[libcxx-commits] [libcxx] [libc++][ranges] optimize the performance of `ranges::starts_with` (PR #84570)

[Louis Dionne via libcxx-commits]

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@@ -50,14 +54,36 @@ struct __fn {
       _Pred __pred   = {},
       _Proj1 __proj1 = {},
       _Proj2 __proj2 = {}) {
-    return __mismatch::__fn::__go(
+    if constexpr (sized_sentinel_for<_Sent1, _Iter1> && sized_sentinel_for<_Sent2, _Iter2>) {
+      auto __n1 = ranges::distance(__first1, __last1);
+      auto __n2 = ranges::distance(__first2, __last2);
+      if (__n2 == 0) {
+        return true;
+      }
+      if (__n2 > __n1) {
+        return false;
+      }
+
+      if constexpr (contiguous_iterator<_Iter1> && contiguous_iterator<_Iter2>) {
+        return ranges::equal(
+            std::move(__first1),
+            ranges::next(__first1, __n2),
+            std::move(__first2),
+            std::move(__last2),
+            std::ref(__pred),
+            std::ref(__proj1),
+            std::ref(__proj2));
+      }
+    }
+
+    return ranges::mismatch(
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ldionne wrote:

Maybe put in a `else` branch? I assume the compiler must instantiate `ranges::mismatch` even if the `if constexpr` above is true if you don't put it inside a branch. I'm not concerned about codegen since this will all fold away, but it would be better for compile-time.

https://github.com/llvm/llvm-project/pull/84570