[Dexter] Improve performance by evaluating expressions only when needed

Currently, Dexter's model for fetching watch values is to build a list of
expressions to watch before running the debugger, then evaluating all of them at
each breakpoint, then finally looking up the values of these expressions at each
line they were expected on. When using dexter on a large project while watching
many different expressions, this is very slow, as Dexter will make a massive
number of calls made to the debugger's API, the vast majority of which are not
being used for anything. This patch fixes this issue by having Dexter only
evaluate expressions at a breakpoint when it will be used by a Dexter command.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D107070

7 files changed, 79 insertions, 26 deletions

diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/command/CommandBase.py b/cross-project-tests/debuginfo-tests/dexter/dex/command/CommandBase.py
index 49e908623dfd..fdeb97f71f29 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/command/CommandBase.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/command/CommandBase.py
@@ -10,8 +10,11 @@ which will then be executed by DExTer during debugging.
 """
 
 import abc
+from collections import namedtuple
 from typing import List
 
+StepExpectInfo = namedtuple('StepExpectInfo', 'expression, path, frame_idx, line_range')
+
 class CommandBase(object, metaclass=abc.ABCMeta):
     def __init__(self):
         self.path = None
diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectProgramState.py b/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectProgramState.py
index 26f97b61590e..24b760ae9146 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectProgramState.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectProgramState.py
@@ -10,7 +10,7 @@ during execution.
 
 from itertools import chain
 
-from dex.command.CommandBase import CommandBase
+from dex.command.CommandBase import CommandBase, StepExpectInfo
 from dex.dextIR import ProgramState, SourceLocation, StackFrame, DextIR
 
 def frame_from_dict(source: dict) -> StackFrame:
@@ -56,9 +56,23 @@ class DexExpectProgramState(CommandBase):
         return __class__.__name__
 
     def get_watches(self):
-        frame_expects = chain.from_iterable(frame.watches
-            for frame in self.expected_program_state.frames)
-        return set(frame_expects)
+        frame_expects = set()
+        for idx, frame in enumerate(self.expected_program_state.frames):
+            path = (frame.location.path if
+                    frame.location and frame.location.path else self.path)
+            line_range = (
+                range(frame.location.lineno, frame.location.lineno + 1)
+                if frame.location and frame.location.lineno else None)
+            for watch in frame.watches:
+                frame_expects.add(
+                    StepExpectInfo(
+                        expression=watch,
+                        path=path,
+                        frame_idx=idx,
+                        line_range=line_range
+                    )
+                )
+        return frame_expects
 
     def eval(self, step_collection: DextIR) -> bool:
         for step in step_collection.steps:
diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectWatchBase.py b/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectWatchBase.py
index e892f01619ca..1c2d544a237d 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectWatchBase.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/command/commands/DexExpectWatchBase.py
@@ -12,11 +12,13 @@
 import abc
 import difflib
 import os
+from collections import namedtuple
 
-from dex.command.CommandBase import CommandBase
+from dex.command.CommandBase import CommandBase, StepExpectInfo
 from dex.command.StepValueInfo import StepValueInfo
 
 
+
 class DexExpectWatchBase(CommandBase):
     def __init__(self, *args, **kwargs):
         if len(args) < 2:
@@ -68,7 +70,7 @@ class DexExpectWatchBase(CommandBase):
 
 
     def get_watches(self):
-        return [self.expression]
+        return [StepExpectInfo(self.expression, self.path, 0, range(self._from_line, self._to_line + 1))]
 
