On Mon, 2011-07-25 at 19:50 -0400, Arnaud Lacombe wrote:
> gcc will only emits the warning at -Os. It seems to me that the
> resulting code clearly ends-up testing an uninitialized value, ie.
> assuming the following test-case:
>
> extern void *e(void);
> extern void *f(void);
> extern void g(void);
>
> void fn(void)
> {
> void *b, *a;
>
> a = e();
> if (a != 0)
> b = f();
> if (a != 0 && b != 0)
> g();
> }
>
> gcc 4.5.1 will generates the following x86-32 assembly:
>
> % gcc -m32 -Wall -Os -c -S -o - kernel/trace/trace_printk.c
> .file "trace_printk.c"
> kernel/trace/trace_printk.c: In function 'fn':
> kernel/trace/trace_printk.c:7:8: warning: 'b' may be used
> uninitialized in this function
> .text
> .globl fn
> .type fn, @function
> fn:
> pushl %ebp
> movl %esp, %ebp
> pushl %esi
> pushl %ebx
> call e
> testl %eax, %eax
> movl %eax, %ebx
> je .L2
> call f
> movl %eax, %esi
> .L2:
> testl %esi, %esi
> je .L1
> testl %ebx, %ebx
> je .L1
> popl %ebx
> popl %esi
> popl %ebp
> jmp g
> .L1:
> popl %ebx
> popl %esi
> popl %ebp
> ret
> .size fn, .-fn
> .ident "GCC: (GNU) 4.5.1 20100924 (Red Hat 4.5.1-4)"
> .section .note.GNU-stack,"",@progbits
>
I wrote a similar program and got the same results for both 4.5.1 and
4.6.0. but only with -Os and -O2 seems fine.
> It seems gcc transforms the conditional from:
>
> if (a != NULL && b != NULL) ...
>
> to
>
> if (b != NULL && a != NULL) ...
>
> In which case the warning is fully valid. I'm not sure what's the C
> standard guarantee in term of conditional test order. gcc 4.7.0 has
> the same behavior.
Yes it seems to be doing this :-/
This is a real bug!
-- Steve
