[Bug tree-optimization/82017] New: missing strlen optimization for chained mempcpy calls

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82017

            Bug ID: 82017
           Summary: missing strlen optimization for chained mempcpy calls
           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: ---

GCC is able to track the length of the string created by the two calls to
memcpy in f() below but it's fails to do the same for the chained calls to
mempcpy in g().  Since mempcpy is specifically designed for such chained
copying it's even more likely to benefit from the strlen optimization than
memcpy.

$ cat t.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout t.c
unsigned f (char *d)
{
  __builtin_memcpy (d, "abc", 3);
  __builtin_memcpy (d + 3, "def", 4);

  return __builtin_strlen (d);   // strlen call folded into a constant
}

unsigned g (char *d)
{
  __builtin_mempcpy (__builtin_mempcpy (d, "abc", 3), "def", 4);
  return __builtin_strlen (d);   // strlen call not folded
}

;; Function f (f, funcdef_no=0, decl_uid=1815, cgraph_uid=0, symbol_order=0)

f (char * d)
{
  char * _1;

  <bb 2> [100.00%] [count: INV]:
  __builtin_memcpy (d_4(D), "abc", 3);
  _1 = d_4(D) + 3;
  __builtin_memcpy (_1, "def", 4);
  return 6;

}



;; Function g (g, funcdef_no=1, decl_uid=1818, cgraph_uid=1, symbol_order=1)

g (char * d)
{
  void * _1;
  long unsigned int _2;
  unsigned int _7;

  <bb 2> [100.00%] [count: INV]:
  _1 = __builtin_mempcpy (d_4(D), "abc", 3);
  __builtin_mempcpy (_1, "def", 4);
  _2 = __builtin_strlen (d_4(D));
  _7 = (unsigned int) _2;
  return _7;

}