[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:

Edit suggestion: "prioritize expensive operations." -> "prefer avoiding expensive ops" or "prefer less expensive ops".

Where does the magic cost scaling factor of 20 come from? I can't match it to the logic this weight function replaces.
It would be prudent to make the new defaults behave close to how they worked before the change. This is in the code path that will likely affect almost everything. We don't want any surprises.


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