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Don't emit an available_externally vtable pointing to linkonce_odr funcs.

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This fixes pr13124.

From the discussion at
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-June/022606.html
we know that we cannot make funcions in a weak_odr vtable also weak_odr. They
should remain linkonce_odr.

The side effect is that we cannot emit a available_externally vtable unless we
also emit a copy of the function. This also has an issue: If codegen is going
to output a function, sema has to mark it used. Given llvm.org/pr9114, it looks
like sema cannot be more aggressive at marking functions used because
of vtables.

This leaves us with a few unpleasant options:

* Marking functions in vtables used if possible. This sounds a bit sloppy, so
  we should avoid it.
* Producing available_externally vtables only when all the functions in it are
  already used or weak_odr. This would cover cases like

--------------------
struct foo {
  virtual ~foo();
};
struct bar : public foo {
  virtual void zed();
};
void f() {
  foo *x(new bar);
  delete x;
}
void g(bar *x) {
  x->~bar(); // force the destructor to be used
}
--------------------------

and

----------------------------------
template<typename T>
struct bar {
  virtual ~bar();
};
template<typename T>
bar<T>::~bar() {
}

// make the destructor weak_odr instead of linkonce_odr
extern template class bar<int>;

void f() {
  bar<int> *x(new bar<int>);
  delete x;
}
----------------------------

These look like corner cases, so it is unclear if it is worth it.

* And finally: Just nuke this optimization. That is what this patch implements.

llvm-svn: 189852
This commit is contained in:
Rafael Espindola
2013-09-03 21:05:13 +00:00
parent f58070baed
commit ee6aa0c62e
4 changed files with 10 additions and 57 deletions
Download Patch File Download Diff File Expand all files Collapse all files

23
clang/lib/CodeGen/CGVTables.cpp
View File

@@ -753,12 +753,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
switch (keyFunction->getTemplateSpecializationKind()) {
case TSK_Undeclared:
case TSK_ExplicitSpecialization:
// When compiling with optimizations turned on, we emit all vtables,
// even if the key function is not defined in the current translation
// unit. If this is the case, use available_externally linkage.
if (!def && CodeGenOpts.OptimizationLevel)
return llvm::GlobalVariable::AvailableExternallyLinkage;
assert(def && "Should not have been asked to emit this");
if (keyFunction->isInlined())
return !Context.getLangOpts().AppleKext ?
llvm::GlobalVariable::LinkOnceODRLinkage :
@@ -777,9 +772,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
llvm::Function::InternalLinkage;
case TSK_ExplicitInstantiationDeclaration:
return !Context.getLangOpts().AppleKext ?
llvm::GlobalVariable::AvailableExternallyLinkage :
llvm::Function::InternalLinkage;
llvm_unreachable("Should not have been asked to emit this");
}
}
@@ -795,7 +788,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
return llvm::GlobalVariable::LinkOnceODRLinkage;
case TSK_ExplicitInstantiationDeclaration:
return llvm::GlobalVariable::AvailableExternallyLinkage;
llvm_unreachable("Should not have been asked to emit this");
case TSK_ExplicitInstantiationDefinition:
return llvm::GlobalVariable::WeakODRLinkage;
@@ -897,16 +890,6 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
/// we define that v-table?
static bool shouldEmitVTableAtEndOfTranslationUnit(CodeGenModule &CGM,
const CXXRecordDecl *RD) {
// If we're building with optimization, we always emit v-tables
// since that allows for virtual function calls to be devirtualized.
// If the v-table is defined strongly elsewhere, this definition
// will be emitted available_externally.
//
// However, we don't want to do this in -fapple-kext mode, because
// kext mode does not permit devirtualization.
if (CGM.getCodeGenOpts().OptimizationLevel && !CGM.getLangOpts().AppleKext)
return true;
return !CGM.getVTables().isVTableExternal(RD);
}