* [PATCH] copy_n for input iterator, avoid in-loop jumps
@ 2017-08-24 11:34 Petr Ovtchenkov
0 siblings, 0 replies; only message in thread
From: Petr Ovtchenkov @ 2017-08-24 11:34 UTC (permalink / raw)
To: libstdc++; +Cc: gcc-patches
Reword loop in copy_n specialization for input iterator.
Avoid condition check and jumps within loop.
Pay attention that input iterator incremented n - 1 times,
while output iterator incremented n times.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50119 and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81857
---
libstdc++-v3/include/bits/stl_algo.h | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)
diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index c2ac031..bf043cd 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -757,17 +757,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_OutputIterator __result, input_iterator_tag)
{
if (__n > 0)
- {
- while (true)
- {
- *__result = *__first;
- ++__result;
- if (--__n > 0)
- ++__first;
- else
- break;
- }
- }
+ {
+ *__result = *__first;
+ ++__result;
+ --__n;
+ }
+ for (; __n > 0; --__n)
+ {
+ ++__first;
+ *__result = *__first;
+ ++__result;
+ }
return __result;
}
--
2.10.1
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