[ Spinoff from PR29721. ]

Consider the following test-case:
...
$ cat test.c
void
foo (const char *s)
{
}

int
main (void)
{
  foo ("foo");
  return 0;
}
...

Compiled without debug info:
...
$ gcc test.c
...

Disassembly of main:
...
0000000000400578 <main>:
  400578:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
  40057c:       910003fd        mov     x29, sp
  400580:       90000000        adrp    x0, 400000 <_init-0x408>
  400584:       91192000        add     x0, x0, #0x648
  400588:       97fffff7        bl      400564 <foo>
  40058c:       52800000        mov     w0, #0x0                        // #0
  400590:       a8c17bfd        ldp     x29, x30, [sp], #16
  400594:       d65f03c0        ret
...

Now, let's do "info frame" at each insn in main (so, using ni to skip over foo), as well as printing $sp and $fp:
...
$ gdb a.out
Reading symbols from a.out...
(gdb) b *main
Breakpoint 1 at 0x400578
(gdb) r
Starting program: a.out 

Breakpoint 1, 0x0000000000400578 in main ()
(gdb) display /x $sp
1: /x $sp = 0xfffffffff3b0
(gdb) display /x $fp
2: /x $fp = 0xfffffffff3b0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400578 in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3b0, args: 
 Locals at 0xfffffffff3b0, Previous frame's sp is 0xfffffffff3b0
(gdb) ni
0x000000000040057c in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x40057c in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x29 at 0xfffffffff3a0, x30 at 0xfffffffff3a8
(gdb) ni
0x0000000000400580 in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400580 in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x29 at 0xfffffffff3a0, x30 at 0xfffffffff3a8
(gdb) ni
0x0000000000400584 in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400584 in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x29 at 0xfffffffff3a0, x30 at 0xfffffffff3a8
(gdb) ni
0x0000000000400588 in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400588 in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x29 at 0xfffffffff3a0, x30 at 0xfffffffff3a8
(gdb) ni
0x000000000040058c in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x40058c in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x29 at 0xfffffffff3a0, x30 at 0xfffffffff3a8
(gdb) ni
0x0000000000400590 in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400590 in main; saved pc = 0xfffff7e5bfa0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x29 at 0xfffffffff3a0, x30 at 0xfffffffff3a8
(gdb) ni
0x0000000000400594 in main ()
1: /x $sp = 0xfffffffff3b0
2: /x $fp = 0xfffffffff3b0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3c0:
 pc = 0x400594 in main; saved pc = 0x4004a4
 Arglist at 0xfffffffff3b0, args: 
 Locals at 0xfffffffff3b0, Previous frame's sp is 0xfffffffff3c0
 Saved registers:
  x29 at 0xfffffffff3b0, x30 at 0xfffffffff3b8
(gdb) ni
0x0000fffff7e5bfa0 in __libc_start_main () from /lib64/libc.so.6
1: /x $sp = 0xfffffffff3b0
2: /x $fp = 0xfffffffff3b0
...

The "frame at" bit lists 0xfffffffff3b0 except at the last insn.

The frame address is calculated here in aarch64_make_prologue_cache_1:
...
  unwound_fp = get_frame_register_unsigned (this_frame, cache->framereg);
  if (unwound_fp == 0)
    return;

  cache->prev_sp = unwound_fp + cache->framesize;
...

For insns after the prologue, we have cache->framereg == fp and cache->framesize == 16, so unwound_fp + cache->framesize gives the wrong answer once fp has been restored to entry value by the before-last insn.