[PATCH] Help compiler detect invalid code

[François Dumont]

On 9/27/19 1:24 PM, Jonathan Wakely wrote:
> On 20/09/19 07:08 +0200, François Dumont wrote:
>> I already realized that previous patch will be too controversial to 
>> be accepted.
>>
>> In this new version I just implement a real memmove in __memmove so 
>
> A real memmove doesn't just work backwards, it needs to detect any
> overlaps and work forwards *or* backwards, as needed.
ok, good to know, I understand now why using __builtin_memcopy didn't 
show any performance enhancement when I tested it !
>
> I think your change will break this case:
>
> #include <algorithm>
>
> constexpr int f(int i, int j, int k)
> {
> Â int arr[5] = { 0, 0, i, j, k };
> Â std::move(arr+2, arr+5, arr);
> Â return arr[0] + arr[1] + arr[2];
> }
>
> static_assert( f(1, 2, 3) == 6 );
>
> This is valid because std::move only requires that the result iterator
> is not in the input range, but it's OK for the two ranges to overlap.
>
> I haven't tested it, but I think with your change the array will end
> up containing {3, 2, 3, 2, 3} instead of {1, 2, 3, 2, 3}.
>
Indeed, I've added a std::move constexpr test in this new proposal which 
demonstrate that.

C++ Standard clearly states that [copy|move]_backward is done backward. 
So in this new proposal I propose to add a __memcopy used in copy/move 
and keep __memmove for *_backward algos. Both are using 
__builtin_memmove as before.


 Â Â Â  * include/bits/stl_algobase.h (__memmove): Return void, loop as long as
 Â Â Â  __n != 0.
 Â Â Â  (__memcopy): New.
 Â Â Â  (__copy_move<_IsMove, true, 
std::random_access_iterator_tag>::__copy_m):
 Â Â Â  Adapt to use latter.
 Â Â Â  (__copy_move_backward_a): Remove std::is_constant_evaluated block.
 Â Â Â  * testsuite/25_algorithms/copy/constexpr.cc (test): Add check on copied
 Â Â Â  values.
 Â Â Â  * testsuite/25_algorithms/copy_backward/constexpr.cc (test): Likewise
 Â Â Â  and rename in test1.
 Â Â Â  (test2): New.
 Â Â Â  * testsuite/25_algorithms/copy/constexpr_neg.cc: New.
 Â Â Â  * testsuite/25_algorithms/copy_backward/constexpr.cc: New.
 Â Â Â  * testsuite/25_algorithms/equal/constexpr_neg.cc: New.
 Â Â Â  * testsuite/25_algorithms/move/constexpr.cc: New.
 Â Â Â  * testsuite/25_algorithms/move/constexpr_neg.cc: New.

Tested under Linux x86_64.

Ok to commit ?

François

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