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libstdc++: Also disable caching of reverse_view::begin() for common_ranges

Message ID 20200228163459.1815748-1-ppalka@redhat.com
State New
Headers show
Series libstdc++: Also disable caching of reverse_view::begin() for common_ranges | expand

Commit Message

Patrick Palka Feb. 28, 2020, 4:34 p.m. UTC 
When the underlying range models common_range, then reverse_view::begin() is
O(1) without caching.  So we should disable the cache in this case too.

libstdc++-v3/ChangeLog:

	* include/std/ranges (reverse_view::_S_needs_cached_begin): Set to false
	whenever the underlying range models common_range.
---
 libstdc++-v3/include/std/ranges | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Comments

Jonathan Wakely Feb. 28, 2020, 4:48 p.m. UTC | #1 
On 28/02/20 11:34 -0500, Patrick Palka wrote:
>When the underlying range models common_range, then reverse_view::begin() is
>O(1) without caching.  So we should disable the cache in this case too.
>
>libstdc++-v3/ChangeLog:
>
>	* include/std/ranges (reverse_view::_S_needs_cached_begin): Set to false
>	whenever the underlying range models common_range.

OK, thanks.
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Patch

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 2f773130979..19d3da950e7 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3177,7 +3177,8 @@  namespace views
     private:
       _Vp _M_base = _Vp();
 
-      static constexpr bool _S_needs_cached_begin = !random_access_range<_Vp>;
+      static constexpr bool _S_needs_cached_begin
+	= !common_range<_Vp> && !random_access_range<_Vp>;
       [[no_unique_address]]
 	__detail::__maybe_empty_t<_S_needs_cached_begin,
 				  __detail::_CachedPosition<_Vp>> _M_cached_begin;