CodeGen: handle llvm.used properly for COFF

`llvm.used` contains a list of pointers to named values which the
compiler, assembler, and linker are required to treat as if there is a
reference that they cannot see.  Ensure that the symbols are preserved
by adding an explicit `-include` reference to the linker command.

llvm-svn: 323017

2 files changed, 35 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 2a20c64e82c..87d91377dc9 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1434,6 +1434,7 @@ bool AsmPrinter::doFinalization(Module &M) {
     // Emit /EXPORT: flags for each exported global as necessary.
     const auto &TLOF = getObjFileLowering();
     std::string Flags;
+
     for (const GlobalValue &GV : M.global_values()) {
       raw_string_ostream OS(Flags);
       TLOF.emitLinkerFlagsForGlobal(OS, &GV);
@@ -1444,6 +1445,35 @@ bool AsmPrinter::doFinalization(Module &M) {
       }
       Flags.clear();
     }
+
+    // Emit /INCLUDE: flags for each used global as necessary.
+    if (const auto *LU = M.getNamedGlobal("llvm.used")) {
+      assert(LU->hasInitializer() &&
+             "expected llvm.used to have an initializer");
+      assert(isa<ArrayType>(LU->getValueType()) &&
+             "expected llvm.used to be an array type");
+      if (const auto *A = cast<ConstantArray>(LU->getInitializer())) {
+        for (const Value *Op : A->operands()) {
+          const auto *GV =
+              cast<GlobalValue>(Op->stripPointerCastsNoFollowAliases());
+          // Global symbols with internal linkage are not visible to the linker,
+          // and thus would cause an error when the linker tried to preserve the
+          // symbol due to the `/include:` directive.
+          if (GV->hasInternalLinkage())
+            continue;
+
+          raw_string_ostream OS(Flags);
+          TLOF.emitLinkerFlagsForUsed(OS, GV);
+          OS.flush();
+
+          if (!Flags.empty()) {
+            OutStreamer->SwitchSection(TLOF.getDrectveSection());
+            OutStreamer->EmitBytes(Flags);
+          }
+          Flags.clear();
+        }
+      }
+    }
   }
 
   // Allow the target to emit any magic that it wants at the end of the file,
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index bdc4bc0302b..0e90df901fd 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -1250,6 +1250,11 @@ void TargetLoweringObjectFileCOFF::emitLinkerFlagsForGlobal(
   emitLinkerFlagsForGlobalCOFF(OS, GV, getTargetTriple(), getMangler());
 }
 
+void TargetLoweringObjectFileCOFF::emitLinkerFlagsForUsed(
+    raw_ostream &OS, const GlobalValue *GV) const {
+  emitLinkerFlagsForUsedCOFF(OS, GV, getTargetTriple(), getMangler());
+}
+
 //===----------------------------------------------------------------------===//
 //                                  Wasm
 //===----------------------------------------------------------------------===//