     @property
     def line_range(self):
@@ -149,11 +151,11 @@ class DexExpectWatchBase(CommandBase):
         return differences
 
     def eval(self, step_collection):
+        assert os.path.exists(self.path)
         for step in step_collection.steps:
             loc = step.current_location
 
             if (loc.path and os.path.exists(loc.path) and
-                os.path.exists(self.path) and
                 os.path.samefile(loc.path, self.path) and
                 loc.lineno in self.line_range):
                 try:
diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/DebuggerBase.py b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/DebuggerBase.py
index c31be774d0fa..a795b3b0a28e 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/DebuggerBase.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/DebuggerBase.py
@@ -13,11 +13,26 @@ import traceback
 import unittest
 
 from types import SimpleNamespace
+from dex.command.CommandBase import StepExpectInfo
 from dex.dextIR import DebuggerIR, FrameIR, LocIR, StepIR, ValueIR
 from dex.utils.Exceptions import DebuggerException
 from dex.utils.Exceptions import NotYetLoadedDebuggerException
 from dex.utils.ReturnCode import ReturnCode
 
+def watch_is_active(watch_info: StepExpectInfo, path, frame_idx, line_no):
+    _, watch_path, watch_frame_idx, watch_line_range = watch_info
+    # If this watch should only be active for a specific file...
+    if watch_path and os.path.isfile(watch_path):
+        # If the current path does not match the expected file, this watch is
+        # not active.
+        if not (path and os.path.isfile(path) and
+                os.path.samefile(path, watch_path)):
+            return False
+    if watch_frame_idx != frame_idx:
+        return False
+    if watch_line_range and line_no not in list(watch_line_range):
+        return False
+    return True
 
 class DebuggerBase(object, metaclass=abc.ABCMeta):
     def __init__(self, context):
diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/dbgeng/dbgeng.py b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/dbgeng/dbgeng.py
index c95aa54f7e6b..2b13398d0f30 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/dbgeng/dbgeng.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/dbgeng/dbgeng.py
@@ -9,7 +9,7 @@ import sys
 import os
 import platform
 
-from dex.debugger.DebuggerBase import DebuggerBase
+from dex.debugger.DebuggerBase import DebuggerBase, watch_is_active
 from dex.dextIR import FrameIR, LocIR, StepIR, StopReason, ValueIR
 from dex.dextIR import ProgramState, StackFrame, SourceLocation
 from dex.utils.Exceptions import DebuggerException, LoadDebuggerException
@@ -133,8 +133,14 @@ class DbgEng(DebuggerBase):
                                                            column=0),
                                    watches={})
           for expr in map(
-              lambda watch, idx=i: self.evaluate_expression(watch, idx),
-              watches):
+              # Filter out watches that are not active in the current frame,
+              # and then evaluate all the active watches.
+              lambda watch_info, idx=i:
+                self.evaluate_expression(watch_info.expression, idx),
+              filter(
+                  lambda watch_info, idx=i, line_no=loc.lineno, path=loc.path:
+                    watch_is_active(watch_info, path, idx, line_no),
+                  watches)):
               state_frame.watches[expr.expression] = expr
           state_frames.append(state_frame)
 
diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/lldb/LLDB.py b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/lldb/LLDB.py
index e8e893995874..410e31f2f5a2 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/lldb/LLDB.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/lldb/LLDB.py
@@ -12,7 +12,7 @@ import os
 from subprocess import CalledProcessError, check_output, STDOUT
 import sys
 
-from dex.debugger.DebuggerBase import DebuggerBase
+from dex.debugger.DebuggerBase import DebuggerBase, watch_is_active
 from dex.dextIR import FrameIR, LocIR, StepIR, StopReason, ValueIR
 from dex.dextIR import StackFrame, SourceLocation, ProgramState
 from dex.utils.Exceptions import DebuggerException, LoadDebuggerException
@@ -208,6 +208,7 @@ class LLDB(DebuggerBase):
                 'column': sb_line.GetColumn()
             }
             loc = LocIR(**loc_dict)
+            valid_loc_for_watch = loc.path and os.path.exists(loc.path)
 
