[AST][Introspection] Avoid creating temporary strings when comparing LocationCalls.

[njames93]

Summary

Adds a bit of boiler plate, but removing the need to build temporary strings to compare the calls definitely worth it.

[steveire]

clang/lib/Tooling/NodeIntrospection.cpp

75

Would it make sense to compare the sizes (leftsize < rightsize) return true etc and only loop if the containers are the same size?

[njames93]

clang/lib/Tooling/NodeIntrospection.cpp

75

That wouldn't work as we are comparing lexicographically. If leftsize was smaller than rightsize, but the first item in left was zzzz, and first in right was aaaa. We'd want right to be marked as less then left.

[steveire]

clang/lib/Tooling/NodeIntrospection.cpp

75

I don't think we would want that. All we need is to be able to put these things in a std set. We only need uniquenes and deterministic order. We don't need or want any particular order at all.

Besides, your implementation orders elements as they are formatted for c++, which could differ from the order the calls would be in if casts are omitted (js, python).

Besides, https://en.m.wikipedia.org/wiki/Lexicographic_order "One variant applies to sequences of different lengths by comparing the lengths of the sequences before considering their elements."

It should be possible to implement this without creating the local vectors. Just iterate in parallel through the location calls, comparing names, and if one chain ends before the other return early. That would seem to have all the properties we need, right?

[njames93]

Rebase and remove Args from comparison.
Add quick check for skippig common prefix calls.

[steveire]

Your implementation is getting very complicated and it requires many comments.

Also, if we get to this point in the execution of RangeLessThan::operator(), you're creating and populating two vectors for every two elements compared. I know it's llvm::SmallVector and maybe not so expensive etc, but doing something is more expensive than not doing it.

You're trying to sort elements (which have the same location/range) alphabetically according to how it will be sorted after c++ formatting. Sorting according to c++ formatting means sorting incorrectly according to JS/python. This is simple to demonstrate:

TEST(Introspection, SourceLocations_CallContainer3) {

  SourceLocationMap slm;
  SharedLocationCall Prefix;
  auto call1 = llvm::makeIntrusiveRefCnt<LocationCall>(
                                        llvm::makeIntrusiveRefCnt<LocationCall>(
                                        llvm::makeIntrusiveRefCnt<LocationCall>(
                                        Prefix, "dddd"), "eeee", LocationCall::IsCast), "ffff");
  auto call2 = llvm::makeIntrusiveRefCnt<LocationCall>(
                                        llvm::makeIntrusiveRefCnt<LocationCall>(
                                        llvm::makeIntrusiveRefCnt<LocationCall>(
                                        Prefix, "dddd"), "gggg", LocationCall::IsCast), "bbbb");
  slm.insert(
      std::make_pair(SourceLocation(), call1));
  slm.insert(
      std::make_pair(SourceLocation(), call2));

  for (auto& item : slm)
  {
    llvm::errs() << LocationCallFormatterCpp::format(*item.second) << "\n";
  }
  for (auto& item : slm)
  {
    llvm::errs() << LocationCallFormatterSimple::format(*item.second) << "\n";
  }
}

output:

dddd().eeee().ffff()
dddd().gggg().bbbb()
dddd().ffff()
dddd().bbbb()

So, the cost of creating two vectors for each invocation of lessThan that reaches this point delivers no benefit.

Here is an implementation which does not create those vectors:

diff --git a/clang/lib/Tooling/NodeIntrospection.cpp b/clang/lib/Tooling/NodeIntrospection.cpp
index 8e2d446c3efe..6e846acf9ecf 100644
--- a/clang/lib/Tooling/NodeIntrospection.cpp
+++ b/clang/lib/Tooling/NodeIntrospection.cpp
@@ -40,6 +40,22 @@ std::string LocationCallFormatterCpp::format(const LocationCall &Call) {
   return Result;
 }
 
+bool locationLessThan(const LocationCall *LHS, const LocationCall *RHS)
+{
+  if (!LHS && !RHS)
+    return false;
+  if (LHS && !RHS)
+    return true;
+  if (!LHS && RHS)
+    return false;
+  auto compareResult = LHS->name().compare(RHS->name());
+  if (compareResult < 0)
+    return true;
+  if (compareResult > 0)
+    return false;
+  return locationLessThan(LHS->on(), RHS->on());
+}
+
 namespace internal {
 bool RangeLessThan::operator()(
     std::pair<SourceRange, SharedLocationCall> const &LHS,
@@ -54,15 +70,13 @@ bool RangeLessThan::operator()(
   else if (LHS.first.getEnd() != RHS.first.getEnd())
     return false;
 
