Emulate @weak functions on Windows and don't emit COMDATs for ELF anymore

Emulation for @weak global variables is still left out but should be
analogous.

The test adaptations are mostly a revert of 3893840f. The testcase has
shown that @weak hasn't worked properly for ELF (linker apparently
prefers the version in the 1st object file, independent of whether it's
weak or not), because the functions are emitted in COMDATs.
clang emits COMDATs for templates and inline functions only, not for
regular functions.

gen/functions.cpp

@@ -54,6 +54,7 @@
 #include "llvm/IR/CFG.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetOptions.h"
+#include "llvm/Transforms/Utils/Cloning.h"
 #include <iostream>
 
 bool isAnyMainFunction(FuncDeclaration *fd) {
@@ -892,6 +893,62 @@ bool eraseDummyAfterReturnBB(llvm::BasicBlock *bb) {
   return false;
 }
 
+/**
+ * LLVM doesn't really support weak linkage for MSVC targets, it just prevents
+ * inlining. We can emulate it though, by conceptually renaming the defined
+ * function, only declaring the original function and embedding a linker
+ * directive in the object file, instructing the linker to fall back to the weak
+ * implementation if there's no strong definition.
+ * The object file still needs to be pulled in by the linker for the directive
+ * to be found.
+ */
+void emulateWeakAnyLinkageForMSVC(LLFunction *func, LINK linkage) {
+  const bool isWin32 = !global.params.is64bit;
+
+  std::string mangleBuffer;
+  llvm::StringRef finalMangle = func->getName();
+  if (finalMangle[0] == '\1') {
+    finalMangle = finalMangle.substr(1);
+  } else if (isWin32) {
+    // implicit underscore prefix for Win32
+    mangleBuffer = ("_" + finalMangle).str();
+    finalMangle = mangleBuffer;
+  }
+
+  std::string finalWeakMangle = finalMangle;
+  if (linkage == LINKcpp) {
+    assert(finalMangle.startswith("?"));
+    // prepend `__weak_` to first identifier
+    size_t offset = finalMangle.startswith("??$") ? 3 : 1;
+    finalWeakMangle.insert(offset, "__weak_");
+  } else if (linkage == LINKd) {
+    const size_t offset = isWin32 ? 1 : 0;
+    assert(finalMangle.substr(offset).startswith("_D"));
+    // prepend a `__weak` package
+    finalWeakMangle.insert(offset + 2, "6__weak");
+  } else {
+    // prepend `__weak_`
+    const size_t offset = isWin32 && finalMangle.startswith("_") ? 1 : 0;
+    finalWeakMangle.insert(offset, "__weak_");
+  }
+
+  const std::string linkerOption =
+      ("/ALTERNATENAME:" + finalMangle + "=" + finalWeakMangle).str();
+  gIR->addLinkerOption(llvm::StringRef(linkerOption));
+
+  // work around LLVM assertion when cloning a function's debuginfos
+  func->setSubprogram(nullptr);
+
+  llvm::ValueToValueMapTy dummy;
+  auto clone = llvm::CloneFunction(func, dummy);
+  clone->setName("\1" + finalWeakMangle);
+  setLinkage({LLGlobalValue::ExternalLinkage, func->hasComdat()}, clone);
+
+  // reduce the original definition to a declaration
+  setLinkage({LLGlobalValue::ExternalLinkage, false}, func);
+  func->deleteBody();
+}
+
 } // anonymous namespace
 
 void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
@@ -1274,6 +1331,11 @@ void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
     auto fn = gIR->module.getFunction(fd->mangleString);
     gIR->dcomputetarget->addKernelMetadata(fd, fn);
   }
+
+  if (func->getLinkage() == LLGlobalValue::WeakAnyLinkage &&
+      global.params.targetTriple->isWindowsMSVCEnvironment()) {
+    emulateWeakAnyLinkageForMSVC(func, fd->linkage);
+  }
 }
 
