[llvm] [LoopInterchange] Stop performing unprofitable interchange (PR #127473)

[Sjoerd Meijer via llvm-commits]

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@@ -1184,11 +1218,27 @@ std::optional<bool> LoopInterchangeProfitability::isProfitableForVectorization(
   return std::optional<bool>(!DepMatrix.empty());
 }
 
-bool LoopInterchangeProfitability::isProfitable(
-    const Loop *InnerLoop, const Loop *OuterLoop, unsigned InnerLoopId,
-    unsigned OuterLoopId, CharMatrix &DepMatrix,
-    const DenseMap<const Loop *, unsigned> &CostMap,
-    std::unique_ptr<CacheCost> &CC) {
+// The bubble-sort fashion algorithm is adopted to sort the loop nest, so the
+// comparison function should ideally induce a strict weak ordering required by
+// some standard C++ libraries. In particular, isProfitable should hold the
+// following properties.
+//
+// Asymmetry: If isProfitable(a, b) is true then isProfitable(b, a) is false.
----------------
sjoerdmeijer wrote:

Question on the assymetry: does this really need to hold? Can it not be the case that `isProfitable(a, b)` and `isProfitable(b, a)` are both profitable, or both neutral, or both not really good? I haven't tried to come up with examples, but I thought about this as a way of saying the cost-model saying "`don't know`"?

https://github.com/llvm/llvm-project/pull/127473