Hi! 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. 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. Best regards, Ilya