 ////////////////////////////////////////////////////////////////////////////////

gen/moduleinfo.cpp

@@ -310,6 +310,6 @@ llvm::GlobalVariable *genModuleInfo(Module *m) {
   // Create a global symbol with the above initialiser.
   LLGlobalVariable *moduleInfoSym = getIrModule(m)->moduleInfoSymbol();
   b.finalize(moduleInfoSym);
-  setLinkage({LLGlobalValue::ExternalLinkage, supportsCOMDAT()}, moduleInfoSym);
+  setLinkage({LLGlobalValue::ExternalLinkage, needsCOMDAT()}, moduleInfoSym);
   return moduleInfoSym;
 }

gen/pgo_ASTbased.cpp

@@ -773,7 +773,7 @@ void CodeGenPGO::setFuncName(llvm::StringRef Name,
     // If Linkage is private, and the function is in a comdat "any" group, set
     // the linkage to internal to prevent LLVM from erroring with "comdat global
     // value has private linkage".
-    if (supportsCOMDAT() &&
+    if (needsCOMDAT() &&
         FuncNameVar->getLinkage() == llvm::GlobalValue::PrivateLinkage) {
       FuncNameVar->setLinkage(llvm::GlobalValue::InternalLinkage);
     }

gen/rttibuilder.cpp

@@ -85,7 +85,7 @@ void RTTIBuilder::push_void_array(llvm::Constant *CI, Type *valtype,
   mangleToBuffer(mangle_sym, &initname);
   initname.writestring(".rtti.voidarr.data");
 
-  const LinkageWithCOMDAT lwc(TYPEINFO_LINKAGE_TYPE, supportsCOMDAT());
+  const LinkageWithCOMDAT lwc(TYPEINFO_LINKAGE_TYPE, needsCOMDAT());
 
   auto G = new LLGlobalVariable(gIR->module, CI->getType(), true, lwc.first, CI,
                                 initname.peekChars());
@@ -111,7 +111,7 @@ void RTTIBuilder::push_array(llvm::Constant *CI, uint64_t dim, Type *valtype,
   initname.writestring(tmpStr.c_str());
   initname.writestring(".data");
 
-  const LinkageWithCOMDAT lwc(TYPEINFO_LINKAGE_TYPE, supportsCOMDAT());
+  const LinkageWithCOMDAT lwc(TYPEINFO_LINKAGE_TYPE, needsCOMDAT());
 
   auto G = new LLGlobalVariable(gIR->module, CI->getType(), true, lwc.first, CI,
                                 initname.peekChars());

gen/tollvm.cpp

@@ -223,25 +223,25 @@ LinkageWithCOMDAT DtoLinkage(Dsymbol *sym) {
     linkage = LLGlobalValue::WeakAnyLinkage;
   }
 
-  return {linkage, supportsCOMDAT()};
+  return {linkage, needsCOMDAT()};
 }
 
-bool supportsCOMDAT() {
-  const auto &triple = *global.params.targetTriple;
-  return !(triple.isOSBinFormatMachO() ||
-#if LDC_LLVM_VER >= 500
-           triple.isOSBinFormatWasm()
-#else
-           triple.getArch() == llvm::Triple::wasm32 ||
-           triple.getArch() == llvm::Triple::wasm64
-#endif
-  );
+bool needsCOMDAT() {
+  /* For MSVC targets (and probably MinGW too), linkonce[_odr] and weak[_odr]
+   * linkages don't work and need to be emulated via COMDATs to prevent multiple
+   * definition errors when linking.
+   * Simply emit all functions in COMDATs, not just templates, for aggressive
+   * linker stripping (/OPT:REF and /OPT:ICF with MS linker/LLD), analogous to
+   * using /Gy with the MS compiler.
+   * https://docs.microsoft.com/en-us/cpp/build/reference/opt-optimizations?view=vs-2019
+   */
+  return global.params.targetTriple->isOSBinFormatCOFF();
 }
 
 void setLinkage(LinkageWithCOMDAT lwc, llvm::GlobalObject *obj) {
   obj->setLinkage(lwc.first);
-  if (lwc.second)
-    obj->setComdat(gIR->module.getOrInsertComdat(obj->getName()));
+  obj->setComdat(lwc.second ? gIR->module.getOrInsertComdat(obj->getName())
+                            : nullptr);
 }
 
 void setLinkageAndVisibility(Dsymbol *sym, llvm::GlobalObject *obj) {

gen/tollvm.h

@@ -69,7 +69,7 @@ LLValue *DtoDelegateEquals(TOK op, LLValue *lhs, LLValue *rhs);
 typedef std::pair<llvm::GlobalValue::LinkageTypes, bool> LinkageWithCOMDAT;
 LinkageWithCOMDAT DtoLinkage(Dsymbol *sym);
 
