[Bug tdep/31486] [gdb/tdep, aarch64] Hang when hitting access watchpoint on M1

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

https://sourceware.org/bugzilla/show_bug.cgi?id=31486

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
There's a similar issue with lldb (version 17.0.6), with less serious failure
mode though.

When setting a write watchpoint on data.u.size8twice[1], execution stops for
reason "trace":
...
$ lldb outputs/gdb.base/watchpoint-unaligned/watchpoint-unaligned
(lldb) target create
"outputs/gdb.base/watchpoint-unaligned/watchpoint-unaligned"
Current executable set to
'/home/vries/gdb/outputs/gdb.base/watchpoint-unaligned/watchpoint-unaligned'
(aarch64).
(lldb) b main
Breakpoint 1: where = watchpoint-unaligned`main + 8 at
watchpoint-unaligned.c:65:3, address = 0x0000000000410208
(lldb) r
Process 1075081 launched:
'/home/vries/gdb/outputs/gdb.base/watchpoint-unaligned/watchpoint-unaligned'
(aarch64)
Process 1075081 stopped
* thread #1, name = 'watchpoint-unal', stop reason = breakpoint 1.1
    frame #0: 0x0000000000410208 watchpoint-unaligned`main at
watchpoint-unaligned.c:65:3
   62  
   63     assert (sizeof (data) == 8 + 3 * 8);
   64  
-> 65     write_size8twice ();
   66  
   67     while (size)
   68       {
(lldb) watch set var -w write data.u.size8twice[1]
Watchpoint created: Watchpoint 1: addr = 0x00440048 size = 8 state = enabled
type = w
    declare @
'/home/vries/gdb/src/gdb/testsuite/gdb.base/watchpoint-unaligned.c:35:3'
    watchpoint spec = 'data.u.size8twice[1]'
    new value: 0
(lldb) c
Process 1075081 resuming
Process 1075081 stopped
* thread #1, name = 'watchpoint-unal', stop reason = trace
    frame #0: 0x00000000004101f0 watchpoint-unaligned`write_size8twice at
watchpoint-unaligned.c:48:3
   45  
   46   #ifdef __aarch64__
   47     volatile void *p = &data.u.size8twice[offset];
-> 48     asm volatile ("stp %1, %2, [%0]"
   49                   : /* output */
   50                   : "r" (p), "r" (first), "r" (second) /* input */
   51                   : "memory" /* clobber */);
...

In contrast, when setting a write watchpoint on data.u.size8twice[0], execution
stops for reason "watchpoint":
...
(lldb) c
Process 1075169 resuming

Watchpoint 1 hit:
old value: 0
new value: 1
Process 1075169 stopped
* thread #1, name = 'watchpoint-unal', stop reason = watchpoint 1
    frame #0: 0x00000000004101f4 watchpoint-unaligned`write_size8twice at
watchpoint-unaligned.c:56:1
   53     data.u.size8twice[offset] = first;
   54     data.u.size8twice[offset + 1] = second;
   55   #endif
-> 56   }
   57  
   58   int
   59   main (void)
(lldb) 
...

This is using a write watchpoint, but it's the same for a regular watchpoint. 
Note that gdb handles a regualar watchpoint by reporting a watchpoint stop in
both (data.u.size8twice[0] and data.u.size8twice[1]) cases.

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