[llvm] [TableGen] Rework error reporting for duplicate Feature/Processor. (PR #102257)

[Tomas Matheson via llvm-commits]

================
@@ -289,20 +286,12 @@ unsigned SubtargetEmitter::FeatureKeyValues(
     printFeatureMask(OS, ImpliesList, FeatureMap);
 
     OS << " },\n";
-    ++NumFeatures;
-
-    auto [It, Inserted] = UniqueFeatures.insert({CommandLineName, Feature});
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tmatheson-arm wrote:

It seems we have gone from:
- one O(n) loop over the features at line 272, with an O(1) check of `UniqueFeatures`,

to:
- one complete sort of the features (n*O(log(n)) I think?) plus two O(n) loops over the features (`std::adjacent_find` + line 272),

which looks like a performance regression. Having said that, it probably doesn't really matter for this workload.

https://github.com/llvm/llvm-project/pull/102257