subreg vs vec_select

[Segher Boessenkool]

Hi Ilya,

On Wed, Sep 09, 2020 at 11:50:56AM +0200, Ilya Leoshkevich via Gcc wrote:
> I have a vector pseudo containing a single 128-bit value (V1TFmode) and
> I need to access its last 64 bits (DFmode). Which of the two options
> is better?
> 
> (subreg:DF (reg:V1TF) 8)
> 
> or
> 
> (vec_select:DF (subreg:V2DF (reg:V1TF) 0) (parallel [(const_int 1)]))
> 
> If I use the first one, I run into a problem with set_noop_p (): it
> thinks that
> 
> (set (subreg:DF (reg:TF %f0) 8) (subreg:DF (reg:V1TF %f0) 8))
> 
> is a no-op, because it doesn't check the mode after stripping the
> subreg:
> 
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/rtlanal.c;h=5ae38b79#l1616
> 
> However this is not correct, because SET_DEST is the second register in
> a register pair, and SET_SRC is half of a vector register that overlaps
> the first register in the corresponding pair. So it looks as if mode
> needs to be considered there.

Yes.

> This helps:
> 
> --- a/gcc/rtlanal.c
> +++ b/gcc/rtlanal.c
> @@ -1619,6 +1619,8 @@ set_noop_p (const_rtx set)
>         return 0;
>        src = SUBREG_REG (src);
>        dst = SUBREG_REG (dst);
> +      if (GET_MODE (src) != GET_MODE (dst))
> +       return 0;
>      }
> 
> but I'm not sure whether I'm not missing something about subreg
> semantics in the first place.

You probably should just see if both modes are the same number of hard
registers?  HARD_REGNO_NREGS.


Segher