[Bug tree-optimization/84069] New: missing strlen optimization after constant memcpy with offset

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84069

            Bug ID: 84069
           Summary: missing strlen optimization after constant memcpy with
                    offset
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

With pr83501 fixed by r256475, GCC 8 is able to fold the call to strlen() in
function f0() below into a constant, and replace the whole body of the function
with just a return statement.  Unfortunately (as pointed out in
https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01879.html), it is not able to
perform the same transformation in the equivalent function f1() in the test
case.

$ cat t.c && /ssd/build/gcc-svn/gcc/xgcc -B /ssd/build/gcc-svn/gcc -O2 -S -Wall
-fdump-tree-optimized=/dev/stdout t.c

int f0 (void)   // folded into 'return 5'
{
  const char s[] = "1234";
  char d[sizeof s + 1];

  *d = '0';
  __builtin_strcpy (d + 1, s);
  return __builtin_strlen (d);   // folded into 5
}

int f1 (void)   // not folded
{
  const char s[] = "1234";
  char d[sizeof s + 1];

  *d = '0';
  __builtin_memcpy (d + 1, s, sizeof s);
  return __builtin_strlen (d);   // not folded
}

;; Function f0 (f0, funcdef_no=0, decl_uid=1950, cgraph_uid=0, symbol_order=0)

f0 ()
{
  <bb 2> [local count: 1073741825]:
  return 5;

}



;; Function f1 (f1, funcdef_no=1, decl_uid=1955, cgraph_uid=1, symbol_order=1)

f1 ()
{
  char d[6];
  const char s[5];
  long unsigned int _1;
  int _6;

  <bb 2> [local count: 1073741825]:
  s = "1234";
  d[0] = 48;
  MEM[(char * {ref-all})&d + 1B] = MEM[(char * {ref-all})&s];
  _1 = __builtin_strlen (&d);
  _6 = (int) _1;
  s ={v} {CLOBBER};
  d ={v} {CLOBBER};
  return _6;

}