[Bug target/105928] New: [AArch64] 64-bit constants with same high/low halves can use ADD lsl 32 (-Os at least)

[peter at cordes dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105928

            Bug ID: 105928
           Summary: [AArch64] 64-bit constants with same high/low halves
                    can use ADD lsl 32 (-Os at least)
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: peter at cordes dot ca
  Target Milestone: ---
            Target: arm64-*-*

void foo(unsigned long *p) {
    *p = 0xdeadbeefdeadbeef;
}

cleverly compiles to https://godbolt.org/z/b3oqao5Kz

        mov     w1, 48879
        movk    w1, 0xdead, lsl 16
        stp     w1, w1, [x0]
        ret

But producing the value in a register uses more than 3 instructions:

unsigned long constant(){
    return 0xdeadbeefdeadbeef;
}

        mov     x0, 48879
        movk    x0, 0xdead, lsl 16
        movk    x0, 0xbeef, lsl 32
        movk    x0, 0xdead, lsl 48
        ret

At least with -Os, and maybe at -O2 or -O3 if it's efficient, we could be doing
a shifted ADD or ORR to broadcast a zero-extended 32-bit value to 64-bit.

        mov     x0, 48879
        movk    x0, 0xdead, lsl 16
        add     x0, x0, x0, lsl 32

Some CPUs may fuse sequences of movk, and shifted operands for ALU ops may take
extra time in some CPUs, so this might not actually be optimal for performance,
but it is smaller for -Os and -Oz.

We should also be using that trick for stores to _Atomic or volatile long*,
where we currently do MOV + 3x MOVK, then an STR, with ARMv8.4-a which
guarantees atomicity.


---

ARMv8.4-a and later guarantees atomicity for ldp/stp within an aligned 16-byte
chunk, so we should use MOV/MOVK / STP there even for volatile or
__ATOMIC_RELAXED.  But presumably that's a different part of GCC's internals,
so I'll report that separately.