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

[Justin Bogner via llvm-commits]

================
@@ -46,21 +49,79 @@ static void updateFunctionFlags(ComputedShaderFlags &CSF,
   }
 }
 
-void ModuleShaderFlags::initialize(const Module &M) {
-  // Collect shader flags for each of the functions
-  for (const auto &F : M.getFunctionList()) {
-    if (F.isDeclaration())
+void ModuleShaderFlags::initialize(Module &M) {
+  CallGraph CG(M);
+
+  // Compute Shader Flags Mask for all functions using post-order visit of SCC
+  // of the call graph.
+  for (scc_iterator<CallGraph *> SCCI = scc_begin(&CG); !SCCI.isAtEnd();
+       ++SCCI) {
+    const std::vector<CallGraphNode *> &CurSCC = *SCCI;
+
+    // Union of shader masks of all functions in CurSCC
+    ComputedShaderFlags SCCSF;
+    for (CallGraphNode *CGN : CurSCC) {
+      Function *F = CGN->getFunction();
+      if (!F)
+        continue;
+
+      if (F->isDeclaration())
+        continue;
+
+      ComputedShaderFlags CSF;
+      for (const auto &BB : *F)
+        for (const auto &I : BB)
+          updateFunctionFlags(CSF, I);
+      // Insert shader flag mask for function F
+      FunctionFlags.insert({F, CSF});
+      // Update combined shader flags mask for all functions of the module
+      CombinedSFMask.merge(CSF);
+      // Update combined shader flags mask for all functions in this SCC
+      SCCSF.merge(CSF);
+    }
+
+    if (CurSCC.size() < 2)
       continue;
-    ComputedShaderFlags CSF;
-    for (const auto &BB : F)
-      for (const auto &I : BB)
-        updateFunctionFlags(CSF, I);
-    // Insert shader flag mask for function F
-    FunctionFlags.push_back({&F, CSF});
-    // Update combined shader flags mask
-    CombinedSFMask.merge(CSF);
+
+    // Shader flags mask of each of the functions in an SCC of the call graph is
+    // the union of all functions in the SCC. Update shader flags masks of
+    // functions in SCC accordingly.
+    for (CallGraphNode *CGN : CurSCC) {
+      Function *F = CGN->getFunction();
+      if (!F)
+        continue;
+      mergeFunctionShaderFlags(F, SCCSF);
+    }
+  }
+
+  // Propagate Shader Flag Masks to callers with another post-order call graph
+  // walk
----------------
bogner wrote:

Wouldn't it be simpler to propagate the mask to callers in the first loop? Since we're iterating in SCC order, the loop over instructions above should just be able to query the flags for any call it encounters.

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