Memory overwrite?

[bill@valiant (R.A.L Williams)]

In article <3g6fi6$fuv@gnat.cs.nyu.edu> Robert Dewar wrote:
: Mark asks if GNAT can also find uninitialized variables, like GCC.

No, actually it was me

: GNAT IS GCC! Use the GCC option, and you will get the output you want!
: \x1a

Well, no! I used these two programs...

with GNAT.IO; 
use GNAT.IO;

procedure CRASH is
  subtype CONSTRAINED_INT is INTEGER range 2 .. 5;
  I : INTEGER;
  J : CONSTRAINED_INT;
  X : array(CONSTRAINED_INT) of INTEGER;
begin
  for I in CONSTRAINED_INT'RANGE loop
    X(I) := I * 2;
    PUT(I); PUT(" : "); PUT(X(I)); NEW_LINE;
  end loop;
  PUT_LINE("The next command may cause a CONSTRAINT_ERROR");
  PUT(J); PUT(" : "); PUT(X(J)); NEW_LINE;
end;

compiled with 'gnat -c -O -Wuninitialized crash.adb' gave no
errors and no warnings; whereas

#include <stdio.h>

main()
{
  int i, j, x[4];

  for (i = 0; i < sizeof(x)/sizeof(int); i++)
  {
    x[i] = (i + 2) * 2;
    printf("%d : %d\n", i, x[i]);
  }
  printf("The next statement may cause a crash\n");
  printf("%d : %d\n", j, x[j]);
}

compiled with 'gcc -O -Wuninitialized -o crash-c crash-c.c'
told me:
  crash-c.c: In function `main':
  crash-c.c:5: warning: `j' might be used uninitialized in this function

Obviously this is a compiler issue, NOT a language issue. There is
no reason that I know of why future versions of GNAT shouldn't generate
similar warnings in these circumstances.

Interestingly, the -Wuninitialized option only works when optimization
is enabled. Does this imply that there isn't much optimization in
GNAT yet. (Can't complain, it is free and available and you cant say that
about any other Ada95 compilers).

Bill Williams