[llvm] [VPlan] Allow folding not (cmp eq) -> icmp ne with other select users (PR #154497)

[Luke Lau via llvm-commits]

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@@ -1107,13 +1107,30 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
       return Def->replaceAllUsesWith(A);
 
     // Try to fold Not into compares by adjusting the predicate in-place.
-    if (isa<VPWidenRecipe>(A) && A->getNumUsers() == 1) {
+    auto CanFold = [&A](VPUser *U) {
+      return match(
+          U, m_CombineOr(m_Not(m_Specific(A)),
+                         m_Select(m_Specific(A), m_VPValue(), m_VPValue())));
+    };
+    if (isa<VPWidenRecipe>(A) && all_of(A->users(), CanFold)) {
       auto *WideCmp = cast<VPWidenRecipe>(A);
       if (WideCmp->getOpcode() == Instruction::ICmp ||
           WideCmp->getOpcode() == Instruction::FCmp) {
         WideCmp->setPredicate(
             CmpInst::getInversePredicate(WideCmp->getPredicate()));
-        Def->replaceAllUsesWith(WideCmp);
+        for (VPUser *U : to_vector(WideCmp->users())) {
+          auto *R = cast<VPSingleDefRecipe>(U);
+          if (match(R, m_Select(m_Specific(WideCmp), m_VPValue(X),
+                                m_VPValue(Y)))) {
+            // select (icmp pred), x, y -> select (icmp inv_pred), y, x
+            R->setOperand(1, Y);
+            R->setOperand(2, X);
+          } else {
+            // not (icmp pred) -> icmp inv_pred
+            assert(match(R, m_Not(m_Specific(WideCmp))) && "Unexpected user");
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lukel97 wrote:

No worries, stuff getting left behind in an assert is a common footgun so I'm glad you're checking for it :)

https://github.com/llvm/llvm-project/pull/154497