[Bug middle-end/78328] New: wrong value in -Walloca-larger-than note

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78328

            Bug ID: 78328
           Summary: wrong value in -Walloca-larger-than note
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The -Walloca-larger-than warning prints the wrong maximum value of the
function's argument in its note.  For example, in the following, the argument
can be as large as UINT_MAX but the note prints 13.  This seems to be caused by
the code that tries to handle casts in alloca_call_type where VR_ANTI_RANGE
isn't being handled correctly for ranges resulting from expressions like the
one below (in this case, ~[13, 13]).

$ cat c.c && gcc -O2 -S -Wall -Wextra -Walloca-larger-than=128 c.c
void f (void*);

void g (unsigned n)
{
  if (n == 7 || n == 13) n = 11;
  f (__builtin_alloca (n));
}
c.c: In function ‘g’:
c.c:6:3: warning: argument to ‘alloca’ may be too large [-Walloca-larger-than=]
   f (__builtin_alloca (n));
   ^~~~~~~~~~~~~~~~~~~~~~~~
c.c:6:3: note: limit is 128 bytes, but argument may be as large as 13