[Bug target/114284] New: [14 Regression] arm: Load of volatile short gets miscompiled (loaded twice)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114284

            Bug ID: 114284
           Summary: [14 Regression] arm: Load of volatile short gets
                    miscompiled (loaded twice)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

The following is a wrong code regression in GCC 14:

volatile short x;
short foo() {
  return x;
}

with -march=armv8-m.base -O2 on the trunk we get:

foo:
        movw    r3, #:lower16:.LANCHOR0
        movt    r3, #:upper16:.LANCHOR0
        ldrh    r2, [r3]
        movs    r0, #0
        ldrsh   r0, [r3, r0]
        bx      lr

i.e. x is loaded twice, but with GCC 13 we get:

foo:
        movw    r3, #:lower16:.LANCHOR0
        movt    r3, #:upper16:.LANCHOR0
        ldrh    r0, [r3]
        sxth    r0, r0
        bx      lr

I suppose ideally we would have just one ldrsh, but the GCC 13 code is OK.