[Bug rtl-optimization/104387] New: aarch64: Redundant SXTH for “bag of bits” moves

[rsandifo at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104387

            Bug ID: 104387
           Summary: aarch64: Redundant SXTH for “bag of bits” moves
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64*-*-*

This PR is about another case in which we generate a redundant SXTH.
Specifically:

void f(short *x, short y)
{
  x[0] = y;
  x[1] = y;
}

generates:

        sxth    w1, w1
        strh    w1, [x0]
        strh    w1, [x0, 2]

even though the RTL makes it clear that the upper 16 bits of the
promoted “y” are unused.

This is related to PR64537, but I think it's worth keeping as a
separate PR since the fix isn't necessarily the same.