[llvm] [SLP] Order clustered load base pointers by ascending offsets (PR #100653)

[David Green via llvm-commits]

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@@ -4842,10 +4842,27 @@ static bool clusterSortPtrAccesses(ArrayRef<Value *> VL, Type *ElemTy,
   if (!AnyConsecutive)
     return false;
 
-  for (auto &Base : Bases) {
-    for (auto &T : Base.second)
+  // If we have a better order, also sort the base pointers by increasing
+  // (variable) values if possible, to try and keep the order more regular.
+  SmallVector<std::pair<Value *, Value *>> SortedBases;
+  for (auto &Base : Bases)
+    SortedBases.emplace_back(Base.first,
+                             Base.first->stripInBoundsConstantOffsets());
+  llvm::stable_sort(SortedBases, [](std::pair<Value *, Value *> V1,
+                                    std::pair<Value *, Value *> V2) {
+    const Value *V = V2.second;
+    while(auto *Gep = dyn_cast<GetElementPtrInst>(V)) {
+      if (Gep->getOperand(0) == V1.second)
+        return true;
+      V = Gep->getOperand(0);
+    }
+    return false;
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davemgreen wrote:

I (unconfidently) believe so. I had a previous version where it would only look one step for adjacent nodes that was certainly not transitive, but this version doesn't have those issues.

https://github.com/llvm/llvm-project/pull/100653