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From: "rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/106069] [12/13 Regression] wrong code with -O
 -fno-tree-forwprop -maltivec on ppc64le
Date: Wed, 03 Aug 2022 09:20:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106069

--- Comment #25 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.o=
rg> ---
AIUI the rules are:

- GCC vector lane numbers always correspond to memory array indices.
  For example, lane 0 always comes first in memory.

- On big-endian targets, vector loads and stores are assumed to put the
  first memory element at the most significant end of the vector register.

So lane 0 refers to the most-significant register element on big-endian
targets and to the least-significant register element on little-endian
targets.  So:

  (vec_select:V4SI (reg:V4SI R)
    [(const_int 2) (const_int 6) (const_int 3) (const_int 7)])

describes a different option on big-endian and little-endian but:

  (vec_select:V4SI (mem:V4SI M)
    [(const_int 2) (const_int 6) (const_int 3) (const_int 7)])

is endian-independent.=