Bring back the ability opt out of padding zero-byte functions by not providing a nop instruction.

[dsanders]

Summary

No test case since I'm not aware of an in-tree target that needs this.

[dsanders]

Hi Hans,

r301047 broke one of our out-of-tree targets by removing the opt-out that was available in the previous version of the code. Would it be possible to bring it back?

[hans]

Sure, but it's pretty much guaranteed to break again if there is no test..

[dsanders]

Thanks. Hopefully it won't break very often.

llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::EmitFunctionBody() {
   // 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.
   const Triple &TT = TM.getTargetTriple();
   if (!HasAnyRealCode && (MAI->hasSubsectionsViaSymbols() ||
                           (TT.isOSWindows() && TT.isOSBinFormatCOFF()))) {
     MCInst Noop;
     MF->getSubtarget().getInstrInfo()->getNoop(Noop);
+
+    // Targets can opt-out of emitting the noop here by leaving the opcode
+    // unspecified.
+    if (Noop.getOpcode()) {
       OutStreamer->AddComment("avoids zero-length function");
       OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
     }
+  }
 
   const Function *F = MF->getFunction();
   for (const auto &BB : *F) {
     if (!BB.hasAddressTaken())
       continue;
     MCSymbol *Sym = GetBlockAddressSymbol(&BB);
     if (Sym->isDefined())
       continue;