[llvm] [DXIL] Add support for root signature flag element in DXContainer (PR #123147)

[Damyan Pepper via llvm-commits]

================
@@ -144,6 +150,30 @@ void DXContainerGlobals::addSignature(Module &M,
   Globals.emplace_back(buildSignature(M, OutputSig, "dx.osg1", "OSG1"));
 }
 
+void DXContainerGlobals::addRootSignature(Module &M,
+                                          SmallVector<GlobalValue *> &Globals) {
+
+  auto &RSA = getAnalysis<RootSignatureAnalysisWrapper>();
+  std::optional<ModuleRootSignature> MaybeRootSignature = RSA.getResult();
+
+  if (!MaybeRootSignature.has_value())
+    return;
+
+  ModuleRootSignature MRS = MaybeRootSignature.value();
----------------
damyanp wrote:

Although ModuleRootSignature is cheap to copy at the moment, that isn't necessarily going to always be the case once we actually implement everything else.  The code as it is now has two copies, Justin's proposal has one copy, but really there should be 0 copies.

This verison has 0 copies:

```
  if (!RSA.getResult())
    return;

  RSH.Flags = RFA.getResult()->Flag;
```



https://github.com/llvm/llvm-project/pull/123147