C++14: Disable sized deallocation by default due to ABI breakage

There are no widely deployed standard libraries providing sized
deallocation functions, so we have to punt and ask the user if they want
us to use sized deallocation. In the future, when such libraries are
deployed, we can teach the driver to detect them and enable this
feature.

N3536 claimed that a weak thunk from sized to unsized deallocation could
be emitted to avoid breaking backwards compatibility with standard
libraries not providing sized deallocation. However, this approach and
other variations don't work in practice.

With the weak function approach, the thunk has to have default
visibility in order to ensure that it is overridden by other DSOs
providing sized deallocation. Weak, default visibility symbols are
particularly expensive on MachO, so John McCall was considering
disabling this feature by default on Darwin. It also changes behavior
ELF linking behavior, causing certain otherwise unreferenced object
files from an archive to be pulled into the link.

Our second approach was to use an extern_weak function declaration and
do an inline conditional branch at the deletion call site. This doesn't
work because extern_weak only works on MachO if you have some archive
providing the default value of the extern_weak symbol. Arranging to
provide such an archive has the same challenges as providing the symbol
in the standard library. Not to mention that extern_weak doesn't really
work on COFF.

Reviewers: rsmith, rjmccall

Differential Revision: http://reviews.llvm.org/D8467

llvm-svn: 232788

1 files changed, 0 insertions, 22 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index dabec15b784..9e80f0a0a1f 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -802,20 +802,6 @@ static void TryMarkNoThrow(llvm::Function *F) {
   F->setDoesNotThrow();
 }
 
-static void EmitSizedDeallocationFunction(CodeGenFunction &CGF,
-                                          const FunctionDecl *UnsizedDealloc) {
-  // This is a weak discardable definition of the sized deallocation function.
-  CGF.CurFn->setLinkage(llvm::Function::LinkOnceAnyLinkage);
-  if (CGF.CGM.supportsCOMDAT())
-    CGF.CurFn->setComdat(
-        CGF.CGM.getModule().getOrInsertComdat(CGF.CurFn->getName()));
-
-  // Call the unsized deallocation function and forward the first argument
-  // unchanged.
-  llvm::Constant *Unsized = CGF.CGM.GetAddrOfFunction(UnsizedDealloc);
-  CGF.Builder.CreateCall(Unsized, &*CGF.CurFn->arg_begin());
-}
-
 void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
                                    const CGFunctionInfo &FnInfo) {
   const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
@@ -891,14 +877,6 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
     emitImplicitAssignmentOperatorBody(Args);
   } else if (Stmt *Body = FD->getBody()) {
     EmitFunctionBody(Args, Body);
-  } else if (FunctionDecl *UnsizedDealloc =
-                 FD->getCorrespondingUnsizedGlobalDeallocationFunction()) {
-    // Global sized deallocation functions get an implicit weak definition if
-    // they don't have an explicit definition, if allowed.
-    assert(getLangOpts().DefineSizedDeallocation &&
-           "Can't emit unallowed definition.");
-    EmitSizedDeallocationFunction(*this, UnsizedDealloc);
-
   } else
     llvm_unreachable("no definition for emitted function");