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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Sun Apr 14 12:28:12 PDT 2019

I believe there is probably a separate instruction in LLVM for thumb2 add.
Probably starting with t2 instead of t.

The definition of tADDi8 looks like this. Where tGPR specifically means

  def tADDi8 :                    // A8.6.4 T2
    T1sItGenEncodeImm<{1,1,0,?,?}, (outs tGPR:$Rdn),
                      (ins tGPR:$Rn, imm0_255:$imm8), IIC_iALUi,
                      "add", "\t$Rdn, $imm8",
                      [(set tGPR:$Rdn, (add tGPR:$Rn, imm8_255:$imm8))]>,


On Sun, Apr 14, 2019 at 12:21 PM Jie Zhou <jzhou41 at cs.rochester.edu> wrote:

> Sorry for not being specific enough. ARMv7-M includes Thumb and Thumb2.
> It has 12 regular registers (R0 - R12), and R8 - R12 are used.
> I can generate mov instruction that from/ R8-R12 to/from R0-R6.
> From this ARM page
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0068b/ch03s03s01.html
> R9 - R12 have their conventional usage, but I don’t if this is the reason
> we cannot
> use them arbitrarily. The fact that we can use it to generate mov
> instructions but
> not add/sub and push/pop confuses me.
> - Jie
> On Apr 14, 2019, at 14:55, Craig Topper <craig.topper at gmail.com> wrote:
> 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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