From c30d5101f14f7e8a1c0f80c8663b54de2a7640fd Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2@gmail.com>
Date: Tue, 24 Nov 2020 12:29:08 -0500
Subject: [libc++] Optimize the number of assignments in std::exclusive_scan

Reported in https://twitter.com/blelbach/status/1169807347142676480

Differential Revision: https://reviews.llvm.org/D67273
---
 libcxx/include/numeric | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/libcxx/include/numeric b/libcxx/include/numeric
index 5c10dd3c1e67..ed06fcc49d5b 100644
--- a/libcxx/include/numeric
+++ b/libcxx/include/numeric
@@ -335,14 +335,17 @@ exclusive_scan(_InputIterator __first, _InputIterator __last,
 {
     if (__first != __last)
     {
-        _Tp __saved = __init;
-        do
+        _Tp __tmp(__b(__init, *__first));
+        while (true)
         {
-            __init = __b(__init, *__first);
-            *__result = __saved;
-            __saved = __init;
+            *__result = _VSTD::move(__init);
             ++__result;
-        } while (++__first != __last);
+            ++__first;
+            if (__first == __last)
+                break;
+            __init = _VSTD::move(__tmp);
+            __tmp = __b(__init, *__first);
+        }
     }
     return __result;
 }
-- 
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