Support attribute used in member funcs of class templates

[rafauler]

Summary

As PR17480 describes, clang does not support the used attribute
for member functions of class templates. This means that if the member
function is not used, its definition is never instantiated. This patch
changes clang to emit the definition if it has the used attribute.

Test Plan: Added a testcase

[rafauler]

Removing facebook tags

[Meinersbur]

Thanks for looking into my almost 6 year old bug!

test/CodeGenCXX/attr-used-member-function-implicit-instantiation.cpp

2

Could you mention PR17480 in this test file as well?

9

Any reason to use the preprocessor here instead of __attribute__ directly?

17

CHECK: instead of CHECK-DAG should be sufficient.

[rafauler]

No problem, thanks for your suggestions!

[aaron.ballman]

lib/Sema/SemaTemplateInstantiateDecl.cpp

2182

const auto *?

2184

const MemberSpecializationInfo *?

2186

const auto *?

2191

You can elide the last argument.

test/CodeGenCXX/attr-used-member-function-implicit-instantiation.cpp

2

Also, can you bump this comment down below the RUN lines?

[rafauler]

Rebase + Aaron's suggestions. Thanks for the suggestions!

[aaron.ballman]

LGTM aside from some formatting nits.

lib/Sema/SemaTemplateInstantiateDecl.cpp

2185

Elide braces

2187

Elide braces

test/CodeGenCXX/attr-used-member-function-implicit-instantiation.cpp

3

I'd go ahead and leave this on the same line as the previous RUN line. We're often less picky about 80-col limits for RUN lines unless they're obnoxiously long. You can drop the -O0 from the RUN line as well (I believe).

7

You should run the patch through clang-format -- the indentation looks incorrect here.

[rafauler]

Thanks for reviewing!

[davezarzycki]

This change broke building Swift on my Linux box. The compiler confusingly complains about the *same* template being both explicitly specialized and implicitly instantiated:

/home/dave/s/u/swift/stdlib/public/runtime/ProtocolConformance.cpp:34:38: error: explicit specialization of 'dump' after instantiation
template <> void ProtocolDescriptor::dump() const {
                                     ^
/home/dave/s/u/swift/stdlib/public/runtime/ProtocolConformance.cpp:34:18: note: implicit instantiation first required here
template <> void ProtocolDescriptor::dump() const {
                 ^

Is this expected?

[rafauler]

Hi davezarzycki, thanks for reporting this! This is not expected, I'll revert this diff for now. Sorry for this.

[phosek]

This broke the 2-stage LTO build of LLVM, the error is:

ld.lld: /usr/local/google/home/phosek/clang-llvm/llvm-project/llvm/lib/IR/Constants.cpp:995: static llvm::Constant *llvm::ConstantArray::getImpl(llvm::ArrayType *, ArrayRef<llvm::Constant *>): Assertion `V[i]->getType() == Ty->getElementType() && "Wrong type in array element initializer"' failed.

The invocation this came from is:

/usr/local/google/home/phosek/clang-llvm/llvm-project/llvm-build/fuchsia-xyz/./bin/clang++  -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -std=c++11 -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -flto -O3 -gline-tables-only  -ldl -lpthread -fuse-ld=lld -Wl,--color-diagnostics -Wl,-allow-shlib-undefined    -Wl,-O3 -Wl,--gc-sections tools/llvm-cov/CMakeFiles/llvm-cov.dir/llvm-cov.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/gcov.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/CodeCoverage.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/CoverageExporterJson.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/CoverageExporterLcov.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/CoverageFilters.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/CoverageReport.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/CoverageSummaryInfo.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/SourceCoverageView.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/SourceCoverageViewHTML.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/SourceCoverageViewText.cpp.o tools/llvm-cov/CMakeFiles/llvm-cov.dir/TestingSupport.cpp.o  -o bin/llvm-cov  -Wl,-rpath,"\$ORIGIN/../lib" lib/libLLVMCore.a lib/libLLVMSupport.a lib/libLLVMObject.a lib/libLLVMCoverage.a lib/libLLVMProfileData.a lib/libLLVMObject.a lib/libLLVMBitReader.a lib/libLLVMMCParser.a lib/libLLVMMC.a lib/libLLVMDebugInfoCodeView.a lib/libLLVMDebugInfoMSF.a lib/libLLVMCore.a lib/libLLVMBinaryFormat.a lib/libLLVMSupport.a -lz -lrt -ldl -lm lib/libLLVMDemangle.a

lib/Sema/SemaTemplateInstantiateDecl.cpp

TemplateDeclInstantiator::VisitCXXMethodDecl(CXXMethodDecl *D,
 
   // Otherwise, add the declaration.  We don't need to do this for
   // class-scope specializations because we'll have matched them with
   // the appropriate template.
   } else if (!IsClassScopeSpecialization) {
     Owner->addDecl(Method);
   }
 
+  // PR17480: Honor the used attribute to instantiate member function
+  // definitions
+  if (Method->hasAttr<UsedAttr>()) {
+    if (auto *A = dyn_cast<CXXRecordDecl>(Owner)) {
+      SourceLocation Loc;
+      if (MemberSpecializationInfo *MSInfo = A->getMemberSpecializationInfo()) {
+        Loc = MSInfo->getPointOfInstantiation();
+      } else if (ClassTemplateSpecializationDecl *Spec =
+                     dyn_cast<ClassTemplateSpecializationDecl>(A)) {
+        Loc = Spec->getPointOfInstantiation();
+      }
+
+      SemaRef.MarkFunctionReferenced(Loc, Method, /*MightBeOdrUse=*/true);
+    }
+  }
+
   return Method;
 }
 
 Decl *TemplateDeclInstantiator::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
   return VisitCXXMethodDecl(D);
 }
 
 Decl *TemplateDeclInstantiator::VisitCXXDestructorDecl(CXXDestructorDecl *D) {

test/CodeGenCXX/attr-used-member-function-implicit-instantiation.cpp

+// Check that PR17480 is fixed: __attribute__((used)) ignored in templated
+// classes
+
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm -O0 -o - %s \
+// RUN:  | FileCheck %s
+
+namespace InstantiateUsedMemberDefinition {
+  template <typename T> struct S {
+    int __attribute__((used)) f() {
+      return 0;
+    }
+  };
+
+  void test() {
+    // Check that InstantiateUsedMemberDefinition::S<int>::f() is defined
+    // as a result of the S class template implicit instantiation
+    // CHECK: define linkonce_odr i32 @_ZN31InstantiateUsedMemberDefinition1SIiE1fEv
+    S<int> inst;
+  }
+}