[llvm] [DirectX] Propagate shader flags mask of callees to callers (PR #118306)

[S. Bharadwaj Yadavalli via llvm-commits]

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@@ -81,16 +101,31 @@ void ComputedShaderFlags::print(raw_ostream &OS) const {
   OS << ";\n";
 }
 
-/// Return the shader flags mask of the specified function Func.
-const ComputedShaderFlags &
-ModuleShaderFlags::getFunctionFlags(const Function *Func) const {
+auto ModuleShaderFlags::getFunctionShaderFlagInfo(const Function *Func) const {
   const auto Iter = llvm::lower_bound(
       FunctionFlags, Func,
       [](const std::pair<const Function *, ComputedShaderFlags> FSM,
          const Function *FindFunc) { return (FSM.first < FindFunc); });
   assert((Iter != FunctionFlags.end() && Iter->first == Func) &&
          "No Shader Flags Mask exists for function");
-  return Iter->second;
+  return Iter;
+}
+
+/// Merge mask NewSF to that of Func, if different.
+/// Return true if mask of Func is changed, else false.
+bool ModuleShaderFlags::mergeFunctionShaderFlags(
+    const Function *Func, const ComputedShaderFlags NewSF) {
+  const auto FuncSFInfo = getFunctionShaderFlagInfo(Func);
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bharadwajy wrote:

> As written, this is probably a `const` local variable (copy)... which you then `const_cast` below. This is a prime example of why `auto` should be used judiciously.

Yeah, the root cause for the use of `const_cast` and `auto` return type stems from the desire to define the functionality to search through the `FunctionFlags` in a single place. The idea is to use the same function (which is `getFunctionShaderFlagsinfo()`) to (a) query for the shader flags of a `Function *` and (b) to locate the `std::pair<>` for merge/update the shader flags of a given `Function *`.

Will try out a different refactoring while keeping that goal intact.   

https://github.com/llvm/llvm-project/pull/118306