[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

lldb/bindings/headers.swig

@@ -77,4 +77,5 @@
 #include "lldb/API/SBValueList.h"
 #include "lldb/API/SBVariablesOptions.h"
 #include "lldb/API/SBWatchpoint.h"
+#include "lldb/API/SBWatchpointOptions.h"
 %}

lldb/bindings/interface/SBWatchpointOptionsDocstrings.i

@@ -0,0 +1,12 @@
+%feature("docstring",
+"A container for options to use when creating watchpoints."
+) lldb::SBWatchpointOptions;
+
+%feature("docstring", "Sets whether the watchpoint should stop on read accesses."
+) lldb::SBWatchpointOptions::SetWatchpointTypeRead;
+%feature("docstring", "Gets whether the watchpoint should stop on read accesses."
+) lldb::SBWatchpointOptions::GetWatchpointTypeRead;
+%feature("docstring", "Sets whether the watchpoint should stop on write accesses. eWatchpointWriteTypeOnModify is the most commonly useful mode, where lldb will stop when the watched value has changed. eWatchpointWriteTypeAlways will stop on any write to the watched region, even if it's the value is the same."
+) lldb::SBWatchpointOptions::SetWatchpointTypeWrite;
+%feature("docstring", "Gets whether the watchpoint should stop on write accesses, returning WatchpointWriteType to indicate the type of write watching that is enabled, or eWatchpointWriteTypeDisabled."
+) lldb::SBWatchpointOptions::GetWatchpointTypeWrite;

lldb/bindings/interfaces.swig

@@ -80,6 +80,7 @@
 %include "./interface/SBValueListDocstrings.i"
 %include "./interface/SBVariablesOptionsDocstrings.i"
 %include "./interface/SBWatchpointDocstrings.i"
+%include "./interface/SBWatchpointOptionsDocstrings.i"
 
 /* API headers */
 %include "lldb/API/SBAddress.h"
@@ -152,6 +153,7 @@
 %include "lldb/API/SBValueList.h"
 %include "lldb/API/SBVariablesOptions.h"
 %include "lldb/API/SBWatchpoint.h"
+%include "lldb/API/SBWatchpointOptions.h"
 
 /* Extensions for SB classes */
 %include "./interface/SBAddressExtensions.i"

lldb/include/lldb/API/SBTarget.h

@@ -21,6 +21,7 @@
 #include "lldb/API/SBType.h"
 #include "lldb/API/SBValue.h"
 #include "lldb/API/SBWatchpoint.h"
+#include "lldb/API/SBWatchpointOptions.h"
 
 namespace lldb_private {
 namespace python {
@@ -828,8 +829,13 @@ public:
 
   lldb::SBWatchpoint FindWatchpointByID(lldb::watch_id_t watch_id);
 
+  LLDB_DEPRECATED("WatchAddress deprecated, use WatchpointCreateByAddress")
   lldb::SBWatchpoint WatchAddress(lldb::addr_t addr, size_t size, bool read,
-                                  bool write, SBError &error);
+                                  bool modify, SBError &error);
+
+  lldb::SBWatchpoint
+  WatchpointCreateByAddress(lldb::addr_t addr, size_t size,
+                            lldb::SBWatchpointOptions options, SBError &error);
 
   bool EnableAllWatchpoints();
 

lldb/include/lldb/API/SBWatchpointOptions.h

@@ -0,0 +1,43 @@
+//===-- SBWatchpointOptions.h -----------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_API_SBWATCHPOINTOPTIONS_H
+#define LLDB_API_SBWATCHPOINTOPTIONS_H
+
+#include "lldb/API/SBDefines.h"
+
+class WatchpointOptionsImpl;
+
+namespace lldb {
+
+class LLDB_API SBWatchpointOptions {
+public:
+  SBWatchpointOptions();
+
+  SBWatchpointOptions(const lldb::SBWatchpointOptions &rhs);
+
+  ~SBWatchpointOptions();
+
+  const SBWatchpointOptions &operator=(const lldb::SBWatchpointOptions &rhs);
+
+  /// Stop when the watched memory region is read.
+  void SetWatchpointTypeRead(bool read);
+  bool GetWatchpointTypeRead() const;
+
+  /// Stop when the watched memory region is written to/modified
+  void SetWatchpointTypeWrite(lldb::WatchpointWriteType write_type);
+  lldb::WatchpointWriteType GetWatchpointTypeWrite() const;
+
+private:
+  // This auto_pointer is made in the constructor and is always valid.
+  mutable std::unique_ptr<WatchpointOptionsImpl> m_opaque_up;
+};
+
+} // namespace lldb
+
+#endif // LLDB_API_SBWATCHPOINTOPTIONS_H

