Re: [RFH] subreg of a vector without going through memory

[Marc Glisse <marc dot glisse at inria dot fr>]

On Sun, 4 Nov 2012, Marc Glisse wrote:

> Hello,
>
> trying to make some progress on PR 53101, I wrote the attached patch
> (it might be completely wrong for big endian, I don't know)
> (it is also missing a check that it isn't a paradoxical subreg)
>
> 	* simplify-rtx.c (simplify_subreg): For vectors, create a VEC_SELECT.
>
> However, when I compile this code on x86_64:
>
> typedef double v4 __attribute__((vector_size(32)));
> typedef double v2 __attribute__((vector_size(16)));
> v2 f(v4 x){
>  return *(v2*)&x;
> }
>
>
> I see in the *.combine dump:
>
> [...]
> Trying 6 -> 7:
> Successfully matched this instruction:
> (set (reg:V2DF 60 [ <retval> ])
>    (vec_select:V2DF (reg/v:V4DF 61 [ x ])
>        (parallel [
>                (const_int 0 [0])
>                (const_int 1 [0x1])
>            ])))
> rejecting combination of insns 6 and 7
> original costs 4 + 16 = 20
> replacement cost 32

This cost comes from the x86 target:

    case VEC_SELECT:
    case VEC_CONCAT:
    case VEC_MERGE:
    case VEC_DUPLICATE:
      /* ??? Assume all of these vector manipulation patterns are
         recognizable.  In which case they all pretty much have the
         same cost.  */
     *total = cost->fabs;
     return true;

If the canonical form of a subvector is vec_select, I guess the cost needs 
updating, and if it is subreg, the target should learn how to handle it 
properly?

> [...]
> (note 4 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)
> (insn 2 4 3 2 (set (reg/v:V4DF 61 [ x ])
>        (reg:V4DF 21 xmm0 [ x ])) v.cc:3 1123 {*movv4df_internal}
>     (expr_list:REG_DEAD (reg:V4DF 21 xmm0 [ x ])
>        (nil)))
> (note 3 2 6 2 NOTE_INSN_FUNCTION_BEG)
> (insn 6 3 7 2 (set (reg:OI 63 [ x ])
>        (subreg:OI (reg/v:V4DF 61 [ x ]) 0)) v.cc:4 60 {*movoi_internal_avx}
>     (expr_list:REG_DEAD (reg/v:V4DF 61 [ x ])
>        (nil)))
> (insn 7 6 11 2 (set (reg:V2DF 60 [ <retval> ])
>        (subreg:V2DF (reg:OI 63 [ x ]) 0)) v.cc:4 1124 {*movv2df_internal}
>     (expr_list:REG_DEAD (reg:OI 63 [ x ])
>        (nil)))
> (insn 11 7 14 2 (set (reg/i:V2DF 21 xmm0)
>        (reg:V2DF 60 [ <retval> ])) v.cc:5 1124 {*movv2df_internal}
>     (expr_list:REG_DEAD (reg:V2DF 60 [ <retval> ])
>        (nil)))
> (insn 14 11 0 2 (use (reg/i:V2DF 21 xmm0)) v.cc:5 -1
>     (nil))
>
>
>
> I am surprised by that high replacement cost that prevents the change. Is my 
> approach wrong? Is there an issue with the evaluation of costs?
>
> The approach was suggested by Richard B:
> http://gcc.gnu.org/ml/gcc-patches/2012-05/msg00197.html

--
Marc Glisse