[Bug tree-optimization/89043] New: strcat (strcpy (d, a), b) not folded to stpcpy (strcpy (d, a), b)

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89043

            Bug ID: 89043
           Summary: strcat (strcpy (d, a), b) not folded to stpcpy (strcpy
                    (d, a), b)
           Product: gcc
           Version: 9.0
            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 folds a small subset of calls to C strcpy to the POSIX stpcpy when doing so
is profitable, such as in function f below.  But it doesn't do the same folding
in the common case when strcat is being called with the result of strcpy as the
first argument, as in function g below.

$ cat u.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout u.c
extern char* stpcpy (char*, const char*);
extern char* strcat (char*, const char*);
extern char* strcpy (char*, const char*);
extern char* strchr (const char*, int);


extern char *a, *b, *d;

char* f (char *d, const char *s)
{
  strcpy (d, s);          // folded to 'p = stpcpy (d, s);'
  return strchr (d, 0);   // folded to 'return p;'
}

void g (void)
{
  // could be folded to 'strcpy (stpcpy (d, a));'
  strcat (strcpy (d, a), b);
}

;; Function f (f, funcdef_no=0, decl_uid=1922, cgraph_uid=1, symbol_order=0)

f (char * d, const char * s)
{
  char * _7;

  <bb 2> [local count: 1073741824]:
  _7 = __builtin_stpcpy (d_2(D), s_3(D));
  return _7;

}



;; Function g (g, funcdef_no=1, decl_uid=1925, cgraph_uid=2, symbol_order=1)

g ()
{
  char * b.0_1;
  char * a.1_2;
  char * d.2_3;
  char * _4;

  <bb 2> [local count: 1073741824]:
  b.0_1 = b;
  a.1_2 = a;
  d.2_3 = d;
  _4 = strcpy (d.2_3, a.1_2);
  strcat (_4, b.0_1); [tail call]
  return;

}