Hi,
so if I understand it right, it should be safe to simply replace memmove
by memcpy. I wonder if we can get rid of the count != 0 check at least
for glibc systems. In general push_back now need inline-insns-auto to
be 33 to be inlined at -O2
jh@ryzen4:/tmp> cat ~/tt.C
#include <vector>
typedef unsigned int uint32_t;
struct pair_t {uint32_t first, second;};
struct pair_t pair;
void
test()
{
std::vector<pair_t> stack;
stack.push_back (pair);
while (!stack.empty()) {
pair_t cur = stack.back();
stack.pop_back();
if (!cur.first)
{
cur.second++;
stack.push_back (cur);
}
if (cur.second > 10000)
break;
}
}
int
main()
{
for (int i = 0; i < 10000; i++)
test();
}
jh@ryzen4:/tmp> ~/trunk-install/bin/g++ ~/tt.C -O2 --param
max-inline-insns-auto=32 ; time ./a.out
real 0m0.399s
user 0m0.399s
sys 0m0.000s
jh@ryzen4:/tmp> ~/trunk-install/bin/g++ ~/tt.C -O2 --param
max-inline-insns-auto=33 ; time ./a.out
real 0m0.039s
user 0m0.039s
sys 0m0.000s
Current inline limit is 15. We can save
- 2 insns if inliner knows that conditional guarding
builtin_unreachable will die (I have patch for this)
- 4 isnsn if we work out that on 64bit hosts allocating vector with
2^63 elements is impossible
- 2 insns if we allow NULL parameter on memcpy
- 2 insns if we allos NULL parameter on delete
So thi is 23 instructions. Inliner has hinting which could make
push_back reasonable candidate for -O2 inlining and then we could be
able to propagate interesitng stuff across repeated calls to push_back.
libstdc++-v3/ChangeLog:
* include/bits/stl_uninitialized.h (relocate_a_1): Use memcpy instead
of memmove.
diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h
b/libstdc++-v3/include/bits/stl_uninitialized.h
index 1282af3bc43..a9b802774c6 100644
--- a/libstdc++-v3/include/bits/stl_uninitialized.h
+++ b/libstdc++-v3/include/bits/stl_uninitialized.h
@@ -1119,14 +1119,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#ifdef __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
{
- // Can't use memmove. Wrap the pointer so that __relocate_a_1
+ // Can't use memcpy. Wrap the pointer so that __relocate_a_1
// resolves to the non-trivial overload above.
__gnu_cxx::__normal_iterator<_Tp*, void> __out(__result);
__out = std::__relocate_a_1(__first, __last, __out, __alloc);
return __out.base();
}
#endif
- __builtin_memmove(__result, __first, __count * sizeof(_Tp));
+ __builtin_memcpy(__result, __first, __count * sizeof(_Tp));
}
return __result + __count;
}
