[lldb] Avoid (unlimited) GetNumChildren calls when printing values (#93946)

For some data formatters, even getting the number of children can be an
expensive operations (e.g., needing to walk a linked list to determine
the number of elements). This is then wasted work when we know we will
be printing only small number of them.

This patch replaces the calls to GetNumChildren (at least those on the
"frame var" path) with the calls to the capped version, passing the
value of `max-children-count` setting (plus one)

Author: labath

lldb/source/DataFormatters/FormatManager.cpp

@@ -9,6 +9,7 @@
 #include "lldb/DataFormatters/FormatManager.h"
 
 #include "lldb/Core/Debugger.h"
+#include "lldb/Core/ValueObject.h"
 #include "lldb/DataFormatters/FormattersHelpers.h"
 #include "lldb/DataFormatters/LanguageCategory.h"
 #include "lldb/Interpreter/ScriptInterpreter.h"
@@ -448,16 +449,19 @@ lldb::Format FormatManager::GetSingleItemFormat(lldb::Format vector_format) {
 }
 
 bool FormatManager::ShouldPrintAsOneLiner(ValueObject &valobj) {
+  TargetSP target_sp = valobj.GetTargetSP();
   // if settings say no oneline whatsoever
-  if (valobj.GetTargetSP().get() &&
-      !valobj.GetTargetSP()->GetDebugger().GetAutoOneLineSummaries())
+  if (target_sp && !target_sp->GetDebugger().GetAutoOneLineSummaries())
     return false; // then don't oneline
 
   // if this object has a summary, then ask the summary
   if (valobj.GetSummaryFormat().get() != nullptr)
     return valobj.GetSummaryFormat()->IsOneLiner();
 
-  auto num_children = valobj.GetNumChildren();
+  const size_t max_num_children =
+      (target_sp ? *target_sp : Target::GetGlobalProperties())
+          .GetMaximumNumberOfChildrenToDisplay();
+  auto num_children = valobj.GetNumChildren(max_num_children);
   if (!num_children) {
     llvm::consumeError(num_children.takeError());
     return true;

lldb/source/DataFormatters/ValueObjectPrinter.cpp

@@ -14,6 +14,8 @@
 #include "lldb/Target/Language.h"
 #include "lldb/Target/Target.h"
 #include "lldb/Utility/Stream.h"
+#include "llvm/Support/MathExtras.h"
+#include <cstdint>
 
 using namespace lldb;
 using namespace lldb_private;
@@ -628,22 +630,21 @@ ValueObjectPrinter::GetMaxNumChildrenToPrint(bool &print_dotdotdot) {
   if (m_options.m_pointer_as_array)
     return m_options.m_pointer_as_array.m_element_count;
 
-  auto num_children_or_err = synth_valobj.GetNumChildren();
+  const uint32_t max_num_children =
+      m_options.m_ignore_cap ? UINT32_MAX
+                             : GetMostSpecializedValue()
+                                   .GetTargetSP()
+                                   ->GetMaximumNumberOfChildrenToDisplay();
+  // Ask for one more child than the maximum to see if we should print "...".
+  auto num_children_or_err = synth_valobj.GetNumChildren(
+      llvm::SaturatingAdd(max_num_children, uint32_t(1)));
   if (!num_children_or_err)
     return num_children_or_err;
-  uint32_t num_children = *num_children_or_err;
-  print_dotdotdot = false;
-  if (num_children) {
-    const size_t max_num_children = GetMostSpecializedValue()
-                                        .GetTargetSP()
-                                        ->GetMaximumNumberOfChildrenToDisplay();
-
-    if (num_children > max_num_children && !m_options.m_ignore_cap) {
-      print_dotdotdot = true;
-      return max_num_children;
-    }
+  if (*num_children_or_err > max_num_children) {
+    print_dotdotdot = true;
+    return max_num_children;
   }
-  return num_children;
+  return num_children_or_err;
 }
 
 void ValueObjectPrinter::PrintChildrenPostamble(bool print_dotdotdot) {

lldb/test/API/functionalities/data-formatter/synthcapping/TestSyntheticCapping.py

@@ -68,6 +68,11 @@ def cleanup():
                 "r = 34",
             ],
         )
+        # num_children() should be called with at most max_num_children=257
+        # (target.max-children-count + 1)
+        self.expect(
+            "script fooSynthProvider.reset_max_num_children_max()", substrs=["257"]
+        )
 
         # check that capping works
         self.runCmd("settings set target.max-children-count 2", check=False)
@@ -80,9 +85,15 @@ def cleanup():
                 "...",
             ],
         )
+        self.expect(
+            "script fooSynthProvider.reset_max_num_children_max()", substrs=["3"]
+        )
 
         self.expect("frame variable f00_1", matching=False, substrs=["r = 34"])
 
         self.runCmd("settings set target.max-children-count 256", check=False)
 
         self.expect("frame variable f00_1", matching=True, substrs=["r = 34"])
+        self.expect(
+            "script fooSynthProvider.reset_max_num_children_max()", substrs=["257"]
+        )

lldb/test/API/functionalities/data-formatter/synthcapping/fooSynthProvider.py

@@ -2,11 +2,24 @@
 
 
 class fooSynthProvider:
+    # For testing purposes, we'll keep track of the maximum value of
+    # max_num_children we've been called with.
+    MAX_NUM_CHILDREN_MAX = 0
+
+    @classmethod
+    def reset_max_num_children_max(cls):
+        old_value = fooSynthProvider.MAX_NUM_CHILDREN_MAX
+        fooSynthProvider.MAX_NUM_CHILDREN_MAX = 0
+        return old_value
+
     def __init__(self, valobj, dict):
         self.valobj = valobj
         self.int_type = valobj.GetType().GetBasicType(lldb.eBasicTypeInt)
 
-    def num_children(self):
+    def num_children(self, max_num_children):
+        fooSynthProvider.MAX_NUM_CHILDREN_MAX = max(
+            fooSynthProvider.MAX_NUM_CHILDREN_MAX, max_num_children
+        )
         return 3
 
     def get_child_at_index(self, index):