[Bug tree-optimization/86396] New: fold calls to strtod() into constants where possible

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86396

            Bug ID: 86396
           Summary: fold calls to strtod() into constants where possible
           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: ---

The canonical C interface to obtain the values of infinity and NaN are the
strtod(), strtof(), and strtold() family of functions.  The C standard defines
other functions such as atof(), nan() or fscanf() (and macros such as INFINITY
and NAN) but they are all specified in terms of strtod() et alia.  GCC provides
__builtin_nan(const char*) and __builtin_inf(), but as the test case below
shows it doesn't make use of these built-ins to fold calls to strtod().  It
would be a worthwhile improvement to add this feature, not just for infinity
and NaN, but for all other constant arguments.  For example, there is no reason
why the call atof("3.14") or any other call with a constant string that is
representable in the return type couldn't be folded into the corresponding
constant.

$ cat c.c && gcc -O2 -S -fdump-tree-optimized=/dev/stdout c.c
extern double atof (const char*);
extern double strtod (const char*, char **);
extern double nan (const char*);

double f0 (void)
{
  return atof ("nan");   // not folded but could be
}

double f1 (void)
{
  return nan ("0");   // folded into NaN
}

double f2 (void)
{
  return strtod ("inf", 0);   // not folded but could be
}

double f3 (void)
{
  return strtod ("nan", 0);   // not folded but could be
}


;; Function f0 (f0, funcdef_no=0, decl_uid=1905, cgraph_uid=1, symbol_order=0)

f0 ()
{
  double _3;

  <bb 2> [local count: 1073741825]:
  _3 = atof ("nan"); [tail call]
  return _3;

}



;; Function f1 (f1, funcdef_no=1, decl_uid=1908, cgraph_uid=2, symbol_order=1)

f1 ()
{
  <bb 2> [local count: 1073741825]:
  return  Nan;

}



;; Function f2 (f2, funcdef_no=2, decl_uid=1911, cgraph_uid=3, symbol_order=2)

f2 ()
{
  double _3;

  <bb 2> [local count: 1073741825]:
  _3 = strtod ("inf", 0B); [tail call]
  return _3;

}



;; Function f3 (f3, funcdef_no=3, decl_uid=1914, cgraph_uid=4, symbol_order=3)

f3 ()
{
  double _3;

  <bb 2> [local count: 1073741825]:
  _3 = strtod ("nan", 0B); [tail call]
  return _3;

}