[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, 23 insertions, 7 deletions

diff --git a/lldb/source/Commands/CommandObjectWatchpoint.cpp b/lldb/source/Commands/CommandObjectWatchpoint.cpp
index a4929ea..83c2fb8 100644
--- a/lldb/source/Commands/CommandObjectWatchpoint.cpp
+++ b/lldb/source/Commands/CommandObjectWatchpoint.cpp
@@ -803,7 +803,7 @@ public:
             "Set a watchpoint on a variable. "
             "Use the '-w' option to specify the type of watchpoint and "
             "the '-s' option to specify the byte size to watch for. "
-            "If no '-w' option is specified, it defaults to write. "
+            "If no '-w' option is specified, it defaults to modify. "
             "If no '-s' option is specified, it defaults to the variable's "
             "byte size. "
             "Note that there are limited hardware resources for watchpoints. "
@@ -878,9 +878,9 @@ protected:
       return false;
     }
 
-    // If no '-w' is specified, default to '-w write'.
+    // If no '-w' is specified, default to '-w modify'.
     if (!m_option_watchpoint.watch_type_specified) {
-      m_option_watchpoint.watch_type = OptionGroupWatchpoint::eWatchWrite;
+      m_option_watchpoint.watch_type = OptionGroupWatchpoint::eWatchModify;
     }
 
     // We passed the sanity check for the command. Proceed to set the
@@ -947,7 +947,23 @@ protected:
     }
 
     // Now it's time to create the watchpoint.
-    uint32_t watch_type = m_option_watchpoint.watch_type;
+    uint32_t watch_type = 0;
+    switch (m_option_watchpoint.watch_type) {
+    case OptionGroupWatchpoint::eWatchModify:
+      watch_type |= LLDB_WATCH_TYPE_MODIFY;
+      break;
+    case OptionGroupWatchpoint::eWatchRead:
+      watch_type |= LLDB_WATCH_TYPE_READ;
+      break;
+    case OptionGroupWatchpoint::eWatchReadWrite:
+      watch_type |= LLDB_WATCH_TYPE_READ | LLDB_WATCH_TYPE_WRITE;
+      break;
+    case OptionGroupWatchpoint::eWatchWrite:
+      watch_type |= LLDB_WATCH_TYPE_WRITE;
+      break;
+    case OptionGroupWatchpoint::eWatchInvalid:
+      break;
+    };
 
     error.Clear();
     WatchpointSP watch_sp =
@@ -999,7 +1015,7 @@ public:
             "Use the '-l' option to specify the language of the expression. "
             "Use the '-w' option to specify the type of watchpoint and "
             "the '-s' option to specify the byte size to watch for. "
-            "If no '-w' option is specified, it defaults to write. "
+            "If no '-w' option is specified, it defaults to modify. "
             "If no '-s' option is specified, it defaults to the target's "
             "pointer byte size. "
             "Note that there are limited hardware resources for watchpoints. "
@@ -1013,7 +1029,7 @@ public:
         R"(
 Examples:
 
-(lldb) watchpoint set expression -w write -s 1 -- foo + 32
+(lldb) watchpoint set expression -w modify -s 1 -- foo + 32
 
     Watches write access for the 1-byte region pointed to by the address 'foo + 32')");
 
@@ -1073,7 +1089,7 @@ protected:
 
     // If no '-w' is specified, default to '-w write'.
     if (!m_option_watchpoint.watch_type_specified) {
-      m_option_watchpoint.watch_type = OptionGroupWatchpoint::eWatchWrite;
+      m_option_watchpoint.watch_type = OptionGroupWatchpoint::eWatchModify;
     }
 
     // We passed the sanity check for the command. Proceed to set the