lldb/include/lldb/Breakpoint/Watchpoint.h

@@ -76,12 +76,14 @@ public:
   
   bool WatchpointRead() const;
   bool WatchpointWrite() const;
+  bool WatchpointModify() const;
   uint32_t GetIgnoreCount() const;
   void SetIgnoreCount(uint32_t n);
   void SetWatchpointType(uint32_t type, bool notify = true);
   void SetDeclInfo(const std::string &str);
   std::string GetWatchSpec();
   void SetWatchSpec(const std::string &str);
+  bool WatchedValueReportable(const ExecutionContext &exe_ctx);
 
   // Snapshot management interface.
   bool IsWatchVariable() const;
@@ -212,7 +214,8 @@ private:
   // again, we check the count, if it is more than 1, it means the user-
   // supplied actions actually want the watchpoint to be disabled!
   uint32_t m_watch_read : 1, // 1 if we stop when the watched data is read from
-      m_watch_write : 1;     // 1 if we stop when the watched data is written to
+      m_watch_write : 1,     // 1 if we stop when the watched data is written to
+      m_watch_modify : 1;    // 1 if we stop when the watched data is changed
   uint32_t m_ignore_count;      // Number of times to ignore this watchpoint
   std::string m_decl_str;       // Declaration information, if any.
   std::string m_watch_spec_str; // Spec for the watchpoint.

lldb/include/lldb/Interpreter/OptionGroupWatchpoint.h

@@ -30,13 +30,15 @@ public:
 
   void OptionParsingStarting(ExecutionContext *execution_context) override;
 
-  // Note:
-  // eWatchRead == LLDB_WATCH_TYPE_READ; and
-  // eWatchWrite == LLDB_WATCH_TYPE_WRITE
+  /// eWatchRead == LLDB_WATCH_TYPE_READ
+  /// eWatchWrite == LLDB_WATCH_TYPE_WRITE
+  /// eWatchModify == LLDB_WATCH_TYPE_MODIFY
+  /// eWatchReadWrite == LLDB_WATCH_TYPE_READ | LLDB_WATCH_TYPE_WRITE
   enum WatchType {
     eWatchInvalid = 0,
     eWatchRead,
     eWatchWrite,
+    eWatchModify,
     eWatchReadWrite
   };
 

lldb/include/lldb/lldb-defines.h

@@ -44,8 +44,10 @@
 #define LLDB_WATCH_ID_IS_VALID(uid) ((uid) != (LLDB_INVALID_WATCH_ID))
 #define LLDB_WATCH_TYPE_READ (1u << 0)
 #define LLDB_WATCH_TYPE_WRITE (1u << 1)
+#define LLDB_WATCH_TYPE_MODIFY (1u << 2)
 #define LLDB_WATCH_TYPE_IS_VALID(type)                                         \
-  ((type & LLDB_WATCH_TYPE_READ) || (type & LLDB_WATCH_TYPE_WRITE))
+  ((type & LLDB_WATCH_TYPE_READ) || (type & LLDB_WATCH_TYPE_WRITE) ||          \
+   (type & LLDB_WATCH_TYPE_MODIFY))
 
 // Generic Register Numbers
 #define LLDB_REGNUM_GENERIC_PC 0    // Program Counter

lldb/include/lldb/lldb-enumerations.h

@@ -431,6 +431,21 @@ FLAGS_ENUM(WatchpointEventType){
     eWatchpointEventTypeThreadChanged = (1u << 11),
     eWatchpointEventTypeTypeChanged = (1u << 12)};
 
