https://gcc.gnu.org/g:1729147f2b1a05ad2a4e8e2d91d1d2ba914039d6
commit r15-5694-g1729147f2b1a05ad2a4e8e2d91d1d2ba914039d6
Author: Jan Hubicka <hubi...@ucw.cz>
Date: Tue Nov 26 13:52:09 2024 +0100
improve std::deque::_M_reallocate_map
Looking into reason why we still do throw_bad_alloc in clang binary I
noticed
that quite few calls come from deque::_M_reallocate_map. This patch adds
unreachable to limit the size of realloc_map. _M_reallocate_map is called
only
if new size is smaller then max_size. map is an array holding pointers to
entries of fixed size.
Since rellocation is done by doubling the map size, I think the maximal
size of
map allocated is max_size / deque_buf_size rounded up times two. This
should
be also safe for overflows since we have extra bit.
map size is always at least 8. Theoretically this computation may be wrong
for
very large T, but in that case callers should never reallocate.
On the testcase I get:
jh@shroud:~> ~/trunk-install-new4/bin/g++ -O2 dq.C -c ; size -A dq.o | grep
text
.text 284 0
.text._ZNSt5dequeIiSaIiEE17_M_reallocate_mapEmb 485 0
.text.unlikely 10 0
jh@shroud:~> ~/trunk-install-new5/bin/g++ -O2 dq.C -c ; size -A dq.o | grep
text
.text 284 0
.text._ZNSt5dequeIiSaIiEE17_M_reallocate_mapEmb 465 0
.text.unlikely 10 0
so this saves about 20 bytes of rellocate_map, which I think is worthwhile.
Curiously enough gcc14 does:
jh@shroud:~> g++ -O2 dq.C -c ; size -A dq.o | grep text
.text 604 0
.text.unlikely 10 0
which is 145 bytes smaller. Obvoius difference is that _M_reallocate_map
gets inlined.
Compiling gcc14 preprocessed file with trunk gives:
jh@shroud:~> g++ -O2 dq.C -S ; size -A dq.o | grep text
.text 762 0
.text.unlikely 10 0
So inlining is due to changes at libstdc++ side, but code size growth is
due to
something else.
For clang this reduced number of thris_bad_new_array_length from 121 to 61.
libstdc++-v3/ChangeLog:
* include/bits/deque.tcc (std::deque::_M_reallocate_map): Add
__builtin_unreachable check to declare that maps are not very large.
* include/bits/stl_deque.h (std::deque::size): Add
__builtin_unreachable
to check for maximal size of map.
gcc/testsuite/ChangeLog:
* g++.dg/tree-ssa/deque-1.C: New test.
* g++.dg/tree-ssa/deque-2.C: New test.
Diff:
---
gcc/testsuite/g++.dg/tree-ssa/deque-1.C | 9 +++++++++
gcc/testsuite/g++.dg/tree-ssa/deque-2.C | 10 ++++++++++
libstdc++-v3/include/bits/deque.tcc | 4 ++++
libstdc++-v3/include/bits/stl_deque.h | 7 ++++++-
4 files changed, 29 insertions(+), 1 deletion(-)
diff --git a/gcc/testsuite/g++.dg/tree-ssa/deque-1.C
b/gcc/testsuite/g++.dg/tree-ssa/deque-1.C
new file mode 100644
index 000000000000..c639ebb1a5f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/deque-1.C
@@ -0,0 +1,9 @@
+// { dg-do compile }
+// { dg-options "-O1 -fdump-tree-optimized" }
+#include <deque>
+void
+test(std::deque<int> &q, int v)
+{
+ q.push_back (v);
+}
+// { dg-final { scan-tree-dump-not "throw_bad_alloc" "optimized" } }
diff --git a/gcc/testsuite/g++.dg/tree-ssa/deque-2.C
b/gcc/testsuite/g++.dg/tree-ssa/deque-2.C
new file mode 100644
index 000000000000..7e268b3f018d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/deque-2.C
@@ -0,0 +1,10 @@
+// { dg-do compile }
+// { dg-options "-O3 -fdump-tree-optimized" }
+#include <deque>
+std::deque<int *>
+test2(std::deque<int *> &q)
+{
+ return q;
+}
+// rethrow is OK, but throw is not.
+// { dg-final { scan-tree-dump-not {[^e]throw} "optimized" } }
diff --git a/libstdc++-v3/include/bits/deque.tcc
b/libstdc++-v3/include/bits/deque.tcc
index deb010a0ebb5..ee03c917a295 100644
--- a/libstdc++-v3/include/bits/deque.tcc
+++ b/libstdc++-v3/include/bits/deque.tcc
@@ -956,6 +956,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
+ std::max(this->_M_impl._M_map_size,
__nodes_to_add) + 2;
+ const size_t __bufsz = __deque_buf_size(sizeof(_Tp));
+ if (__new_map_size > ((max_size() + __bufsz - 1) / __bufsz) * 2)
+ __builtin_unreachable();
+
_Map_pointer __new_map = this->_M_allocate_map(__new_map_size);
__new_nstart = __new_map + (__new_map_size - __new_num_nodes) / 2
+ (__add_at_front ? __nodes_to_add : 0);
diff --git a/libstdc++-v3/include/bits/stl_deque.h
b/libstdc++-v3/include/bits/stl_deque.h
index c617933bd81c..dd1212793ded 100644
--- a/libstdc++-v3/include/bits/stl_deque.h
+++ b/libstdc++-v3/include/bits/stl_deque.h
@@ -1266,7 +1266,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_NODISCARD
size_type
size() const _GLIBCXX_NOEXCEPT
- { return this->_M_impl._M_finish - this->_M_impl._M_start; }
+ {
+ size_type __sz = this->_M_impl._M_finish - this->_M_impl._M_start;
+ if (__sz > max_size ())
+ __builtin_unreachable ();
+ return __sz;
+ }
/** Returns the size() of the largest possible %deque. */
_GLIBCXX_NODISCARD
