[llvm] [VPlan] Explicitly reassociate header mask in logical and (PR #180898)

[via llvm-commits]

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@@ -1614,6 +1605,31 @@ void VPlanTransforms::simplifyRecipes(VPlan &Plan) {
   }
 }
 
+/// Reassociate (headermask && x) && y -> headermask && (x && y) to allow the
+/// header mask to be simplified further, e.g. in optimizeEVLMasks.
+static void reassociateHeaderMask(VPlan &Plan) {
+  VPValue *HeaderMask = vputils::findHeaderMask(Plan);
+  if (!HeaderMask)
+    return;
+  SmallVector<VPUser *> Worklist(HeaderMask->users());
+  while (!Worklist.empty()) {
+    auto *R = dyn_cast<VPSingleDefRecipe>(Worklist.pop_back_val());
+    if (!R)
+      continue;
+    VPValue *X, *Y;
+    if (match(R, m_LogicalAnd(m_Specific(HeaderMask), m_VPValue())))
+      Worklist.append(R->user_begin(), R->user_end());
+    else if (match(R, m_LogicalAnd(
+                          m_LogicalAnd(m_Specific(HeaderMask), m_VPValue(X)),
+                          m_VPValue(Y)))) {
+      VPBuilder Builder(R);
+      Worklist.append(R->user_begin(), R->user_end());
+      R->replaceAllUsesWith(
+          Builder.createLogicalAnd(HeaderMask, Builder.createLogicalAnd(X, Y)));
----------------
ayalz wrote:

Slightly more logical to first update all users of R and then append them to worklist?
```suggestion
      R->replaceAllUsesWith(
          Builder.createLogicalAnd(HeaderMask, Builder.createLogicalAnd(X, Y)));
      Worklist.append(R->user_begin(), R->user_end());
```

https://github.com/llvm/llvm-project/pull/180898