+enum WatchpointWriteType {
+  /// Don't stop when the watched memory region is written to.
+  eWatchpointWriteTypeDisabled,
+  /// Stop on any write access to the memory region, even if
+  /// the value doesn't change.  On some architectures, a write
+  /// near the memory region may be falsely reported as a match,
+  /// and notify this spurious stop as a watchpoint trap.
+  eWatchpointWriteTypeAlways,
+  /// Stop on a write to the memory region that changes its value.
+  /// This is most likely the behavior a user expects, and is the
+  /// behavior in gdb.  lldb can silently ignore writes near the
+  /// watched memory region that are reported as accesses to lldb.
+  eWatchpointWriteTypeOnModify
+};
+
 /// Programming language type.
 ///
 /// These enumerations use the same language enumerations as the DWARF

lldb/include/lldb/lldb-forward.h

@@ -284,6 +284,7 @@ class VariableList;
 class Watchpoint;
 class WatchpointList;
 class WatchpointOptions;
+class WatchpointSetOptions;
 struct CompilerContext;
 struct LineEntry;
 struct PropertyDefinition;

lldb/source/API/CMakeLists.txt

@@ -91,6 +91,7 @@ add_lldb_library(liblldb SHARED ${option_framework}
   SBValueList.cpp
   SBVariablesOptions.cpp
   SBWatchpoint.cpp
+  SBWatchpointOptions.cpp
   SBUnixSignals.cpp
   SystemInitializerFull.cpp
   ${lldb_python_wrapper}

lldb/source/API/SBTarget.cpp

@@ -1322,27 +1322,39 @@ SBWatchpoint SBTarget::FindWatchpointByID(lldb::watch_id_t wp_id) {
 }
 
 lldb::SBWatchpoint SBTarget::WatchAddress(lldb::addr_t addr, size_t size,
-                                          bool read, bool write,
+                                          bool read, bool modify,
                                           SBError &error) {
   LLDB_INSTRUMENT_VA(this, addr, size, read, write, error);
 
+  SBWatchpointOptions options;
+  options.SetWatchpointTypeRead(read);
+  options.SetWatchpointTypeWrite(eWatchpointWriteTypeOnModify);
+  return WatchpointCreateByAddress(addr, size, options, error);
+}
+
+lldb::SBWatchpoint
+SBTarget::WatchpointCreateByAddress(lldb::addr_t addr, size_t size,
+                                    SBWatchpointOptions options,
+                                    SBError &error) {
+  LLDB_INSTRUMENT_VA(this, addr, size, options, error);
+
   SBWatchpoint sb_watchpoint;
   lldb::WatchpointSP watchpoint_sp;
   TargetSP target_sp(GetSP());
-  if (target_sp && (read || write) && addr != LLDB_INVALID_ADDRESS &&
-      size > 0) {
+  uint32_t watch_type = 0;
+  if (options.GetWatchpointTypeRead())
+    watch_type |= LLDB_WATCH_TYPE_READ;
+  if (options.GetWatchpointTypeWrite() == eWatchpointWriteTypeAlways)
+    watch_type |= LLDB_WATCH_TYPE_WRITE;
+  if (options.GetWatchpointTypeWrite() == eWatchpointWriteTypeOnModify)
+    watch_type |= LLDB_WATCH_TYPE_MODIFY;
+  if (watch_type == 0) {
+    error.SetErrorString("Can't create a watchpoint that is neither read nor "
+                         "write nor modify.");
+    return sb_watchpoint;
+  }
+  if (target_sp && addr != LLDB_INVALID_ADDRESS && size > 0) {
     std::lock_guard<std::recursive_mutex> guard(target_sp->GetAPIMutex());
-    uint32_t watch_type = 0;
-    if (read)
-      watch_type |= LLDB_WATCH_TYPE_READ;
-    if (write)
-      watch_type |= LLDB_WATCH_TYPE_WRITE;
-    if (watch_type == 0) {
-      error.SetErrorString(
-          "Can't create a watchpoint that is neither read nor write.");
-      return sb_watchpoint;
-    }
-
     // Target::CreateWatchpoint() is thread safe.
     Status cw_error;
     // This API doesn't take in a type, so we can't figure out what it is.

