[PATCH 0/3] tcg: Improve vector tail clearing

[Richard Henderson]

Something I noticed while looking at AdvSIMD dumps, while
testing changes common with SVE2.

If we're going to load a zero into a vector register for
clearing the high bits of the SVE register, we might as
well use that zero to store the 8 bytes at the top of the
AdvSIMD register as well.

Output assembly goes from e.g.

  00:   48 c7 85 08 10 00 00 00   movq   $0x0,0x1008(%rbp)
        00 00 00
  0b:   c5 f9 ef c0               vpxor  %xmm0,%xmm0,%xmm0
  0f:   c5 fe 7f 85 10 10 00 00   vmovdqu %ymm0,0x1010(%rbp)
  17:   c5 fa 7f 85 30 10 00 00   vmovdqu %xmm0,0x1030(%rbp)

to

  00:   c5 f9 ef c0               vpxor  %xmm0,%xmm0,%xmm0
  04:   c5 f9 d6 85 08 10 00 00   vmovq  %xmm0,0x1008(%rbp)
  0c:   c5 fe 7f 85 10 10 00 00   vmovdqu %ymm0,0x1010(%rbp)
  14:   c5 fa 7f 85 30 10 00 00   vmovdqu %xmm0,0x1030(%rbp)

Saves a few bytes now, and more when we can do better with
loading constants into registers, where we can share the
vpxor between instructions.

The target/arm patches are not aided by the tcg patch, but
are not dependent on it.


r~


Richard Henderson (3):
  tcg: Improve vector tail clearing
  target/arm: Use tcg_gen_gvec_mov for clear_vec_high
  target/arm: Use clear_vec_high more effectively

 target/arm/translate-a64.c | 69 ++++++++++++++++++--------------
 tcg/tcg-op-gvec.c          | 82 +++++++++++++++++++++++++++++---------
 2 files changed, 101 insertions(+), 50 deletions(-)

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2.20.1