Always_inline codegen rewrite.

Current implementation may end up emitting an undefined reference for
an "inline __attribute__((always_inline))" function by generating an
"available_externally alwaysinline" IR function for it and then failing to
inline all the calls. This happens when a call to such function is in dead
code. As the inliner is an SCC pass, it does not process dead code.

Libc++ relies on the compiler never emitting such undefined reference.

With this patch, we emit a pair of
1. internal alwaysinline definition (called F.alwaysinline)
2a. A stub F() { musttail call F.alwaysinline }
  -- or, depending on the linkage --
2b. A declaration of F.

The frontend ensures that F.inlinefunction is only used for direct
calls, and the stub is used for everything else (taking the address of
the function, really). Declaration (2b) is emitted in the case when
"inline" is meant for inlining only (like __gnu_inline__ and some
other cases).

This approach, among other nice properties, ensures that alwaysinline
functions are always internal, making it impossible for a direct call
to such function to produce an undefined symbol reference.

This patch is based on ideas by Chandler Carruth and Richard Smith.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247465 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 4 insertions, 8 deletions

diff --git a/lib/CodeGen/CGCXX.cpp b/lib/CodeGen/CGCXX.cpp
index ad30364986..07eb91582a 100644
--- a/lib/CodeGen/CGCXX.cpp
+++ b/lib/CodeGen/CGCXX.cpp
@@ -109,6 +109,9 @@ bool CodeGenModule::TryEmitBaseDestructorAsAlias(const CXXDestructorDecl *D) {
       D->getType()->getAs<FunctionType>()->getCallConv())
     return true;
 
+  if (BaseD->hasAttr<AlwaysInlineAttr>())
+    return true;
+
   return TryEmitDefinitionAsAlias(GlobalDecl(D, Dtor_Base),
                                   GlobalDecl(BaseD, Dtor_Base),
                                   false);
@@ -161,14 +164,7 @@ bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
 
   // Instead of creating as alias to a linkonce_odr, replace all of the uses
   // of the aliasee.
-  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage) &&
-     (TargetLinkage != llvm::GlobalValue::AvailableExternallyLinkage ||
-      !TargetDecl.getDecl()->hasAttr<AlwaysInlineAttr>())) {
-    // FIXME: An extern template instantiation will create functions with
-    // linkage "AvailableExternally". In libc++, some classes also define
-    // members with attribute "AlwaysInline" and expect no reference to
-    // be generated. It is desirable to reenable this optimisation after
-    // corresponding LLVM changes.
+  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage)) {
     Replacements[MangledName] = Aliasee;
     return false;
   }