixed offset

Code Generation X86
1

Hello everyone,

I am currently struggling with one problem while developing a customized compiler with LLVM for my current project, and I wonder whether there is someone who can offer a piece of advice.

I am trying to reserve a fixed size (16 bytes) of space in an X86 stack frame at a fixed offset.

(I found a similar discussion here: https://groups.google.com/g/llvm-dev/c/_fQMdoS9KIY).

Initially, I tried to allocate a stack object using MachineFrameInfo::CreateFixedObject() and MachineFrameInfo::CreateFixedSpillStackObject() in MachineFunctionPass that is pipelined in addPreRegAlloc() in llvm/lib/Target/X86/X86TargetMachine.cpp.

It worked fine for a very simple application, but I found that it does not reserve the stack space when there are spilled general-purpose registers in the stack. I need to reserve a stack space before the spilled registers. Is there any good way of doing this?

I would appreciate any pointer, hint, or tip.

Thank you so much,

Inho Cho

2

Hi Inho Cho,

it depends on where you need the space, and how you access it. The frame layout is created in the FrameLowering classes, so I would recommend to have a look at X86FrameLowering.

  • You can use the constructor parameter LocalAreaOffset to reserve a space between the SP on function entry, and the local area.
  • You can also use the CreateFixedObject(), e.g. in assignCalleeSavedSpillSlots().
  • You should also check orderFrameObjects() in the X86FrameLowering class, because this functions reorders stack objects.

Regards,
Kai

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