[Bug driver/71905] New: bogus -Wlarger-than=N and -Wframe-larger-than warnings on small objects with large N

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71905

            Bug ID: 71905
           Summary: bogus -Wlarger-than=N and -Wframe-larger-than warnings
                    on small objects with large N
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

As discussed in the thread Re: RFA: new pass to warn on questionable uses of
alloca() and VLAs (https://gcc.gnu.org/ml/gcc-patches/2016-07/msg00953.html),
the -Wlarger-than=N and -Wframe-larger-than=N options accept arguments in
excess of 32-bits but apparently store their arguments as an unsigned int and
int, respectively, allowing slicing and conversion errors.

Likewise, the checkers for the options interpret the arguments as 32-bit
integers resulting in incorrect warnings when the N argument exceeds UINT_MAX
or INT_MAX, respectively.

If storing and correctly interpreting very large arguments isn't possible, the
driver should detect such arguments, and either issue a warning and disable the
corresponding option or issue an error.

(As a data point, Clang treats option arguments as 32-bit unsigned int and
issues the following warning when it's out of bounds:

clang (LLVM option parsing): for the -warn-stack-size option: '4294967316'
value invalid for uint argument!)

$ cat x.c && /build/gcc-walloca/gcc/xgcc -B /build/gcc-walloca/gcc  -S -Wall
-Wextra -Wpedantic -Wlarger-than=$((256*256*256*256+20))
-Wframe-larger-than=$((256*256*256*129)) x.c
void f (void*);

char a [32];

void g (void)
{
  char b [32];
  f (b);
}

x.c:3:6: warning: size of ‘a’ is 32 bytes [-Wlarger-than=]
 char a [32];
      ^
x.c: In function ‘g’:
x.c:7:8: warning: size of ‘b’ is 32 bytes [-Wlarger-than=]
   char b [32];
        ^
x.c:9:1: warning: the frame size of 32 bytes is larger than -2130706432 bytes
[-Wframe-larger-than=]
 }
 ^