[clang] Annotating C++'s `operator new` with more attributes

Summary:
Right now we annotate C++'s `operator new` with `noalias` attribute,
which very much is healthy for optimizations.

However as per [[ http://eel.is/c++draft/basic.stc.dynamic.allocation | `[basic.stc.dynamic.allocation]` ]],
there are more promises on global `operator new`, namely:
* non-`std::nothrow_t` `operator new` *never* returns `nullptr`
* If `std::align_val_t align` parameter is taken, the pointer will also be `align`-aligned
* ~~global `operator new`-returned pointer is `__STDCPP_DEFAULT_NEW_ALIGNMENT__`-aligned ~~ It's more caveated than that.

Supplying this information may not cause immediate landslide effects
on any specific benchmarks, but it for sure will be healthy for optimizer
in the sense that the IR will better reflect the guarantees provided in the source code.

The caveat is `-fno-assume-sane-operator-new`, which currently prevents emitting `noalias`
attribute, and is automatically passed by Sanitizers ([[ https://bugs.llvm.org/show_bug.cgi?id=16386 | PR16386 ]]) - should it also cover these attributes?
The problem is that the flag is back-end-specific, as seen in `test/Modules/explicit-build-flags.cpp`.
But while it is okay to add `noalias` metadata in backend, we really should be adding at least
the alignment metadata to the AST, since that allows us to perform sema checks on it.

Reviewers: erichkeane, rjmccall, jdoerfert, eugenis, rsmith

Reviewed By: rsmith

Subscribers: xbolva00, jrtc27, atanasyan, nlopes, cfe-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D73380

1 files changed, 0 insertions, 9 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index d4ed486b4079..652efd857064 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -1875,15 +1875,6 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
     // default, only if it is invoked by a new-expression or delete-expression.
     F->addAttribute(llvm::AttributeList::FunctionIndex,
                     llvm::Attribute::NoBuiltin);
-
-    // A sane operator new returns a non-aliasing pointer.
-    // FIXME: Also add NonNull attribute to the return value
-    // for the non-nothrow forms?
-    auto Kind = FD->getDeclName().getCXXOverloadedOperator();
-    if (getCodeGenOpts().AssumeSaneOperatorNew &&
-        (Kind == OO_New || Kind == OO_Array_New))
-      F->addAttribute(llvm::AttributeList::ReturnIndex,
-                      llvm::Attribute::NoAlias);
   }
 
   if (isa<CXXConstructorDecl>(FD) || isa<CXXDestructorDecl>(FD))