[lldb] Add 'modify' type watchpoints, make it default (#66308)

Watchpoints in lldb can be either 'read', 'write', or 'read/write'. This
is exposing the actual behavior of hardware watchpoints. gdb has a
different behavior: a "write" type watchpoint only stops when the
watched memory region *changes*.

A user is using a watchpoint for one of three reasons:

1. Want to find what is changing/corrupting this memory.
2. Want to find what is writing to this memory.
3. Want to find what is reading from this memory.

I believe (1) is the most common use case for watchpoints, and it
currently can't be done in lldb -- the user needs to continue every time
the same value is written to the watched-memory manually. I think gdb's
behavior is the correct one. There are some use cases where a developer
wants to find every function that writes/reads to/from a memory region,
regardless of value, I want to still allow that functionality.

This is also a bit of groundwork for my large watchpoint support
proposal
https://discourse.llvm.org/t/rfc-large-watchpoint-support-in-lldb/72116
where I will be adding support for AArch64 MASK watchpoints which watch
power-of-2 memory regions. A user might ask to watch 24 bytes, and a
MASK watchpoint stub can do this with a 32-byte MASK watchpoint if it is
properly aligned. And we need to ignore writes to the final 8 bytes of
that watched region, and not show those hits to the user.

This patch adds a new 'modify' watchpoint type and it is the default.

rdar://108234227

1 files changed, 6 insertions, 4 deletions

diff --git a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
index 2fc446b..9d4e0a9 100644
--- a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
@@ -3108,14 +3108,16 @@ static GDBStoppointType GetGDBStoppointType(Watchpoint *wp) {
   assert(wp);
   bool watch_read = wp->WatchpointRead();
   bool watch_write = wp->WatchpointWrite();
+  bool watch_modify = wp->WatchpointModify();
 
-  // watch_read and watch_write cannot both be false.
-  assert(watch_read || watch_write);
-  if (watch_read && watch_write)
+  // watch_read, watch_write, watch_modify cannot all be false.
+  assert((watch_read || watch_write || watch_modify) &&
+         "watch_read, watch_write, watch_modify cannot all be false.");
+  if (watch_read && (watch_write || watch_modify))
     return eWatchpointReadWrite;
   else if (watch_read)
     return eWatchpointRead;
-  else // Must be watch_write, then.
+  else // Must be watch_write or watch_modify, then.
     return eWatchpointWrite;
 }