[Bug c++/82505] New: g++ -O3 -funroll-loops generates weird code

[barto at cambridgesemantics dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82505

            Bug ID: 82505
           Summary: g++ -O3 -funroll-loops generates weird code
           Product: gcc
           Version: 7.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barto at cambridgesemantics dot com
  Target Milestone: ---

Created attachment 42334
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42334&action=edit
Assembly output from g++ -O3 -funroll-loops -S

In the following code, the first two tests generate much the same code when
compiled
with g++ -O3 -funroll-loops.
The third test generates an excess of loads and moves. Assembly attached.

extern char *dest;
extern char *src;

int t0()
{
  for (int i = 0; i < 5; ++i)
  {
    dest[i] = src[i];
  }
}

int t1()
{
    dest[0] = src[0];
    dest[1] = src[1];
    dest[2] = src[2];
    dest[3] = src[3];
    dest[4] = src[4];
}

int t2()
{
  for (int i = 0; i < 5; ++i)
  {
    *dest = *src;
    ++dest;
    ++src;
  }
}