https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89617
Bug ID: 89617
Summary: memmove used even after runtime guard against overlap
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jzwinck at gmail dot com
Target Milestone: ---
For the following example, GCC 8.3 uses memmove unless compiling with
-DRESTRICT. With -DRESTRICT it optimizes the copy to just 4 mov instructions.
The copy code is only invoked after checking that the addresses differ, so
moving the call to std::copy out of line just to decorate it with __restrict__
should not be necessary to achieve optimal code generation, given strict
aliasing.
#include <algorithm>
#include <iterator>
typedef int Arr[3];
inline void Copy(Arr& __restrict__ dest, const Arr& __restrict__ src)
{
std::copy(std::begin(src), std::end(src), dest);
}
void f2(Arr& dest, const Arr& src)
{
if (&dest != &src) // make sure there is no aliasing
{
#ifdef RESTRICT
Copy(dest, src); // generates better code, why?
#else
std::copy(std::begin(src), std::end(src), dest);
#endif
}
}
Demo: https://godbolt.org/z/mT18Hb
Discussion:
https://stackoverflow.com/questions/55015151/can-arrays-overlap-alias-or-is-gcc-being-overly-cautious
