[PATCH] CodeGen: Don't emit a thread-wrapper if we can't touch the backing variable
David Majnemer
david.majnemer at gmail.com
Thu Jun 12 02:17:04 PDT 2014
Hi rsmith, rjmccall,
OS X TLS has all accesses going through the thread-wrapper function and
gives the backing thread-local variable internal linkage. This means
that thread-wrappers must have WeakAnyLinkage so that references to the
internal thread-local variables do not get propagated to other code.
It also means that translation units which do not provide a definition
for the thread-local variable cannot attempt to emit a thread-wrapper
because the thread wrapper will attempt to reference the backing
variable.
http://reviews.llvm.org/D4109
Files:
lib/CodeGen/ItaniumCXXABI.cpp
test/CodeGenCXX/tls-init-funcs.cpp
Index: lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- lib/CodeGen/ItaniumCXXABI.cpp
+++ lib/CodeGen/ItaniumCXXABI.cpp
@@ -1572,6 +1572,15 @@
CGF.registerGlobalDtorWithAtExit(D, dtor, addr);
}
+static bool
+isThreadLocalBackingVariableAlwaysReferenceable(const VarDecl *VD,
+ CodeGen::CodeGenModule &CGM) {
+ // OS X prefers to have references to thread local variables to go through
+ // the thread wrapper instead of directly referencing the backing variable.
+ return VD->getTLSKind() != VarDecl::TLS_Dynamic ||
+ !CGM.getTarget().getTriple().isMacOSX();
+}
+
/// Get the appropriate linkage for the wrapper function. This is essentially
/// the weak form of the variable's linkage; every translation unit which wneeds
/// the wrapper emits a copy, and we want the linker to merge them.
@@ -1584,12 +1593,11 @@
if (llvm::GlobalValue::isLocalLinkage(VarLinkage))
return VarLinkage;
- // All accesses to the thread_local variable go through the thread wrapper.
- // However, this means that we cannot allow the thread wrapper to get inlined
- // into any functions.
- if (VD->getTLSKind() == VarDecl::TLS_Dynamic &&
- CGM.getTarget().getTriple().isMacOSX())
- return llvm::GlobalValue::WeakAnyLinkage;
+ // The thread wrapper can't be inlined if the backing variable cannot always
+ // be referenced.
+ if (!isThreadLocalBackingVariableAlwaysReferenceable(VD, CGM))
+ return VD->hasDefinition() ? llvm::GlobalValue::WeakAnyLinkage
+ : llvm::GlobalValue::ExternalLinkage;
return llvm::GlobalValue::WeakODRLinkage;
}
@@ -1628,6 +1636,13 @@
const VarDecl *VD = Decls[I].first;
llvm::GlobalVariable *Var = Decls[I].second;
+ // Some targets require that all access to thread local variables go through
+ // the thread wrapper. This means that we cannot attempt to create a thread
+ // wrapper or a thread helper.
+ if (!isThreadLocalBackingVariableAlwaysReferenceable(VD, CGM) &&
+ !VD->hasDefinition())
+ continue;
+
// Mangle the name for the thread_local initialization function.
SmallString<256> InitFnName;
{
Index: test/CodeGenCXX/tls-init-funcs.cpp
===================================================================
--- test/CodeGenCXX/tls-init-funcs.cpp
+++ test/CodeGenCXX/tls-init-funcs.cpp
@@ -2,10 +2,17 @@
// CHECK: @a = internal thread_local global
// CHECK: @_tlv_atexit({{.*}}@_ZN1AD1Ev
+// CHECK: call {{.*}} @_ZTW3ext()
+// CHECK: declare hidden {{.*}} @_ZTW3ext()
// CHECK: define weak hidden {{.*}} @_ZTW1a
struct A {
~A();
};
thread_local A a;
+
+extern thread_local A ext;
+A &get_ext() {
+ return ext;
+}
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