[Bug rtl-optimization/100085] New: Bad code for union transfer from __float128 to vector types

["munroesj at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100085

            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=50595&action=edit
Reduced example of union and __float128 to vector transfer.

GCC 10/9/8/7 will generate poor (-mcpu=power8) 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 Vector
registers. 

But for runtime codes implementing __float128 operations for -mcpu=power8 it is
useful (and faster) to perform some (data_class, conversions, etc) operations
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 vector
register transfer and optimized as such.

But when for GCC and PowerPCle and -mcpu=power8, we are consistently seeing
store/reload sequences. For Power8 this can cause load-hit-store and pipe-line
rejects (33 cycles).

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