[Bug c/33024] gcc allows negatively-sized arrays

["sdyoung at miranda dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #3 from sdyoung at miranda dot org  2007-08-08 17:14 -------
Subject: Re:  gcc allows negatively-sized arrays

On Wed, Aug 08, 2007 at 04:55:08PM -0000, pinskia at gcc dot gnu dot org wrote:
> ------- Comment #2 from pinskia at gcc dot gnu dot org  2007-08-08 16:55 -------
> This is not a bug, VLA's size is unsigned so -2 is converted over to
> 0xFFFFFFFD.  also sizeof gives back an unsigned type.  This is only undefined
> behavior anyways.

  Looking at the assembly, this does not seem to be the case:



  Looking at the assembly, this does not seem to be the case:

        .file   "test.c"
        .section        .rodata
.LC0:
        .string "%d\n"
        .text
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx
        pushl   %ecx
        subl    $32, %esp
        movl    %gs:20, %eax
        movl    %eax, -12(%ebp)
        xorl    %eax, %eax
        movl    %esp, %eax
        movl    %eax, %ebx
        movl    $-2, -16(%ebp)
        movl    -16(%ebp), %edx
        movl    %edx, %eax
        sall    $2, %eax
        addl    $15, %eax
        addl    $15, %eax
        shrl    $4, %eax
        sall    $4, %eax
        subl    %eax, %esp
        leal    8(%esp), %eax
        movl    %eax, -28(%ebp)
        movl    -28(%ebp), %eax
        addl    $15, %eax
        shrl    $4, %eax
        sall    $4, %eax
        movl    %eax, -28(%ebp)
        movl    -28(%ebp), %eax
        movl    %eax, -20(%ebp)
        movl    %edx, %eax
        sall    $2, %eax
        movl    %eax, 4(%esp)
        movl    $.LC0, (%esp)
        call    printf
        movl    %ebx, %esp
        movl    -12(%ebp), %edx
        xorl    %gs:20, %edx
        je      .L3
        call    __stack_chk_fail
.L3:
        leal    -8(%ebp), %esp
        popl    %ecx
        popl    %ebx
        popl    %ebp
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.1.2 (Ubuntu 4.1.2-0ubuntu4)"
        .section        .note.GNU-stack,"",@progbits

  Can you point out where in there 0xFFFFFFFD is popping up? I agree
it's undefined behaviour, but the fact that it doesn't issue a warning
about such obviously wrong behaviour seems bad.

  Steve.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33024