lldb/source/API/SBValue.cpp

@@ -1433,10 +1433,17 @@ lldb::SBWatchpoint SBValue::Watch(bool resolve_location, bool read, bool write,
       return sb_watchpoint;
 
     uint32_t watch_type = 0;
-    if (read)
+    if (read) {
       watch_type |= LLDB_WATCH_TYPE_READ;
-    if (write)
-      watch_type |= LLDB_WATCH_TYPE_WRITE;
+      // read + write, the most likely intention
+      // is to catch all writes to this, not just
+      // value modifications.
+      if (write)
+        watch_type |= LLDB_WATCH_TYPE_WRITE;
+    } else {
+      if (write)
+        watch_type |= LLDB_WATCH_TYPE_MODIFY;
+    }
 
     Status rc;
     CompilerType type(value_sp->GetCompilerType());

lldb/source/API/SBWatchpoint.cpp

@@ -354,7 +354,8 @@ bool SBWatchpoint::IsWatchingWrites() {
     std::lock_guard<std::recursive_mutex> guard(
         watchpoint_sp->GetTarget().GetAPIMutex());
 
-    return watchpoint_sp->WatchpointWrite();
+    return watchpoint_sp->WatchpointWrite() ||
+           watchpoint_sp->WatchpointModify();
   }
 
   return false;

lldb/source/API/SBWatchpointOptions.cpp

@@ -0,0 +1,73 @@
+//===-- SBWatchpointOptions.cpp -------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/API/SBWatchpointOptions.h"
+#include "lldb/Breakpoint/Watchpoint.h"
+#include "lldb/Utility/Instrumentation.h"
+
+#include "Utils.h"
+
+using namespace lldb;
+using namespace lldb_private;
+
+class WatchpointOptionsImpl {
+public:
+  bool m_read = false;
+  bool m_write = false;
+  bool m_modify = false;
+};
+
+
+SBWatchpointOptions::SBWatchpointOptions()
+    : m_opaque_up(new WatchpointOptionsImpl()) {
+  LLDB_INSTRUMENT_VA(this);
+}
+
+SBWatchpointOptions::SBWatchpointOptions(const SBWatchpointOptions &rhs) {
+  LLDB_INSTRUMENT_VA(this, rhs);
+
+  m_opaque_up = clone(rhs.m_opaque_up);
+}
+
+const SBWatchpointOptions &
+SBWatchpointOptions::operator=(const SBWatchpointOptions &rhs) {
+  LLDB_INSTRUMENT_VA(this, rhs);
+
+  if (this != &rhs)
+    m_opaque_up = clone(rhs.m_opaque_up);
+  return *this;
+}
+
+SBWatchpointOptions::~SBWatchpointOptions() = default;
+
+void SBWatchpointOptions::SetWatchpointTypeRead(bool read) {
+  m_opaque_up->m_read = read;
+}
+bool SBWatchpointOptions::GetWatchpointTypeRead() const {
+  return m_opaque_up->m_read;
+}
+
+void SBWatchpointOptions::SetWatchpointTypeWrite(
+    WatchpointWriteType write_type) {
+  if (write_type == eWatchpointWriteTypeOnModify) {
+    m_opaque_up->m_write = false;
+    m_opaque_up->m_modify = true;
+  } else if (write_type == eWatchpointWriteTypeAlways) {
+    m_opaque_up->m_write = true;
+    m_opaque_up->m_modify = false;
+  } else
+    m_opaque_up->m_write = m_opaque_up->m_modify = false;
+}
+
+WatchpointWriteType SBWatchpointOptions::GetWatchpointTypeWrite() const {
+  if (m_opaque_up->m_modify)
+    return eWatchpointWriteTypeOnModify;
+  if (m_opaque_up->m_write)
+    return eWatchpointWriteTypeAlways;
+  return eWatchpointWriteTypeDisabled;
+}

lldb/source/Breakpoint/Watchpoint.cpp

@@ -29,7 +29,8 @@ Watchpoint::Watchpoint(Target &target, lldb::addr_t addr, uint32_t size,
     : StoppointSite(0, addr, size, hardware), m_target(target),
       m_enabled(false), m_is_hardware(hardware), m_is_watch_variable(false),
       m_is_ephemeral(false), m_disabled_count(0), m_watch_read(0),
-      m_watch_write(0), m_ignore_count(0), m_being_created(true) {
+      m_watch_write(0), m_watch_modify(0), m_ignore_count(0),
+      m_being_created(true) {
 
