Re: [PATCH] xtensa: Optimize stack frame adjustment more

[Takayuki 'January June' Suwa <jjsuwa_sys3175@yahoo.co.jp>]

On 2023/01/06 15:26, Max Filippov wrote:
> On Thu, Jan 5, 2023 at 7:35 PM Takayuki 'January June' Suwa
> <jjsuwa_sys3175@yahoo.co.jp> wrote:
>> On second thought, it cannot be a good idea to split addition/subtraction to the stack pointer.
>>
>>> -    4aaf:      b0a192          movi    a9, 0x1b0
>>> -    4ab2:      1f9a            add.n   a1, a15, a9
>>
>>> +    4aaf:      02df12          addmi   a1, a15, 0x200
>>> +    4ab2:      b0c112          addi    a1, a1, -80
>>
>> Because the former is atomic, but the latter is not. (may be interrupted between the two add instructions)
> 
> Oh, right, there are two issues: one is interruption in the absence of
> detailed stack tracking in the DWARF info, which can be fixed by emitting
> a separate note for each a1 change, the other is interruption when
> a1 is in the parent frame, which can be fixed by always moving a1
> down first, e.g. with the following change:
> 
> diff --git a/gcc/config/xtensa/xtensa.cc b/gcc/config/xtensa/xtensa.cc
> index 3b8a7bcda371..29cb91fa7de5 100644
> --- a/gcc/config/xtensa/xtensa.cc
> +++ b/gcc/config/xtensa/xtensa.cc
> @@ -2539,7 +2539,10 @@ xtensa_split_imm_two_addends (HOST_WIDE_INT
> imm, HOST_WIDE_INT v[2])
> 
>   if (xtensa_simm8 (v1) || xtensa_simm8x256 (v1))
>     {
> -      v[0] = v0, v[1] = v1;
> +      if (v0 < 0)
> +       v[0] = v0, v[1] = v1;
> +      else
> +       v[0] = v1, v[1] = v0;
>       return true;
>     }
> 
> Or both can be fixed by using a scratch register in the middle of the
> addi/addmi sequence.
> 
>> I'll wait for the results of your investigation, but it may be better to withdraw the patch.
> 
> The issue was in the unwinding code in the libgcc_s.so. I haven't figured
> out the exact mechanism, but found that emitting a separate note for each
> a1 change fixes it.
> 

Oh, thank you very much for your detailed investigation.  I will try to correct what you pointed out ASAP.