https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67609
--- Comment #20 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jeffrey A. Law from comment #19)
> Preserving the upper part when setting the low part would be expressed via
> STRICT_LOW_PART. At least that's what I'd expect to see.
According to documentation this is only needed for smaller-than-word subregs:
"When used as an lvalue, @code{subreg} is a word-based accessor.
Storing to a @code{subreg} modifies all the words of @var{reg} that
overlap the @code{subreg}, but it leaves the other words of @var{reg}
alone.
for the case in question, while strict-low-part:
"When storing to a normal @code{subreg} that is smaller than a word,
the other bits of the referenced word are usually left in an undefined
state. This laxity makes it easier to generate efficient code for
such instructions. To represent an instruction that preserves all the
bits outside of those in the @code{subreg}, use @code{strict_low_part}
or @code{zero_extract} around the @code{subreg}.