   if (type && type->IsValid())
     m_type = *type;
@@ -193,7 +194,7 @@ bool Watchpoint::IsWatchVariable() const { return m_is_watch_variable; }
 void Watchpoint::SetWatchVariable(bool val) { m_is_watch_variable = val; }
 
 bool Watchpoint::CaptureWatchedValue(const ExecutionContext &exe_ctx) {
-  ConstString watch_name("$__lldb__watch_value");
+  ConstString g_watch_name("$__lldb__watch_value");
   m_old_value_sp = m_new_value_sp;
   Address watch_address(GetLoadAddress());
   if (!m_type.IsValid()) {
@@ -205,12 +206,47 @@ bool Watchpoint::CaptureWatchedValue(const ExecutionContext &exe_ctx) {
     return false;
   }
   m_new_value_sp = ValueObjectMemory::Create(
-      exe_ctx.GetBestExecutionContextScope(), watch_name.GetStringRef(),
+      exe_ctx.GetBestExecutionContextScope(), g_watch_name.GetStringRef(),
       watch_address, m_type);
-  m_new_value_sp = m_new_value_sp->CreateConstantValue(watch_name);
+  m_new_value_sp = m_new_value_sp->CreateConstantValue(g_watch_name);
   return (m_new_value_sp && m_new_value_sp->GetError().Success());
 }
 
+bool Watchpoint::WatchedValueReportable(const ExecutionContext &exe_ctx) {
+  if (!m_watch_modify || m_watch_read)
+    return true;
+  if (!m_type.IsValid())
+    return true;
+
+  ConstString g_watch_name("$__lldb__watch_value");
+  Address watch_address(GetLoadAddress());
+  ValueObjectSP newest_valueobj_sp = ValueObjectMemory::Create(
+      exe_ctx.GetBestExecutionContextScope(), g_watch_name.GetStringRef(),
+      watch_address, m_type);
+  newest_valueobj_sp = newest_valueobj_sp->CreateConstantValue(g_watch_name);
+  Status error;
+
+  DataExtractor new_data;
+  DataExtractor old_data;
+
+  newest_valueobj_sp->GetData(new_data, error);
+  if (error.Fail())
+    return true;
+  m_new_value_sp->GetData(old_data, error);
+  if (error.Fail())
+    return true;
+
+  if (new_data.GetByteSize() != old_data.GetByteSize() ||
+      new_data.GetByteSize() == 0)
+    return true;
+
+  if (memcmp(new_data.GetDataStart(), old_data.GetDataStart(),
+             old_data.GetByteSize()) == 0)
+    return false; // Value has not changed, user requested modify watchpoint
+
+  return true;
+}
+
 // RETURNS - true if we should stop at this breakpoint, false if we
 // should continue.
 
@@ -268,10 +304,10 @@ void Watchpoint::DumpWithLevel(Stream *s,
          description_level <= lldb::eDescriptionLevelVerbose);
 
   s->Printf("Watchpoint %u: addr = 0x%8.8" PRIx64
-            " size = %u state = %s type = %s%s",
+            " size = %u state = %s type = %s%s%s",
             GetID(), GetLoadAddress(), m_byte_size,
             IsEnabled() ? "enabled" : "disabled", m_watch_read ? "r" : "",
-            m_watch_write ? "w" : "");
+            m_watch_write ? "w" : "", m_watch_modify ? "m" : "");
 
   if (description_level >= lldb::eDescriptionLevelFull) {
     if (!m_decl_str.empty())
@@ -333,10 +369,13 @@ void Watchpoint::SetEnabled(bool enabled, bool notify) {
 void Watchpoint::SetWatchpointType(uint32_t type, bool notify) {
   int old_watch_read = m_watch_read;
   int old_watch_write = m_watch_write;
+  int old_watch_modify = m_watch_modify;
   m_watch_read = (type & LLDB_WATCH_TYPE_READ) != 0;
   m_watch_write = (type & LLDB_WATCH_TYPE_WRITE) != 0;
+  m_watch_modify = (type & LLDB_WATCH_TYPE_MODIFY) != 0;
   if (notify &&
-      (old_watch_read != m_watch_read || old_watch_write != m_watch_write))
+      (old_watch_read != m_watch_read || old_watch_write != m_watch_write ||
+       old_watch_modify != m_watch_modify))
     SendWatchpointChangedEvent(eWatchpointEventTypeTypeChanged);
 }
 
