[Fix] Speedup -Wunsafe-buffer-usage when using clang modules. (llvm#127161)

Each piece of code should have analysis run on it precisely once.
However, if you build a module, and then build another module depending
on it, the header file of the module you depend on will have
`-Wunsafe-buffer-usage` run on it twice. This normally isn't a huge
issue, but in the case of using the standard library as a module, simply
adding the line `#include <cstddef>` increases compile times by 900ms
(from 100ms to 1 second) on my machine. I believe this is because the
standard library has massive modules, of which only a small part is used
(the AST is ~700k lines), and because if what I've been told is correct,
the AST is lazily generated, and `-Wunsafe-buffer-usage` forces it to be
evaluated every time.

See https://issues.chromium.org/issues/351909443 for details and
benchmarks.

Author: matts1

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -2532,14 +2532,25 @@ static void flushDiagnostics(Sema &S, const sema::FunctionScopeInfo *fscope) {
 class CallableVisitor : public DynamicRecursiveASTVisitor {
 private:
   llvm::function_ref<void(const Decl *)> Callback;
+  const Module *const TUModule;
 
 public:
-  CallableVisitor(llvm::function_ref<void(const Decl *)> Callback)
-      : Callback(Callback) {
+  CallableVisitor(llvm::function_ref<void(const Decl *)> Callback,
+                  const Module *const TUModule)
+      : Callback(Callback), TUModule(TUModule) {
     ShouldVisitTemplateInstantiations = true;
     ShouldVisitImplicitCode = false;
   }
 
+  bool TraverseDecl(Decl *Node) override {
+    // For performance reasons, only validate the current translation unit's
+    // module, and not modules it depends on.
+    // See https://issues.chromium.org/issues/351909443 for details.
+    if (Node && Node->getOwningModule() == TUModule)
+      return DynamicRecursiveASTVisitor::TraverseDecl(Node);
+    return true;
+  }
+
   bool VisitFunctionDecl(FunctionDecl *Node) override {
     if (cast<DeclContext>(Node)->isDependentContext())
       return true; // Not to analyze dependent decl
@@ -2622,7 +2633,8 @@ void clang::sema::AnalysisBasedWarnings::IssueWarnings(
                        SourceLocation()) ||
       (!Diags.isIgnored(diag::warn_unsafe_buffer_libc_call, SourceLocation()) &&
        S.getLangOpts().CPlusPlus /* only warn about libc calls in C++ */)) {
-    CallableVisitor(CallAnalyzers).TraverseTranslationUnitDecl(TU);
+    CallableVisitor(CallAnalyzers, TU->getOwningModule())
+        .TraverseTranslationUnitDecl(TU);
   }
 }
 

clang/test/Modules/safe_buffers_optout.cpp

@@ -95,18 +95,10 @@ int textual(int *p) {
 // `safe_buffers_test_optout`, which uses another top-level module
 // `safe_buffers_test_base`. (So the module dependencies form a DAG.)
 
-// No expected warnings from base.h because base.h is a separate
-// module and in a separate TU that is not textually included.  The
-// explicit command that builds base.h has no `-Wunsafe-buffer-usage`.
-
-// [email protected]:3{{unsafe buffer access}}
-// [email protected]:3{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
-// expected-warning@test_sub1.h:5{{unsafe buffer access}}
-// expected-note@test_sub1.h:5{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
-// expected-warning@test_sub1.h:14{{unsafe buffer access}}
-// expected-note@test_sub1.h:14{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
-// expected-warning@test_sub2.h:5{{unsafe buffer access}}
-// expected-note@test_sub2.h:5{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
+// No expected warnings from base.h, test_sub1, or test_sub2 because they are
+// in seperate modules, and the explicit commands that builds them have no
+// `-Wunsafe-buffer-usage`.
+
 int foo(int * p) {
   int x = p[5]; // expected-warning{{unsafe buffer access}} expected-note{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
 #pragma clang unsafe_buffer_usage begin
@@ -129,14 +121,10 @@ int foo(int * p) {
 // `safe_buffers_test_optout`, which uses another top-level module
 // `safe_buffers_test_base`. (So the module dependencies form a DAG.)
 
-// [email protected]:3{{unsafe buffer access}}
-// [email protected]:3{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
-// expected-warning@test_sub1.h:5{{unsafe buffer access}}
-// expected-note@test_sub1.h:5{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
-// expected-warning@test_sub1.h:14{{unsafe buffer access}}
-// expected-note@test_sub1.h:14{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
-// expected-warning@test_sub2.h:5{{unsafe buffer access}}
-// expected-note@test_sub2.h:5{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
+// No expected warnings from base.h, test_sub1, or test_sub2 because they are
+// in seperate modules, and the explicit commands that builds them have no
+// `-Wunsafe-buffer-usage`.
+
 int foo(int * p) {
   int x = p[5]; // expected-warning{{unsafe buffer access}} expected-note{{pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions}}
 #pragma clang unsafe_buffer_usage begin