On 3/29/19 12:39 PM, Jakub Jelinek wrote:
On Fri, Mar 29, 2019 at 12:02:48PM -0400, Ed Smith-Rowland wrote:
This differs from the previous patch in actually testing constexpr :-\ and
in the addition of wrappers for __builtin_memmove and __builtin_memcmp that
supply constexpr branches if C++20 and is_constant_evaluated().
+ void*
+ __memmove(_Tp* __dst, const _Tp* __src, ptrdiff_t __num)
+ {
+#if __cplusplus > 201703L
+ if (is_constant_evaluated())
+ {
+ for(; __num > 0; --__num)
+ {
+ *__dst = *__src;
+ ++__src;
+ ++__dst;
+ }
+ return __dst;
+ }
+ else if (__num)
+#endif
+ return __builtin_memmove(__dst, __src, sizeof(_Tp) * abs(__num));
+ return __dst;
..
const ptrdiff_t _Num = __last - __first;
if (_Num)
- __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
+ __memmove(__result, __first, _Num);
..
const ptrdiff_t _Num = __last - __first;
if (_Num)
- __builtin_memmove(__result - _Num, __first, sizeof(_Tp) * _Num);
+ __memmove(__result - _Num, __first, _Num);
Why the abs in there, that is something that wasn't previously there and
if the compiler doesn't figure out that __last >= __first, it would mean
larger emitted code for the non-constexpr case. As memmove argument is
size_t, wouldn't it be better to make __num just size_t and remove this abs?
Also, wouldn't it be better to have on the other side the __num == 0
handling inside of __memmove, you already have it there for C++2a, but not
for older. You could then drop the if (_Num) guards around __memmove.
memmove needs to be able to work with __last < __first also.
I don't get it, you are replacing calls with __builtin_memmove with
__memmove, and the __builtin_memmove calls didn't do anything like that,
the last argument is size_t and didn't use any abs. So are you saying you
see crashes with the current code (when not in constexpr contexts) that your
patch fixes?
Jakub
If std::copy is intended to work with first < last then yes. OTOH, the
copy_move is just an impl detail for speed.
The std doesn't say that first >= last as a precondition but has
sentences like:
"For each non-negative integer n < (last - first), performs *(result +
n) = *(first + n)."
If this fixes a bug then I should make a pug report, a separate patch,
and probably backport it.
I also took out the else if (__num) in the __memmove since this check is
done at both call sites.
I made __memmove and __memcmp inline so that, certainly for C++ < 20
these don't pessimize.
Retesting.
Ed
int
main()
{
float arr[1000];
float brr[1000];
std::copy(arr + 500, arr, brr + 500);
}