[COFF] Use COFF stubs for extern_weak functions

Summary:
A COFF stub indirects the reference to a symbol through memory. A
.refptr.$sym global variable pointer is created to refer to $sym.
Typically mingw uses these for external global variable declarations,
but we can use them for weak function declarations as well.

Updates the dso_local classification to add a special case for
extern_weak symbols on COFF in both clang and LLVM.

Fixes PR37598

Reviewers: smeenai, mstorsjo

Subscribers: hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D61615

llvm-svn: 360207

4 files changed, 21 insertions, 11 deletions

diff --git a/llvm/lib/Target/TargetMachine.cpp b/llvm/lib/Target/TargetMachine.cpp
index 7a1e01bdab6..5b2bdc37af8 100644
--- a/llvm/lib/Target/TargetMachine.cpp
+++ b/llvm/lib/Target/TargetMachine.cpp
@@ -144,6 +144,12 @@ bool TargetMachine::shouldAssumeDSOLocal(const Module &M,
       isa<GlobalVariable>(GV))
     return false;
 
+  // On COFF, don't mark 'extern_weak' symbols as DSO local. If these symbols
+  // remain unresolved in the link, they can be resolved to zero, which is
+  // outside the current DSO.
+  if (TT.isOSBinFormatCOFF() && GV && GV->hasExternalWeakLinkage())
+    return false;
+
   // Every other GV is local on COFF.
   // Make an exception for windows OS in the triple: Some firmware builds use
   // *-win32-macho triples. This (accidentally?) produced windows relocations
diff --git a/llvm/lib/Target/X86/X86FastISel.cpp b/llvm/lib/Target/X86/X86FastISel.cpp
index 80dcd74a5d2..6fca1acb009 100644
--- a/llvm/lib/Target/X86/X86FastISel.cpp
+++ b/llvm/lib/Target/X86/X86FastISel.cpp
@@ -3503,8 +3503,9 @@ bool X86FastISel::fastLowerCall(CallLoweringInfo &CLI) {
 
     // This will be a direct call, or an indirect call through memory for
     // NonLazyBind calls or dllimport calls.
-    bool NeedLoad =
-        OpFlags == X86II::MO_DLLIMPORT || OpFlags == X86II::MO_GOTPCREL;
+    bool NeedLoad = OpFlags == X86II::MO_DLLIMPORT ||
+                    OpFlags == X86II::MO_GOTPCREL ||
+                    OpFlags == X86II::MO_COFFSTUB;
     unsigned CallOpc = NeedLoad
                            ? (Is64Bit ? X86::CALL64m : X86::CALL32m)
                            : (Is64Bit ? X86::CALL64pcrel32 : X86::CALLpcrel32);
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 56f02c412ae..26619571821 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -3969,10 +3969,10 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
       Callee = DAG.getTargetGlobalAddress(
           GV, dl, getPointerTy(DAG.getDataLayout()), G->getOffset(), OpFlags);
 
-      if (OpFlags == X86II::MO_GOTPCREL) {
+      if (isGlobalStubReference(OpFlags)) {
         // Add a wrapper.
-        Callee = DAG.getNode(X86ISD::WrapperRIP, dl,
-          getPointerTy(DAG.getDataLayout()), Callee);
+        Callee = DAG.getNode(getGlobalWrapperKind(GV, OpFlags), dl,
+                             getPointerTy(DAG.getDataLayout()), Callee);
         // Add extra indirection
         Callee = DAG.getLoad(
             getPointerTy(DAG.getDataLayout()), dl, DAG.getEntryNode(), Callee,
@@ -3987,9 +3987,9 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
     Callee = DAG.getTargetExternalSymbol(
         S->getSymbol(), getPointerTy(DAG.getDataLayout()), OpFlags);
 
-    if (OpFlags == X86II::MO_GOTPCREL) {
-      Callee = DAG.getNode(X86ISD::WrapperRIP, dl,
-          getPointerTy(DAG.getDataLayout()), Callee);
+    if (isGlobalStubReference(OpFlags)) {
+      Callee = DAG.getNode(getGlobalWrapperKind(nullptr, OpFlags), dl,
+                           getPointerTy(DAG.getDataLayout()), Callee);
       Callee = DAG.getLoad(
           getPointerTy(DAG.getDataLayout()), dl, DAG.getEntryNode(), Callee,
           MachinePointerInfo::getGOT(DAG.getMachineFunction()));
diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index 177fde45911..d5bb56603df 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -176,10 +176,13 @@ X86Subtarget::classifyGlobalFunctionReference(const GlobalValue *GV,
   if (TM.shouldAssumeDSOLocal(M, GV))
     return X86II::MO_NO_FLAG;
 
+  // Functions on COFF can be non-DSO local for two reasons:
+  // - They are marked dllimport
+  // - They are extern_weak, and a stub is needed
   if (isTargetCOFF()) {
-    assert(GV->hasDLLImportStorageClass() &&
-           "shouldAssumeDSOLocal gave inconsistent answer");
-    return X86II::MO_DLLIMPORT;
+    if (GV->hasDLLImportStorageClass())
+      return X86II::MO_DLLIMPORT;
+    return X86II::MO_COFFSTUB;
   }
 
   const Function *F = dyn_cast_or_null<Function>(GV);