X86: Don't emit zero-byte functions on Windows

Empty functions can lead to duplicate entries in the Guard CF Function
Table of a binary due to multiple functions sharing the same RVA,
causing the kernel to refuse to load that binary.

We had a terrific bug due to this in Chromium.

It turns out we were already doing this for Mach-O in certain
situations. This patch expands the code for that in
AsmPrinter::EmitFunctionBody() and renames
TargetInstrInfo::getNoopForMachoTarget() to simply getNoop() since it
seems it was used for not just Mach-O anyway.

Differential Revision: https://reviews.llvm.org/D32330

llvm-svn: 301040

2 files changed, 11 insertions, 9 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 028c79f3ab6..bf2e5818940 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1046,15 +1046,17 @@ void AsmPrinter::EmitFunctionBody() {
   // If the function is empty and the object file uses .subsections_via_symbols,
   // then we need to emit *something* to the function body to prevent the
   // labels from collapsing together.  Just emit a noop.
-  if ((MAI->hasSubsectionsViaSymbols() && !HasAnyRealCode)) {
+  // Similarly, don't emit empty functions on Windows either. It can lead to
+  // duplicate entries (two functions with the same RVA) in the Guard CF Table
+  // after linking, causing the kernel not to load the binary:
+  // https://developercommunity.visualstudio.com/content/problem/45366/vc-linker-creates-invalid-dll-with-clang-cl.html
+  // FIXME: Hide this behind some API in e.g. MCAsmInfo or MCTargetStreamer.
+  if (!HasAnyRealCode &&
+      (MAI->hasSubsectionsViaSymbols() || TM.getTargetTriple().isOSWindows())) {
     MCInst Noop;
-    MF->getSubtarget().getInstrInfo()->getNoopForMachoTarget(Noop);
+    MF->getSubtarget().getInstrInfo()->getNoop(Noop);
     OutStreamer->AddComment("avoids zero-length function");
-
-    // Targets can opt-out of emitting the noop here by leaving the opcode
-    // unspecified.
-    if (Noop.getOpcode())
-      OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
+    OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
   }
 
   const Function *F = MF->getFunction();
diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index 711144a3474..69b2517e129 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -428,8 +428,8 @@ static const TargetRegisterClass *canFoldCopy(const MachineInstr &MI,
   return nullptr;
 }
 
-void TargetInstrInfo::getNoopForMachoTarget(MCInst &NopInst) const {
-  llvm_unreachable("Not a MachO target");
+void TargetInstrInfo::getNoop(MCInst &NopInst) const {
+  llvm_unreachable("Not implemented");
 }
 
 static MachineInstr *foldPatchpoint(MachineFunction &MF, MachineInstr &MI,