[llvm-dev] RFC: Setting MachineInstr flags through storeRegToStackSlot

Quentin Colombet via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 17 11:23:08 PST 2017

Hi Alex,

I believe solution #3 is the one that makes sense the more sense.

> On Feb 17, 2017, at 7:29 AM, Alex Bradbury via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> On 17 February 2017 at 11:33, Alex Bradbury <asb at asbradbury.org> wrote:
>> ## Problem description
>> One of the responsibilities of a target's implementation of
>> TargetFrameLowering::emitPrologue is to set the frame pointer (if needed).
>> Typically, the frame pointer will be stored to the stack just like the other
>> callee-saved registers, and emitPrologue must insert the instruction to change
>> its value after it was stored to the stack. Mips does this by looking at the
>> number of callee-saved registers spilled for this function and skipping over
>> that many instructions. Other backends such as AArch64 or X86 will skip over
>> all instruction marked with the MachineInstr::FrameSetup flag[1].
> I should meant to say, the exact same issue exists for the
> MachineInstr::FrameDestroy flag, emitEpilogue, and
> loadRegFromStackSlot. The same resolution should be used for this case
> as well.
> Option 2) that I mentioned isn't quite necessary, callers can work out
> the number of MachineInstrs emitted by looking at the change in the
> size of the MachineBasicBlock. This does have the disadvantage of
> forcing all backends using {storeRegTo,loadRegFrom}StackSlot in
> FooTgtFrameLowering to replicate the necessary loops to mark the
> inserted instructions.
> Best,
> Alex
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