commit 99e7df8ddcb7ddc5606a97533de0ab49321df303
author Louis Dionne <ldionne.2@gmail.com> Tue Nov 24 12:29:08 2020 -0500
committer Copybara-Service <copybara-worker@google.com> Tue Dec 01 09:56:00 2020 -0800
tree 1bd1c526c84dce023c4d3e85659652aa548a3ccf
parent 1846f1d18eee6b7720f4d89f89e22159f0e13dc4

[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

GitOrigin-RevId: c30d5101f14f7e8a1c0f80c8663b54de2a7640fd

include/numeric

diff --git a/include/numeric b/include/numeric
index 5c10dd3..ed06fcc 100644
--- a/include/numeric
+++ b/include/numeric
@@ -335,14 +335,17 @@
 {
     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;
 }