[Bug tree-optimization/78327] New: -Walloca-large-than false positives due to bad range info for signed integers in [-TYPE_MAX + N, N]

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78327

            Bug ID: 78327
           Summary: -Walloca-large-than false positives due to bad range
                    info for signed integers in [-TYPE_MAX + N, N]
           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: ---

The following is a test case for the problem brought up in the the thread Re:
anti-ranges of signed variables
(https://gcc.gnu.org/ml/gcc/2016-11/msg00029.html).

In the test case below, n's range in function g is [-125, 2], making the
largest value that (n + 128) can evaluate to and alloca be called with 130. 
Yet -Walloca-larger-than=200 emits a warning for the function with a note
claiming that the computed value may be as large as 255.  That's incorrect.

Recompiling the same test case with -Walloca-larger-than=100 results in a
warning for function h as well (as expected), and with a note correctly
indicating the largest value alloca can be called with there given its range of
[-125, 1]: 129.  (The warning for g still says 255.)

  c.c:14:3: note: limit is 100 bytes, but argument may be as large as 129

The output of -fdump-tree-vrp confirms that the range GCC computes for n in g
is [0, 255], while in h [3, 129].  The same problem seems to affect all signed
integers.


$ cat c.c && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -O2 -S
-Wall -Wextra -Walloca-larger-than=200 c.c
void f (void*);

void g (signed char n)
{
  if (n < -125 || 2 < n) n = 0;

  f (__builtin_alloca (n + 128));
}

void h (signed char n)
{
  if (n < -125 || 1 < n) n = 0;

  f (__builtin_alloca (n + 128));
}

c.c: In function ‘g’:
c.c:7:3: warning: argument to ‘alloca’ may be too large [-Walloca-larger-than=]
   f (__builtin_alloca (n + 128));
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
c.c:7:3: note: limit is 200 bytes, but argument may be as large as 255