[Bug tdep/30011] New: [gdb/tdep, aarch64] Incorrect frame address for last insn (leaf case)

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

https://sourceware.org/bugzilla/show_bug.cgi?id=30011

            Bug ID: 30011
           Summary: [gdb/tdep, aarch64] Incorrect frame address for last
                    insn (leaf case)
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

[ 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 foo:
...
00000000004005e4 <foo>:
  4005e4:       d10043ff        sub     sp, sp, #0x10
  4005e8:       f90007e0        str     x0, [sp, #8]
  4005ec:       d503201f        nop
  4005f0:       910043ff        add     sp, sp, #0x10
  4005f4:       d65f03c0        ret
...

Now, let's do "info frame" at each insn in foo, as well as printing $sp and
$fp:
...
$ gdb a.out
Reading symbols from a.out...
(gdb) b *foo
Breakpoint 1 at 0x400564
(gdb) r
Starting program: a.out 

Breakpoint 1, 0x0000000000400564 in foo ()
(gdb) display /x $sp
1: /x $sp = 0xfffffffff3a0
(gdb) display /x $fp
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
 pc = 0x400564 in foo; saved pc = 0x40058c
 called by frame at 0xfffffffff3b0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3a0
(gdb) si
0x0000000000400568 in foo ()
1: /x $sp = 0xfffffffff390
2: /x $fp = 0xfffffffff390
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
 pc = 0x400568 in foo; saved pc = 0x40058c
 called by frame at 0xfffffffff3b0
 Arglist at 0xfffffffff390, args: 
 Locals at 0xfffffffff390, Previous frame's sp is 0xfffffffff3a0
(gdb) si
0x000000000040056c in foo ()
1: /x $sp = 0xfffffffff390
2: /x $fp = 0xfffffffff390
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
 pc = 0x40056c in foo; saved pc = 0x40058c
 called by frame at 0xfffffffff3b0
 Arglist at 0xfffffffff390, args: 
 Locals at 0xfffffffff390, Previous frame's sp is 0xfffffffff3a0
 Saved registers:
  x0 at 0xfffffffff398
(gdb) si
0x0000000000400570 in foo ()
1: /x $sp = 0xfffffffff390
2: /x $fp = 0xfffffffff390
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
 pc = 0x400570 in foo; saved pc = 0x40058c
 called by frame at 0xfffffffff3b0
 Arglist at 0xfffffffff390, args: 
 Locals at 0xfffffffff390, Previous frame's sp is 0xfffffffff3a0
 Saved registers:
  x0 at 0xfffffffff398
(gdb) si
0x0000000000400574 in foo ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400574 in foo; saved pc = 0x40058c
 called by frame at 0xfffffffff3b0
 Arglist at 0xfffffffff3a0, args: 
 Locals at 0xfffffffff3a0, Previous frame's sp is 0xfffffffff3b0
 Saved registers:
  x0 at 0xfffffffff3a8
(gdb) si
0x000000000040058c in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $fp = 0xfffffffff3a0
...

The "frame at" bit lists 0xfffffffff3a0 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 == sp and
cache->framesize == 16, so unwound_fp + cache->framesize gives the wrong answer
once sp has been restored to entry value by the before-last insn.

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