[llvm] [VPlan] Reassociate header masks (PR #155383)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1119,6 +1120,14 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
     return;
   }
 
+  // Reassociate the header mask so it has more opportunities to be simplified.
+  // (headermask && x) && y -> headermask && (x && y)
+  if (HeaderMask && match(Def, m_LogicalAnd(m_LogicalAnd(m_Specific(HeaderMask),
+                                                         m_VPValue(X)),
+                                            m_VPValue(Y))))
+    return Def->replaceAllUsesWith(
+        Builder.createLogicalAnd(HeaderMask, Builder.createLogicalAnd(X, Y)));
+
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artagnon wrote:

Hm, not sure if we can generalize this to reassociate in a more general case: not sure why header masks are special?

https://github.com/llvm/llvm-project/pull/155383