[Bug tree-optimization/90766] strlen(a + i) missing range for arrays of unknown bound with strings of known length and variable i

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90766

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Blocks|                            |83819

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
For the same reason, although GCC folds the strlen() computation in the first
two functions into the difference N - i (where N is the known length of the
string), it fails to do the same thing in the last instance (ditto when b is a
char*).

$ cat a.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout a.c
typedef __SIZE_TYPE__ size_t;

const char s[] = "123";

size_t f (unsigned i)
{
  return __builtin_strlen (&s[i]);   // folded to 3 - i
}

extern char a[8];

size_t g (unsigned i)
{
  __builtin_strcpy (a, "123");
  return __builtin_strlen (&s[i]);   // folded to 3 - i
}

extern char b[];

size_t h (unsigned i)
{
  __builtin_strcpy (b, "123");
  return __builtin_strlen (&b[i]);   // not folded but could be
}

;; Function f (f, funcdef_no=0, decl_uid=1908, cgraph_uid=1, symbol_order=1)

Removing basic block 5
f (unsigned int i)
{
  sizetype _1;
  size_t iftmp.0_2;
  size_t iftmp.0_4;

  <bb 2> [local count: 1073741824]:
  if (i_3(D) <= 3)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _1 = (sizetype) i_3(D);
  iftmp.0_4 = 3 - _1;

  <bb 4> [local count: 1073741824]:
  # iftmp.0_2 = PHI <iftmp.0_4(3), 0(2)>
  return iftmp.0_2;

}



;; Function g (g, funcdef_no=1, decl_uid=1912, cgraph_uid=2, symbol_order=2)

Removing basic block 5
g (unsigned int i)
{
  sizetype _1;
  size_t iftmp.1_2;
  size_t iftmp.1_6;

  <bb 2> [local count: 1073741824]:
  __builtin_memcpy (&a, "123", 4);
  if (i_5(D) <= 3)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _1 = (sizetype) i_5(D);
  iftmp.1_6 = 3 - _1;

  <bb 4> [local count: 1073741824]:
  # iftmp.1_2 = PHI <iftmp.1_6(3), 0(2)>
  return iftmp.1_2;

}



;; Function h (h, funcdef_no=2, decl_uid=1916, cgraph_uid=3, symbol_order=3)

h (unsigned int i)
{
  char * _1;
  size_t _5;
  sizetype _6;

  <bb 2> [local count: 1073741824]:
  __builtin_memcpy (&b, "123", 4);
  _6 = (sizetype) i_4(D);
  _1 = &b + _6;
  _5 = __builtin_strlen (_1); [tail call]
  return _5;

}


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83819
[Bug 83819] [meta-bug] missing strlen optimizations