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From: "munroesj at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/100085] New: Bad code for union transfer from
 __float128 to vector types
Date: Wed, 14 Apr 2021 18:19:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100085

            Bug ID: 100085
           Summary: Bad code for union transfer from __float128 to vector
                    types
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: munroesj at gcc dot gnu.org
  Target Milestone: ---

Created attachment 50595
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50595&action=3Dedit
Reduced example of union and __float128 to vector transfer.

GCC 10/9/8/7 will generate poor (-mcpu=3Dpower8) code when using a union to
transfer a __float128 scalar to any vector type. __float128 is a scalar type
and not typecast compatible with any vector type. Despite both being in Vec=
tor
registers.=20

But for runtime codes implementing __float128 operations for -mcpu=3Dpower8=
 it is
useful (and faster) to perform some (data_class, conversions, etc) operatio=
ns
directly in vector registers. The only solution for this is to use union to
transfer values between __float128/vector types. This should be a simple ve=
ctor
register transfer and optimized as such.

But when for GCC and PowerPCle and -mcpu=3Dpower8, we are consistently seei=
ng
store/reload sequences. For Power8 this can cause load-hit-store and pipe-l=
ine
rejects (33 cycles).

We don't see this when targeting -mcpu=3Dpower9, but power9 supports hardwa=
re
Float128 instruction. Also we don't see this when targeting BE.=