[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),
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ldionne wrote:

You'd have to compute `ranges::next(__first1, __n2)` before the function call if you want to move `__first1`. Otherwise you could end up using a moved-from iterator in the `ranges::next` call.

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