[clang][Sema] Add instant event when template instantiation is deferred.

[ivanaivanovska]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: None (ivanaivanovska)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/111524.diff

3 Files Affected:

• (modified) clang/lib/Sema/SemaExpr.cpp (+12)
• (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+10)
• (modified) clang/unittests/Support/TimeProfilerTest.cpp (+49-2)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index f930a21ea870ec..e786277d7cffd1 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/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>
 
@@ -18146,6 +18148,16 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
                 std::make_pair(Func, PointOfInstantiation));
             // Notify the consumer that a function was implicitly instantiated.
             Consumer.HandleCXXImplicitFunctionInstantiation(Func);
+
+            if (llvm::isTimeTraceVerbose()) {
+              llvm::timeTraceAddInstantEvent("DeferInstantiation", [&] {
+                std::string Name;
+                llvm::raw_string_ostream OS(Name);
+                Func->getNameForDiagnostic(OS, getPrintingPolicy(),
+                                           /*Qualified=*/true);
+                return Name;
+              });
+            }
           }
         }
       } else {
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 1c35c7d288e325..34558e1a005d5a 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4986,6 +4986,16 @@ 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())) {
diff --git a/clang/unittests/Support/TimeProfilerTest.cpp b/clang/unittests/Support/TimeProfilerTest.cpp
index f53fe71d630bf5..339b470153e64e 100644
--- a/clang/unittests/Support/TimeProfilerTest.cpp
+++ b/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));