[clang][Sema] Add instant event when template instantiation is deferred. (#111524)

While profiling a clang invocation using `-ftime-trace`, now we add
instant events when template instantiation is deferred.
These events include the fully qualified name of the function template
being deferred and therefore could be very verbose. This is therefore
only added in verbose mode (when `TimeTraceVerbose` is enabled).

The point of time when a particular instantiation is deferred can be
used to identify the parent TimeTrace scope (usually another function
instantiation), which is responsible for deferring this instantiation.
This relationship can be used to attribute the cost of a deferred
template instantiation to the function deferring this particular
instantiation.

Author: ivanaivanovska

clang/lib/Sema/SemaExpr.cpp

@@ -18,6 +18,7 @@
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/CXXInheritance.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/EvaluatedExprVisitor.h"
@@ -65,6 +66,7 @@
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/SaveAndRestore.h"
+#include "llvm/Support/TimeProfiler.h"
 #include "llvm/Support/TypeSize.h"
 #include <optional>
 
@@ -18144,6 +18146,15 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
             Func->setInstantiationIsPending(true);
             PendingInstantiations.push_back(
                 std::make_pair(Func, PointOfInstantiation));
+            if (llvm::isTimeTraceVerbose()) {
+              llvm::timeTraceAddInstantEvent("DeferInstantiation", [&] {
+                std::string Name;
+                llvm::raw_string_ostream OS(Name);
+                Func->getNameForDiagnostic(OS, getPrintingPolicy(),
+                                           /*Qualified=*/true);
+                return Name;
+              });
+            }
             // Notify the consumer that a function was implicitly instantiated.
             Consumer.HandleCXXImplicitFunctionInstantiation(Func);
           }

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -5015,6 +5015,16 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
       Function->setInstantiationIsPending(true);
       PendingInstantiations.push_back(
         std::make_pair(Function, PointOfInstantiation));
+
+      if (llvm::isTimeTraceVerbose()) {
+        llvm::timeTraceAddInstantEvent("DeferInstantiation", [&] {
+          std::string Name;
+          llvm::raw_string_ostream OS(Name);
+          Function->getNameForDiagnostic(OS, getPrintingPolicy(),
+                                         /*Qualified=*/true);
+          return Name;
+        });
+      }
     } else if (TSK == TSK_ImplicitInstantiation) {
       if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
           !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {

clang/unittests/Support/TimeProfilerTest.cpp

@@ -238,13 +238,55 @@ Frontend (test.cc)
             buildTraceGraph(Json));
 }
 
+TEST(TimeProfilerTest, ClassTemplateInstantiations) {
+  std::string Code = R"(
+    template<class T>
+    struct S
+    {
+      void foo() {}
+      void bar();
+    };
+
+    template struct S<double>; // explicit instantiation of S<double>
+
+    void user() {
+      S<int> a; // implicit instantiation of S<int>
+      S<float>* b;
+      b->foo(); // implicit instatiation of S<float> and S<float>::foo()
+    }
+  )";
+
+  setupProfiler();
+  ASSERT_TRUE(compileFromString(Code, "-std=c++20", "test.cc"));
+  std::string Json = teardownProfiler();
+  ASSERT_EQ(R"(
+Frontend (test.cc)
+| ParseClass (S)
+| InstantiateClass (S<double>, test.cc:9)
+| InstantiateFunction (S<double>::foo, test.cc:5)
+| ParseDeclarationOrFunctionDefinition (test.cc:11:5)
+| | ParseFunctionDefinition (user)
+| | | InstantiateClass (S<int>, test.cc:3)
+| | | InstantiateClass (S<float>, test.cc:3)
+| | | DeferInstantiation (S<float>::foo)
+| PerformPendingInstantiations
+| | InstantiateFunction (S<float>::foo, test.cc:5)
+)",
+            buildTraceGraph(Json));
+}
+
 TEST(TimeProfilerTest, TemplateInstantiations) {
   std::string B_H = R"(
     template <typename T>
-    T fooB(T t) {
+    T fooC(T t) {
       return T();
     }
 
+    template <typename T>
+    constexpr T fooB(T t) {
+      return fooC(t);
+    }
+
     #define MacroTemp(x) template <typename T> void foo##x(T) { T(); }
   )";
 
@@ -267,14 +309,19 @@ TEST(TimeProfilerTest, TemplateInstantiations) {
   std::string Json = teardownProfiler();
   ASSERT_EQ(R"(
 Frontend (test.cc)
+| ParseFunctionDefinition (fooC)
 | ParseFunctionDefinition (fooB)
 | ParseFunctionDefinition (fooMTA)
 | ParseFunctionDefinition (fooA)
 | ParseDeclarationOrFunctionDefinition (test.cc:3:5)
 | | ParseFunctionDefinition (user)
+| | | DeferInstantiation (fooA<int>)
 | PerformPendingInstantiations
 | | InstantiateFunction (fooA<int>, a.h:7)
-| | | InstantiateFunction (fooB<int>, b.h:3)
+| | | InstantiateFunction (fooB<int>, b.h:8)
+| | | | DeferInstantiation (fooC<int>)
+| | | DeferInstantiation (fooMTA<int>)
+| | | InstantiateFunction (fooC<int>, b.h:3)
 | | | InstantiateFunction (fooMTA<int>, a.h:4)
 )",
             buildTraceGraph(Json));