linux-kernel - Re: [PATCH] TRACING: Fix a copmile warning

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Date:	Mon, 25 Jul 2011 20:35:34 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	gcc-help@....gnu.org, stufever@...il.com,
	linux-kernel@...r.kernel.org,
	Wang Shaoyan <wangshaoyan.pt@...bao.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] TRACING: Fix a copmile warning

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


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