-  return LocationCallFormatterCpp::format(*LHS.second) <
-         LocationCallFormatterCpp::format(*RHS.second);
+  return locationLessThan(LHS.second.get(), RHS.second.get());
 }
 bool RangeLessThan::operator()(
     std::pair<SourceLocation, SharedLocationCall> const &LHS,
     std::pair<SourceLocation, SharedLocationCall> const &RHS) const {
   if (LHS.first == RHS.first)
-    return LocationCallFormatterCpp::format(*LHS.second) <
-           LocationCallFormatterCpp::format(*RHS.second);
+    return locationLessThan(LHS.second.get(), RHS.second.get());
   return LHS.first < RHS.first;
 }
 } // namespace internal

clang/lib/Tooling/NodeIntrospection.cpp

 //===- NodeIntrospection.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
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file contains the implementation of the NodeIntrospection.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/Tooling/NodeIntrospection.h"
 
 #include "clang/AST/AST.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace clang {
 
 namespace tooling {
 
 void LocationCallFormatterCpp::print(const LocationCall &Call,
                                      llvm::raw_ostream &OS) {

 std::string LocationCallFormatterCpp::format(const LocationCall &Call) {
   std::string Result;
   llvm::raw_string_ostream OS(Result);
   print(Call, OS);
   OS.flush();
   return Result;
 }
 
+/// A 3 way comparison of LHS and RHS based on call names.
+static int compareLocationCall(const LocationCall &LHS,
+                               const LocationCall &RHS) {
+  // Expand the call stacks into a vector so we can traverse it in reverse
+  // order.
+  llvm::SmallVector<const LocationCall *> Left, Right;
+  for (const LocationCall *Item = &LHS; Item; Item = Item->on())
+    Left.push_back(Item);
+  for (const LocationCall *Item = &RHS; Item; Item = Item->on())
+    Right.push_back(Item);
+
+  auto LI = Left.rbegin(), LE = Left.rend(), RI = Right.rbegin();
+  // It's common for 2 location calls to start with the same prefix:
+  // getTypeSourceInfo()->getTypeLoc().getAs<..>().getLocalSourceRange()
+  // getTypeSourceInfo()->getTypeLoc().getLocalSourceRange()
+  // We can use pointer identity to quickly try and skip these cases.
+  for (; LI != LE && *LI == *RI; *LI++, *RI++)
+    assert(RI != Right.rend());
+  for (; LI != LE; ++LI, ++RI) {
+    // If we haven't returned up to here, both call stacks are the same up the
+    // here are the same. If we detect that RI is at the end, that means the
+    // rights last item was a SourceLocation/Range accessor. Therefore the
+    // preceding item in left should also return a Loc/Range and it should be
+    // the last item in its stack.
+    assert(RI != Right.rend());
+    if (auto Strcmp = (*LI)->name().compare((*RI)->name()))
+      return Strcmp;
+    // As both calls are the same, they should both either return by value or
+    // both return by pointer.
+    assert((*LI)->returnsPointer() == (*RI)->returnsPointer());
+  }
+  // As before but flipped.
+  assert(RI == Right.rend());
+  return 0;
+}
+
 namespace internal {
 bool RangeLessThan::operator()(
     std::pair<SourceRange, SharedLocationCall> const &LHS,
     std::pair<SourceRange, SharedLocationCall> const &RHS) const {
   if (!LHS.first.isValid() || !RHS.first.isValid())
     return false;
 
   if (LHS.first.getBegin() < RHS.first.getBegin())
     return true;
   else if (LHS.first.getBegin() != RHS.first.getBegin())
     return false;
 
   if (LHS.first.getEnd() < RHS.first.getEnd())
     return true;
   else if (LHS.first.getEnd() != RHS.first.getEnd())
     return false;
 
-  return LocationCallFormatterCpp::format(*LHS.second) <
-         LocationCallFormatterCpp::format(*RHS.second);
+  return compareLocationCall(*LHS.second, *RHS.second) < 0;
 }
 bool RangeLessThan::operator()(
     std::pair<SourceLocation, SharedLocationCall> const &LHS,
     std::pair<SourceLocation, SharedLocationCall> const &RHS) const {
   if (LHS.first == RHS.first)
-    return LocationCallFormatterCpp::format(*LHS.second) <
-           LocationCallFormatterCpp::format(*RHS.second);
+    return compareLocationCall(*LHS.second, *RHS.second) < 0;
   return LHS.first < RHS.first;
 }
 } // namespace internal
 
 } // namespace tooling
 } // namespace clang
 
 #include "clang/Tooling/NodeIntrospection.inc"