[llvm] [VPlan] Process simplifyRecipes via a worklist (PR #133977)

[Luke Lau via llvm-commits]

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@@ -1075,16 +1003,110 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
              m_DerivedIV(m_SpecificInt(0), m_SpecificInt(0), m_VPValue()))) &&
       TypeInfo.inferScalarType(R.getOperand(1)) ==
           TypeInfo.inferScalarType(R.getVPSingleValue()))
-    return R.getVPSingleValue()->replaceAllUsesWith(R.getOperand(1));
+    return R.getOperand(1);
+
+  return nullptr;
 }
 
 void VPlanTransforms::simplifyRecipes(VPlan &Plan, Type &CanonicalIVTy) {
   ReversePostOrderTraversal<VPBlockDeepTraversalWrapper<VPBlockBase *>> RPOT(
       Plan.getEntry());
   VPTypeAnalysis TypeInfo(&CanonicalIVTy);
+  SetVector<VPRecipeBase *> Worklist;
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lukel97 wrote:

Oh thanks for sharing that, I didn't know about that previous PR. Why do we need it to be pop_front/push_back? I believe InstCombine uses pop_back/push_back, i.e. immediately processes anything added to the list.

I also just realised that InstCombine pushes the initial instructions onto the worklist in order, so the perceived order is actually top to bottom. I've updated that here 3ca3643c14a895bf94879ebfe9e85f942b7242ea

https://github.com/llvm/llvm-project/pull/133977