@@ -344,6 +383,8 @@ bool Watchpoint::WatchpointRead() const { return m_watch_read != 0; }
 
 bool Watchpoint::WatchpointWrite() const { return m_watch_write != 0; }
 
+bool Watchpoint::WatchpointModify() const { return m_watch_modify != 0; }
+
 uint32_t Watchpoint::GetIgnoreCount() const { return m_ignore_count; }
 
 void Watchpoint::SetIgnoreCount(uint32_t n) {

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

lldb/source/Interpreter/OptionGroupWatchpoint.cpp

@@ -27,6 +27,11 @@ static constexpr OptionEnumValueElement g_watch_type[] = {
         "write",
         "Watch for write",
     },
+    {
+        OptionGroupWatchpoint::eWatchModify,
+        "modify",
+        "Watch for modifications",
+    },
     {
         OptionGroupWatchpoint::eWatchReadWrite,
         "read_write",

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;
 }
 

lldb/source/Target/StopInfo.cpp

@@ -982,6 +982,12 @@ protected:
             m_should_stop = false;
           }
         }
+
+        // Don't stop if the watched region value is unmodified, and
+        // this is a Modify-type watchpoint.
+        if (m_should_stop && !wp_sp->WatchedValueReportable(exe_ctx))
+          m_should_stop = false;
+
         // Finally, if we are going to stop, print out the new & old values:
         if (m_should_stop) {
           wp_sp->CaptureWatchedValue(exe_ctx);

lldb/source/Target/Target.cpp

@@ -860,7 +860,8 @@ WatchpointSP Target::CreateWatchpoint(lldb::addr_t addr, size_t size,
     size_t old_size = matched_sp->GetByteSize();
     uint32_t old_type =
         (matched_sp->WatchpointRead() ? LLDB_WATCH_TYPE_READ : 0) |
-        (matched_sp->WatchpointWrite() ? LLDB_WATCH_TYPE_WRITE : 0);
+        (matched_sp->WatchpointWrite() ? LLDB_WATCH_TYPE_WRITE : 0) |
+        (matched_sp->WatchpointModify() ? LLDB_WATCH_TYPE_MODIFY : 0);
     // Return the existing watchpoint if both size and type match.
     if (size == old_size && kind == old_type) {
       wp_sp = matched_sp;

lldb/test/API/commands/watchpoints/multiple_hits/main.cpp

@@ -1,5 +1,6 @@
-#include <stdio.h>
 #include <stdint.h>
+#include <stdio.h>
+#include <string.h>
 alignas(16) uint8_t buf[32];
 // This uses inline assembly to generate an instruction that writes to a large
 // block of memory. If it fails on your compiler/architecture, please add
@@ -8,6 +9,7 @@ alignas(16) uint8_t buf[32];
 // this test.
 
 int main() {
+  memset(buf, UINT32_MAX, 8);
 #if defined(__i386__) || defined(__x86_64__)
   asm volatile ("movdqa %%xmm0, %0" : : "m"(buf));
 #elif defined(__arm__)

lldb/test/API/commands/watchpoints/unaligned-watchpoint/main.c

@@ -5,7 +5,7 @@ int main() {
     uint8_t buf[8];
     uint64_t val;
   } a;
-  a.val = 0; // break here
+  a.val = UINT64_MAX; // break here
   for (int i = 0; i < 5; i++) {
     a.val = i;
     printf("a.val is %lu\n", a.val);

lldb/test/API/functionalities/gdb_remote_client/TestNoWatchpointSupportInfo.py

@@ -58,7 +58,9 @@ class TestNoWatchpointSupportInfo(GDBRemoteTestBase):
             print(result.GetOutput())
 
