[analyzer][NFC] Explain why operator new/delete should never be eval-called (#161370)

Downstream, some change triggered an investigation if we could move a
checker callback from check::PostCall to eval::Call. After a lengthy
investigation that lead to ExprEngine::VisitCXXNewExpr we realized that
CXXNewExprs only trigger a PreCall and PostCall, but never an EvalCall.
It also had a FIXME that maybe it should trigger it.

Remember, it called `defaultEvalCall` which either inlines or
conservatively evaluates aka. invalidates the call. But never probes the
checker eval-calls to see if any would step in.

After implementing the changes to trigger the eval call for the
checkers, I realized that it doesn't really make sense because we are
eval-calling user-provided functions, that we can't be really sure about
their semantics, thus there is no generic way to properly implement the
eval call callback.
This touches on an important point. It only ever makes sense to eval
call functions that has a clear spec. such as standard functions, as
implementing the callback would prevent the inlining of that function,
risking regressing analysis quality if the implemented model is not
complete/correct enough.

As a conclusion, I opted for not exposing the eval call event to
checkers, in other words, keep everything as-is, but document my
journey.

CPP-6585

clang/lib/StaticAnalyzer/Checkers/AnalysisOrderChecker.cpp

@@ -129,7 +129,8 @@ public:
       llvm::errs() << " {argno: " << Call.getNumArgs() << '}';
       llvm::errs() << " [" << Call.getKindAsString() << ']';
       llvm::errs() << '\n';
-      return true;
+      // We can't return `true` from this callback without binding the return
+      // value. Let's just fallthrough here and return `false`.
     }
     return false;
   }

clang/lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp

@@ -262,6 +262,15 @@ public:
   /// state. This callback allows a checker to provide domain specific knowledge
   /// about the particular functions it knows about.
   ///
+  /// Note that to evaluate a call, the handler MUST bind the return value if
+  /// its a non-void function. Invalidate the arguments if necessary.
+  ///
+  /// Note that in general, user-provided functions should not be eval-called
+  /// because the checker can't predict the exact semantics/contract of the
+  /// callee, and by having the eval::Call callback, we also prevent it from
+  /// getting inlined, potentially regressing analysis quality.
+  /// Consider using check::PreCall or check::PostCall to allow inlining.
+  ///
   /// \returns true if the call has been successfully evaluated
   /// and false otherwise. Note, that only one checker can evaluate a call. If
   /// more than one checker claims that they can evaluate the same call the

clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp

@@ -909,7 +909,14 @@ void ExprEngine::VisitCXXNewAllocatorCall(const CXXNewExpr *CNE,
   ExplodedNodeSet DstPostCall;
   StmtNodeBuilder CallBldr(DstPreCall, DstPostCall, *currBldrCtx);
   for (ExplodedNode *I : DstPreCall) {
-    // FIXME: Provide evalCall for checkers?
+    // Operator new calls (CXXNewExpr) are intentionally not eval-called,
+    // because it does not make sense to eval-call user-provided functions.
+    // 1) If the new operator can be inlined, then don't prevent it from
+    //    inlining by having an eval-call of that operator.
+    // 2) If it can't be inlined, then the default conservative modeling
+    //    is what we want anyway.
+    // So the best is to not allow eval-calling CXXNewExprs from checkers.
+    // Checkers can provide their pre/post-call callbacks if needed.
     defaultEvalCall(CallBldr, I, *Call);
   }
   // If the call is inlined, DstPostCall will be empty and we bail out now.
@@ -1110,6 +1117,10 @@ void ExprEngine::VisitCXXDeleteExpr(const CXXDeleteExpr *CDE,
   if (AMgr.getAnalyzerOptions().MayInlineCXXAllocator) {
     StmtNodeBuilder Bldr(DstPreCall, DstPostCall, *currBldrCtx);
     for (ExplodedNode *I : DstPreCall) {
+      // Intentionally either inline or conservative eval-call the operator
+      // delete, but avoid triggering an eval-call event for checkers.
+      // As detailed at handling CXXNewExprs, in short, because it does not
+      // really make sense to eval-call user-provided functions.
       defaultEvalCall(Bldr, I, *Call);
     }
   } else {

clang/test/Analysis/cxxctr-evalcall-analysis-order.cpp

@@ -18,16 +18,33 @@ void foo() {
   C C0;
   C C1(42);
   C *C2 = new C{2, 3};
+  delete C2;
 }
 
 // CHECK:  PreCall (C::C) [CXXConstructorCall]
 // CHECK-NEXT:  EvalCall (C::C) {argno: 0} [CXXConstructorCall]
 // CHECK-NEXT:  PostCall (C::C) [CXXConstructorCall]
+
 // CHECK-NEXT:  PreCall (C::C) [CXXConstructorCall]
 // CHECK-NEXT:  EvalCall (C::C) {argno: 1} [CXXConstructorCall]
 // CHECK-NEXT:  PostCall (C::C) [CXXConstructorCall]
+
 // CHECK-NEXT:  PreCall (operator new) [CXXAllocatorCall]
+//    COMMENT: Operator new calls (CXXNewExpr) are intentionally not eval-called,
+//    COMMENT: because it does not make sense to eval call user-provided functions.
+//    COMMENT: 1) If the new operator can be inlined, then don't prevent it from
+//    COMMENT:    inlining by having an eval-call of that operator.
+//    COMMENT: 2) If it can't be inlined, then the default conservative modeling
+//    COMMENT:    is what we anyways want anyway.
+//    COMMENT: So the EvalCall event will not be triggered for operator new calls.
+// CHECK-NOT:   EvalCall
 // CHECK-NEXT:  PostCall (operator new) [CXXAllocatorCall]
+
 // CHECK-NEXT:  PreCall (C::C) [CXXConstructorCall]
 // CHECK-NEXT:  EvalCall (C::C) {argno: 2} [CXXConstructorCall]
 // CHECK-NEXT:  PostCall (C::C) [CXXConstructorCall]
+
+// CHECK-NEXT: PreCall (operator delete) [CXXDeallocatorCall]
+//    COMMENT: Same reasoning as for CXXNewExprs above.
+// CHECK-NOT:  EvalCall
+// CHECK-NEXT: PostCall (operator delete) [CXXDeallocatorCall]