On Thu, 23 Nov 2023 at 15:44, Jan Hubicka <hubi...@ucw.cz> wrote:
>
> 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.
I don't think we can do that. It's still undefined with glibc, and
glibc marks it with __attribute__((nonnull)), and ubsan will diagnose
it.
> 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
I don't think we can do that.
> - 2 insns if we allos NULL parameter on delete
That's allowed, I think we just check first to avoid making a function
call if it's null, because we know operator delete will do nothing.
But if it's hurting inlining, maybe that's the wrong choice.
> 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.
This patch is OK for trunk.
>
> 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;
> }
>
