[llvm] [InstrRef][NFC] Avoid another DenseMap, use a sorted vector (PR #99051)

[Jeremy Morse via llvm-commits]

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@@ -439,17 +454,20 @@ class TransferTracker {
     UseBeforeDefs.clear();
     UseBeforeDefVariables.clear();
 
-    // Map of the preferred location for each value.
-    DenseMap<ValueIDNum, LocationAndQuality> ValueToLoc;
+    // Mapping of the preferred locations for each value. Collected into this
+    // vector then sorted for easy searching.
+    SmallVector<ValueLocPair, 16> ValueToLoc;
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jmorse wrote:

We'd need to know the number of unique `ValueIDNum`s which then requires storage itself, so no easy way AFAIUI. I have in my notes (but don't have the original data any more, was over CTMark) that the median number of unique values used for variable locations in a block was 8, with the vast majority of the distribution being less than 20. So there's at least /some/ data behind all of this.

https://github.com/llvm/llvm-project/pull/99051