             frame = FrameIR(
                 function=function, is_inlined=sb_frame.IsInlined(), loc=loc)
@@ -223,11 +224,18 @@ class LLDB(DebuggerBase):
                                      is_inlined=frame.is_inlined,
                                      location=SourceLocation(**loc_dict),
                                      watches={})
-            for expr in map(
-                lambda watch, idx=i: self.evaluate_expression(watch, idx),
-                watches):
-                state_frame.watches[expr.expression] = expr
-            state_frames.append(state_frame)
+            if valid_loc_for_watch:
+                for expr in map(
+                    # Filter out watches that are not active in the current frame,
+                    # and then evaluate all the active watches.
+                    lambda watch_info, idx=i:
+                        self.evaluate_expression(watch_info.expression, idx),
+                    filter(
+                        lambda watch_info, idx=i, line_no=loc.lineno, loc_path=loc.path:
+                            watch_is_active(watch_info, loc_path, idx, line_no),
+                        watches)):
+                    state_frame.watches[expr.expression] = expr
+                state_frames.append(state_frame)
 
         if len(frames) == 1 and frames[0].function is None:
             frames = []
diff --git a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/visualstudio/VisualStudio.py b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/visualstudio/VisualStudio.py
index b4558e2d8a50..e36b353cef6f 100644
--- a/cross-project-tests/debuginfo-tests/dexter/dex/debugger/visualstudio/VisualStudio.py
+++ b/cross-project-tests/debuginfo-tests/dexter/dex/debugger/visualstudio/VisualStudio.py
@@ -11,10 +11,10 @@ import imp
 import os
 import sys
 from pathlib import PurePath
-from collections import namedtuple
-from collections import defaultdict
+from collections import defaultdict, namedtuple
 
-from dex.debugger.DebuggerBase import DebuggerBase
+from dex.command.CommandBase import StepExpectInfo
+from dex.debugger.DebuggerBase import DebuggerBase, watch_is_active
 from dex.dextIR import FrameIR, LocIR, StepIR, StopReason, ValueIR
 from dex.dextIR import StackFrame, SourceLocation, ProgramState
 from dex.utils.Exceptions import Error, LoadDebuggerException
@@ -244,6 +244,9 @@ class VisualStudio(DebuggerBase, metaclass=abc.ABCMeta):  # pylint: disable=abst
         state_frames = []
 
 
+        loc = LocIR(**self._location)
+        valid_loc_for_watch = loc.path and os.path.exists(loc.path)
+
         for idx, sf in enumerate(stackframes):
             frame = FrameIR(
                 function=self._sanitize_function_name(sf.FunctionName),
@@ -254,20 +257,20 @@ class VisualStudio(DebuggerBase, metaclass=abc.ABCMeta):  # pylint: disable=abst
             if any(name in fname for name in self.frames_below_main):
                 break
 
-
             state_frame = StackFrame(function=frame.function,
                                      is_inlined=frame.is_inlined,
                                      watches={})
 
-            for watch in watches:
-                state_frame.watches[watch] = self.evaluate_expression(
-                    watch, idx)
+            if valid_loc_for_watch and idx == 0:
+                for watch_info in watches:
+                    if watch_is_active(watch_info, loc.path, idx, loc.lineno):
+                        watch_expr = watch_info.expression
+                        state_frame.watches[watch_expr] = self.evaluate_expression(watch_expr, idx)
 
 
             state_frames.append(state_frame)
             frames.append(frame)
 
-        loc = LocIR(**self._location)
         if frames:
             frames[0].loc = loc
             state_frames[0].location = SourceLocation(**self._location)
@@ -298,9 +301,11 @@ class VisualStudio(DebuggerBase, metaclass=abc.ABCMeta):  # pylint: disable=abst
         ]
 
     def evaluate_expression(self, expression, frame_idx=0) -> ValueIR:
-        self.set_current_stack_frame(frame_idx)
+        if frame_idx != 0:
+            self.set_current_stack_frame(frame_idx)
         result = self._debugger.GetExpression(expression)
-        self.set_current_stack_frame(0)
+        if frame_idx != 0:
+            self.set_current_stack_frame(0)
         value = result.Value
 
         is_optimized_away = any(s in value for s in [