-bool supportsCOMDAT();
+bool needsCOMDAT();
 void setLinkage(LinkageWithCOMDAT lwc, llvm::GlobalObject *obj);
 // Sets the linkage of the specified IR global and possibly hides it, both based
 // on the specified D symbol.

gen/typinf.cpp

@@ -666,7 +666,7 @@ void TypeInfoDeclaration_codegen(TypeInfoDeclaration *decl, IRState *p) {
   LLVMDefineVisitor v(gvar);
   decl->accept(&v);
 
-  setLinkage({TYPEINFO_LINKAGE_TYPE, supportsCOMDAT()}, gvar);
+  setLinkage({TYPEINFO_LINKAGE_TYPE, needsCOMDAT()}, gvar);
   if (auto forStructType = forType->isTypeStruct())
     setVisibility(forStructType->sym, gvar);
 }

ir/irclass.cpp

@@ -394,7 +394,7 @@ void IrAggr::defineInterfaceVtbl(BaseClass *b, bool new_instance,
     llvm::Function *thunk = gIR->module.getFunction(thunkIRMangle);
     if (!thunk) {
       const LinkageWithCOMDAT lwc(LLGlobalValue::LinkOnceODRLinkage,
-                                  supportsCOMDAT());
+                                  needsCOMDAT());
       const auto callee = irFunc->getLLVMCallee();
       thunk = LLFunction::Create(
           isaFunction(callee->getType()->getContainedType(0)), lwc.first,

tests/PGO/irbased_indirect_calls.d

@@ -2,17 +2,20 @@
 
 // REQUIRES: PGO_RT
 
+// with LLVM 4.0, @optStrategy apparently doesn't suffice to prevent eliding the hot() call
+// XFAIL: llvm400
+
 // RUN: %ldc -O3 -fprofile-generate=%t.profraw -run %s  \
 // RUN:   &&  %profdata merge %t.profraw -o %t.profdata \
 // RUN:   &&  %ldc -O3 -c -output-ll -of=%t.use.ll -fprofile-use=%t.profdata %s \
 // RUN:   &&  FileCheck %s -check-prefix=PROFUSE < %t.use.ll
 
-import ldc.attributes : weak;
+import ldc.attributes;
 
 extern (C)
 { // simplify name mangling for simpler string matching
 
-    @weak // disable reasoning about this function
+    @optStrategy("none")
     void hot()
     {
     }

tests/codegen/attr_weak.d

@@ -1,20 +1,23 @@
-// Test linking+running a program with @weak function
+// Test linking+running a program with @weak functions
 
 // RUN: %ldc -O3 %S/inputs/attr_weak_input.d -c -of=%t-dir/attr_weak_input%obj
-// RUN: %ldc -O3 %t-dir/attr_weak_input%obj %s -of=%t%exe
+// RUN: %ldc -O3 %s %t-dir/attr_weak_input%obj -of=%t%exe
 // RUN: %t%exe
 
 
 import ldc.attributes;
 
-// Should be overridden by attr_weak_input.d (but only because its object
-// file is specified before this one for the linker).
-// The @weak attribute prevents the optimizer from making any assumptions
-// though, so the call below is not inlined.
+// should take precedence over and not conflict with weak attr_weak_input.return_two
+extern(C) int return_two() {
+    return 123;
+}
+
+// should be overridden by strong attr_weak_input.return_seven
 extern(C) @weak int return_seven() {
-  return 1;
+  return 456;
 }
 
 void main() {
+  assert( return_two() == 123 );
   assert( return_seven() == 7 );
 }

tests/codegen/attributes.d

@@ -20,7 +20,7 @@ import ldc.attributes;
 // CHECK-DAG: @{{.*}}myWeakGlobali{{\"?}} = weak
 @(ldc.attributes.weak) int myWeakGlobal;
 
-// CHECK-DAG: define{{.*}} weak {{.*}}void @{{.*}}weakFunc
+// CHECK-DAG: define{{.*}} {{(weak .*void @.*_D)|(void @.*_D6__weak)}}10attributes8weakFuncFZv
 @weak void weakFunc() {}
 
 //---------------------------------------------------------------------

tests/codegen/inputs/attr_weak_input.d

@@ -1,3 +1,9 @@
+import ldc.attributes;
+
+extern(C) @weak int return_two() {
+  return 2;
+}
+
 extern(C) int return_seven() {
   return 7;
 }