[llvm-dev] [A bug?] Failed to use BuildMI to add R7 - R12 registers for tADDi8 and tPUSH of ARM

[Craig Topper via llvm-dev]

I don't know much about ARM. But it looks like tADDi8 is a Thumb
instruction and it can only use R0-R7.

tPUSH probably as a similar issue. But it's also a store instruction and
doesn't produce a register output. So you should use the form of BuildMI
that doesn't take a register as its last argument.

~Craig


On Sun, Apr 14, 2019 at 11:17 AM Jie Zhou via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
>
> I’m trying to insert some add/sub and push/pop instructions in a
> MachineFunction pass for ARMv7-M. However, I encountered something weird.
> For an add, when I use
>
>         BuildMI(….., TII->get(ARM::tADDi8),
> reg).addReg(reg).addReg(reg).addImm(imm).
>
> if reg is R0 - R7, everything is fine: I would get something like
>
>         adds r1, 4
>
> But if I use R8 - R12 as the reg in the BuildMI, I wouldn’t get the
> correct register in the assembly code. For example, when I pass R8 to it, I
> would get
>
>         adds r0, 4
>
> rather than
>
>         adds r8, 4.
>
> Similar problems happen to push and pop instructions. I can create a
> push/pop if the register list only contains registers R0 - R7, but
> for registers whose number are greater than R7, the generated asm code
> doesn’t have it. For example,
>
>         BuildMI(……, TII->get(ARM::tPUSH), R8)…..
>
> would give me
>
>         push {}
>
> Is this a bug in the LLVM ARM code generator? Or is there a reason why we
> cannot use big-number registers for add/sub and push/pop?
>
> Thanks,
> - Jie
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