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
