[gcc r12-2965] libstdc++: Optimize std::seed_seq construction

Jonathan Wakely redi@gcc.gnu.org
Tue Aug 17 15:55:02 GMT 2021


https://gcc.gnu.org/g:174f9257a75dec93221eca26c236e0a6346c9dfd

commit r12-2965-g174f9257a75dec93221eca26c236e0a6346c9dfd
Author: Antony Polukhin <antoshkka@gmail.com>
Date:   Tue Aug 17 13:50:53 2021 +0100

    libstdc++: Optimize std::seed_seq construction
    
    When std::seed_seq is constructed from random access iterators we can
    detect the internal vector size in O(1). Reserving memory for elements
    in such cases may avoid multiple memory allocations.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/random.tcc (seed_seq::seed_seq): Reserve capacity
            if distance is O(1).
            * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
            line number.
    
    Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/random.tcc                     | 4 ++++
 libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc
index 0be50d90e8a..023fded7f5d 100644
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -3240,6 +3240,7 @@ namespace __detail
   template<typename _IntType, typename>
     seed_seq::seed_seq(std::initializer_list<_IntType> __il)
     {
+      _M_v.reserve(__il.size());
       for (auto __iter = __il.begin(); __iter != __il.end(); ++__iter)
 	_M_v.push_back(__detail::__mod<result_type,
 		       __detail::_Shift<result_type, 32>::__value>(*__iter));
@@ -3248,6 +3249,9 @@ namespace __detail
   template<typename _InputIterator>
     seed_seq::seed_seq(_InputIterator __begin, _InputIterator __end)
     {
+      if _GLIBCXX17_CONSTEXPR (__is_random_access_iter<_InputIterator>::value)
+	_M_v.reserve(std::distance(__begin, __end));
+
       for (_InputIterator __iter = __begin; __iter != __end; ++__iter)
 	_M_v.push_back(__detail::__mod<result_type,
 		       __detail::_Shift<result_type, 32>::__value>(*__iter));
diff --git a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
index 8fba7144d8a..3ab9c44232e 100644
--- a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
+++ b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
@@ -12,4 +12,4 @@ auto x = std::generate_canonical<std::size_t,
 
 // { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 169 }
 
-// { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 3352 }
+// { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 3356 }


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