subreg vs vec_select

[Ilya Leoshkevich]

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