[llvm] [ADT] Make set_subtract more efficient when subtrahend is larger (NFC) (PR #98702)

[Jakub Kuderski via llvm-commits]

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@@ -92,9 +92,27 @@ S1Ty set_difference(const S1Ty &S1, const S2Ty &S2) {
   return Result;
 }
 
+/// set_subtract_vec(A, B) - Compute A := A - B, where B can be a vector.
+///
+template <class S1Ty, class S2Ty>
+void set_subtract_vec(S1Ty &S1, const S2Ty &S2) {
+  for (typename S2Ty::const_iterator SI = S2.begin(), SE = S2.end(); SI != SE;
+       ++SI)
+    S1.erase(*SI);
+}
+
 /// set_subtract(A, B) - Compute A := A - B
 ///
+/// Selects the set to iterate based on the relative sizes of A and B for better
+/// efficiency.
+///
 template <class S1Ty, class S2Ty> void set_subtract(S1Ty &S1, const S2Ty &S2) {
+  if (S1.size() < S2.size()) {
+    for (typename S1Ty::iterator SI = S1.begin(), SE = S1.end(); SI != SE; ++SI)
+      if (S2.count(*SI))
+        S1.erase(SI);
+    return;
+  }
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kuhar wrote:

This makes sense to me in the absence of more standardized type traits we could use.

https://github.com/llvm/llvm-project/pull/98702