[Bug tree-optimization/78450] New: strlen(s) return value can be assumed to be less than the size of s

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78450

            Bug ID: 78450
           Summary: strlen(s) return value can be assumed to be less than
                    the size of s
           Product: gcc
           Version: 7.0
            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: ---

In a call to strlen(p) where p points to two or more constant strings of known
lengths GCC determines the length of the longest string and uses that as the
upper bound of the range of values returned by the call.

Similarly, in a call to strlen(q) where the length of one or more of the
strings pointed to by q is not known but where q points to arrays of known
size, GCC could use the size of the largest array as the upper bound (modulo
flexible array members and such).  However, the test case below shows that GCC
does not take advantage of this possible optimization opportunity.

$ (set -x && cat x.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout x.c | grep -e "^[fg] (" -e abort)
+ cat x.c
const char a[] = "123";
const char b[] = "1234";

void f (int i)
{
  const char *s = i < 0 ? a : b;
  unsigned n = __builtin_strlen (s);
  if (4 < n)
    __builtin_abort ();
}

char a3[3];
char b4[4];

void g (int i)
{
  const char *s = i < 0 ? a3 : b4;
  unsigned n = __builtin_strlen (s);
  if (3 < n)
    __builtin_abort ();
}

+ gcc -O2 -S -Wall -Wextra -Wpedantic -fdump-tree-optimized=/dev/stdout x.c
+ grep --color=auto -e '^[fg] (' -e abort
f (int i)
g (int i)
  __builtin_abort ();