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.
