[Bug target/104110] New: AArch64 unnecessary use of call-preserved register

[nate at thatsmathematics dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104110

            Bug ID: 104110
           Summary: AArch64 unnecessary use of call-preserved register
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nate at thatsmathematics dot com
  Target Milestone: ---

gcc misses an optimization (or in some sense deoptimizes) by using a
call-preserved register to save a trivial constant across a function call.

Source code:

void bar(unsigned);
unsigned foo(unsigned c) {
    bar(1U << c);
    return 1;
}

Output from gcc -O3 on AArch64:

foo:
        stp     x29, x30, [sp, -32]!
        mov     x29, sp
        str     x19, [sp, 16]
        mov     w19, 1
        lsl     w0, w19, w0
        bl      bar
        mov     w0, w19
        ldr     x19, [sp, 16]
        ldp     x29, x30, [sp], 32
        ret

Note that x19 is used unnecessarily to save the constant 1 across the function
call, causing an unnecessary push and pop.  It would have been better to just
use some call-clobbered register for the constant 1 before the function call,
and then a simple `mov w0, 1` afterward.\

Same behavior with -O, -O2, -Os.  Tested on godbolt, affects yesterday's trunk
and all the way back to 5.4.

Might be related to bug 70801 or bug 71768 but I am not sure.