[llvm] [NVPTX] Add TLI hook for load slice cost and implement it (PR #131847)

[Artem Belevich via llvm-commits]

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@@ -3133,6 +3133,20 @@ class TargetLoweringBase {
     return false;
   }
 
+  virtual unsigned getLoadSliceCost(bool ForCodeSize, unsigned Loads,
+                                    unsigned CrossRegisterBanksCopies,
+                                    unsigned Truncates, unsigned ZExts,
+                                    unsigned Shifts) const {
+    // Assume cross register banks copies are as expensive as loads.
+    unsigned ExpensiveOps = Loads + CrossRegisterBanksCopies;
+
+    // Unless we are optimizing for code size, prioritize expensive operations.
+    if (!ForCodeSize)
+      ExpensiveOps = ExpensiveOps * 20;
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Artem-B wrote:

Cost heuristics are prone to cause unexpected surprises, and I do not see a good way to validate the currently selected value without exposing it to the real world use cases. Perhaps we can make that scaling factor tunable as an escape hatch in case the current value produces suboptimal results for some users. 



https://github.com/llvm/llvm-project/pull/131847