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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98221] [11 regression] Wrong unpack operation
 emitted in tree-ssa-forwprop.c
Date: Thu, 10 Dec 2020 10:33:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98221

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
But all GIMPLE operates on "GIMPLE lane order" so this is a defect in how t=
he
backend handles those tree codes at expansion time?

You'll find no mention of BYTES_BIG_ENDIAN in the vectorizer (ok, you find
exactly one - but other UNPACK_LO/HI uses in other places).

>>From looking at tree-vect-generic.c VEC_UNPACK_{,FIX_TRUNC,FLOAT}_{LO,HI}_E=
XPR
are subject to this while supportable_widening_operation adds some more
LO,HI but also one EVEN,ODD case.

(Ab-)using BYTES_BIG_ENDIAN is also a bit odd, if we insist on preserving
this endian dependency on GIMPLE it should be a VECTOR_LANES_BIG_ENDIAN
or so.  But it doesn't seem to be consistently implemented.

The patch looks "obvious" if the semantics are dependent on BYTES_BIG_ENDIAN
but I see that nowhere documented and it's contrary to my expectation of
GIMPLE semantics (GIMPLE semantics assumes memory order).=