[all-commits] [llvm/llvm-project] 805a36: [lldb][AArch64] Simplify handing of scalable regis...

[David Spickett via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 805a36aaf571a8d53781c0c413d1af125137bfa2
      https://github.com/llvm/llvm-project/commit/805a36aaf571a8d53781c0c413d1af125137bfa2
  Author: David Spickett <david.spickett at linaro.org>
  Date:   2023-11-02 (Thu, 02 Nov 2023)

  Changed paths:
    M lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
    M lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
    M lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.h

  Log Message:
  -----------
  [lldb][AArch64] Simplify handing of scalable registers using vg and svg (#70914)

This removes explicit invalidation of vg and svg that was done in
`GDBRemoteRegisterContext::AArch64Reconfigure`. This was in fact
covering up a bug elsehwere.

Register information says that a write to vg also invalidates svg (it
does not unless you are in streaming mode, but we decided to keep it
simple and say it always does).

This invalidation was not being applied until *after* AArch64Reconfigure
was called. This meant that without those manual invalidates this
happened:
* vg is written
* svg is not invalidated
* Reconfigure uses the written vg value
* Reconfigure uses the *old* svg value

I have moved the AArch64Reconfigure call to after we've processed the
invalidations caused by the register write, so we no longer need the
manual invalidates in AArch64Reconfigure.

In addition I have changed the order in which expedited registers as
parsed. These registers come with a stop notification and include,
amongst others, vg and svg.

So now we:
* Parse them and update register values (including vg and svg)
* AArch64Reconfigure, which uses those values, and invalidates every
register, because offsets may have changed.
* Parse the expedited registers again, knowing that none of the values
will have changed due to the scaling.

This means we use the expedited registers during the reconfigure, but
the invalidate does not mean we throw all of them away.

The cost is we parse them twice client side, but this is cheap compared
to a network packet, and is limited to AArch64 targets only.

On a system with SVE and SME, these are the packets sent for a step:
```
(lldb) b-remote.async>  < 803> read packet:
$T05thread:p1f80.1f80;name:main.o;threads:1f80;thread-pcs:000000000040056c<...>a1:0800000000000000;d9:0400000000000000;reason:trace;#fc
intern-state     <  21> send packet: $xfffffffff200,200#5e
intern-state     < 516> read packet:
$e4f2ffffffff000000<...>#71
intern-state     <  15> send packet: $Z0,400568,4#4d
intern-state     <   6> read packet: $OK#9a
dbg.evt-handler  <  16> send packet: $jThreadsInfo#c1
dbg.evt-handler  < 224> read packet:
$[{"name":"main.o","reason":"trace","registers":{"161":"0800000000000000",<...>}],"signal":5,"tid":8064}]]#73
```

You can see there are no extra register reads which means we're using
the expedited registers.

For a write to vg:
```
(lldb) register write vg 4
lldb             <  37> send packet:
$Pa1=0400000000000000;thread:1f80;#4a
lldb             <   6> read packet: $OK#9a
lldb             <  20> send packet: $pa1;thread:1f80;#29
lldb             <  20> read packet: $0400000000000000#04
lldb             <  20> send packet: $pd9;thread:1f80;#34
lldb             <  20> read packet: $0400000000000000#04
```

There is the initial P write, and lldb correctly assumes that SVG is
invalidated by this also so we read back the new vg and svg values
afterwards.