         err = lldb.SBError()
-        wp = target.WatchAddress(0x100, 8, False, True, err)
+        wp_opts = lldb.SBWatchpointOptions()
+        wp_opts.SetWatchpointTypeWrite(lldb.eWatchpointWriteTypeOnModify)
+        wp = target.WatchpointCreateByAddress(0x100, 8, wp_opts, err)
         if self.TraceOn() and (err.Fail() or wp.IsValid == False):
             strm = lldb.SBStream()
             err.GetDescription(strm)

lldb/test/API/functionalities/watchpoint/large-watchpoint/TestLargeWatchpoint.py

@@ -45,7 +45,9 @@ class UnalignedWatchpointTestCase(TestBase):
         # to a 1024 byte boundary to begin with, force alignment.
         wa_256_addr = (array_addr + 1024) & ~(1024 - 1)
         err = lldb.SBError()
-        wp = target.WatchAddress(wa_256_addr, 1024, False, True, err)
+        wp_opts = lldb.SBWatchpointOptions()
+        wp_opts.SetWatchpointTypeWrite(lldb.eWatchpointWriteTypeOnModify)
+        wp = target.WatchpointCreateByAddress(wa_256_addr, 1024, wp_opts, err)
         self.assertTrue(wp.IsValid())
         self.assertSuccess(err)
 

lldb/test/API/functionalities/watchpoint/modify-watchpoints/Makefile

@@ -0,0 +1,3 @@
+C_SOURCES := main.c
+
+include Makefile.rules

lldb/test/API/functionalities/watchpoint/modify-watchpoints/TestModifyWatchpoint.py

@@ -0,0 +1,39 @@
+"""
+Confirm that lldb modify watchpoints only stop
+when the value being watched changes.
+"""
+
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class ModifyWatchpointTestCase(TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def test_modify_watchpoint(self):
+        """Test that a modify watchpoint only stops when the value changes."""
+        self.build()
+        self.main_source_file = lldb.SBFileSpec("main.c")
+        (target, process, thread, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "break here", self.main_source_file
+        )
+
+        self.runCmd("watch set variable value")
+        process.Continue()
+        frame = process.GetSelectedThread().GetFrameAtIndex(0)
+        self.assertEqual(frame.locals["value"][0].GetValueAsUnsigned(), 10)
+
+        process.Continue()
+        frame = process.GetSelectedThread().GetFrameAtIndex(0)
+        self.assertEqual(frame.locals["value"][0].GetValueAsUnsigned(), 5)
+
+        process.Continue()
+        frame = process.GetSelectedThread().GetFrameAtIndex(0)
+        self.assertEqual(frame.locals["value"][0].GetValueAsUnsigned(), 7)
+
+        process.Continue()
+        frame = process.GetSelectedThread().GetFrameAtIndex(0)
+        self.assertEqual(frame.locals["value"][0].GetValueAsUnsigned(), 9)

lldb/test/API/functionalities/watchpoint/modify-watchpoints/main.c

@@ -0,0 +1,20 @@
+#include <stdint.h>
+int main() {
+  int value = 5;
+
+  value = 5; // break here
+  value = 5;
+  value = 5;
+  value = 5;
+  value = 5;
+  value = 5;
+  value = 10;
+  value = 10;
+  value = 10;
+  value = 10;
+  value = 5;
+  value = 7;
+  value = 9;
+
+  return value;
+}

lldb/test/API/functionalities/watchpoint/unaligned-spanning-two-dwords/TestUnalignedSpanningDwords.py

@@ -46,7 +46,9 @@ class UnalignedWatchpointTestCase(TestBase):
         a_bytebuf_6 = frame.GetValueForVariablePath("a.bytebuf[6]")
         a_bytebuf_6_addr = a_bytebuf_6.GetAddress().GetLoadAddress(target)
         err = lldb.SBError()
-        wp = target.WatchAddress(a_bytebuf_6_addr, 4, False, True, err)
+        wp_opts = lldb.SBWatchpointOptions()
+        wp_opts.SetWatchpointTypeWrite(lldb.eWatchpointWriteTypeOnModify)
+        wp = target.WatchpointCreateByAddress(a_bytebuf_6_addr, 4, wp_opts, err)
         self.assertTrue(err.Success())
         self.assertTrue(wp.IsEnabled())
         self.assertEqual(wp.GetWatchSize(), 4)

lldb/test/API/python_api/default-constructor/sb_target.py

@@ -53,7 +53,10 @@ def fuzz_obj(obj):
     obj.GetByteOrder()
     obj.GetTriple()
     error = lldb.SBError()
-    obj.WatchAddress(123, 8, True, True, error)
+    wp_opts = lldb.SBWatchpointOptions()
+    wp_opts.SetWatchpointTypeRead(True)
+    wp_opts.SetWatchpointTypeWrite(lldb.eWatchpointWriteTypeOnModify)
+    obj.WatchpointCreateByAddress(123, 8, wp_opts, error)
     obj.GetBroadcaster()
     obj.GetDescription(lldb.SBStream(), lldb.eDescriptionLevelBrief)
     obj.Clear()

lldb/test/API/python_api/watchpoint/watchlocation/TestTargetWatchAddress.py

@@ -1,5 +1,5 @@
 """
-Use lldb Python SBtarget.WatchAddress() API to create a watchpoint for write of '*g_char_ptr'.
+Use lldb Python SBtarget.WatchpointCreateByAddress() API to create a watchpoint for write of '*g_char_ptr'.
 """
 
 import lldb
@@ -8,7 +8,7 @@ from lldbsuite.test.lldbtest import *
 from lldbsuite.test import lldbutil
 
 
-class TargetWatchAddressAPITestCase(TestBase):
+class TargetWatchpointCreateByAddressPITestCase(TestBase):
     NO_DEBUG_INFO_TESTCASE = True
 
     def setUp(self):
@@ -22,7 +22,7 @@ class TargetWatchAddressAPITestCase(TestBase):
         self.violating_func = "do_bad_thing_with_location"
 
     def test_watch_address(self):
-        """Exercise SBTarget.WatchAddress() API to set a watchpoint."""
+        """Exercise SBTarget.WatchpointCreateByAddress() API to set a watchpoint."""
         self.build()
         exe = self.getBuildArtifact("a.out")
 
@@ -51,8 +51,10 @@ class TargetWatchAddressAPITestCase(TestBase):
         )
         # Watch for write to *g_char_ptr.
         error = lldb.SBError()
-        watchpoint = target.WatchAddress(
-            value.GetValueAsUnsigned(), 1, False, True, error
+        wp_opts = lldb.SBWatchpointOptions()
+        wp_opts.SetWatchpointTypeWrite(lldb.eWatchpointWriteTypeOnModify)
+        watchpoint = target.WatchpointCreateByAddress(
+            value.GetValueAsUnsigned(), 1, wp_opts, error
         )
         self.assertTrue(
             value and watchpoint, "Successfully found the pointer and set a watchpoint"
@@ -90,7 +92,7 @@ class TargetWatchAddressAPITestCase(TestBase):
     @skipIf(archs=["mips", "mipsel", "mips64", "mips64el"])
     @skipIf(archs=["s390x"])  # Likewise on SystemZ
     def test_watch_address_with_invalid_watch_size(self):
-        """Exercise SBTarget.WatchAddress() API but pass an invalid watch_size."""
+        """Exercise SBTarget.WatchpointCreateByAddress() API but pass an invalid watch_size."""
         self.build()
         exe = self.getBuildArtifact("a.out")
 
@@ -124,8 +126,10 @@ class TargetWatchAddressAPITestCase(TestBase):
         if self.getArchitecture() not in ["arm64", "arm64e", "arm64_32"]:
             # Watch for write to *g_char_ptr.
             error = lldb.SBError()
-            watchpoint = target.WatchAddress(
-                value.GetValueAsUnsigned(), 365, False, True, error
+            wp_opts = lldb.SBWatchpointOptions()
+            wp_opts.SetWatchpointTypeWrite(lldb.eWatchpointWriteTypeOnModify)
+            watchpoint = target.WatchpointCreateByAddress(
+                value.GetValueAsUnsigned(), 365, wp_opts, error
             )
             self.assertFalse(watchpoint)
             self.expect(