Date: Tue, 13 Mar 2007 15:06:32 +0100 From: Maxime Henrion <mux@FreeBSD.org> To: Ricardo Nabinger Sanchez <rnsanchez@wait4.org> Cc: freebsd-bugs@FreeBSD.ORG, freebsd-current@FreeBSD.ORG Subject: Re: Bad gcc -O optimization cause core dump. What to do? Message-ID: <20070313140632.GK65356@elvis.mu.org> In-Reply-To: <20070313101312.71d35c32.rnsanchez@wait4.org> References: <20070313121106.GA96293@nagual.pp.ru> <20070313101312.71d35c32.rnsanchez@wait4.org>
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Ricardo Nabinger Sanchez wrote:
> On Tue, 13 Mar 2007 15:11:07 +0300
> Andrey Chernov <ache@freebsd.org> wrote:
>
> > cc -O -S a.c
> > .file "a.c"
> > .text
> > .p2align 2,,3
> > .globl main
> > .type main, @function
> > main:
> > pushl %ebp
> > movl %esp, %ebp
> > subl $8, %esp
> > andl $-16, %esp
> > subl $28, %esp
> > pushl $0
> > call puts
> > leave
> > ret
> > .size main, .-main
> > .ident "GCC: (GNU) 3.4.6 [FreeBSD] 20060825"
>
> Confirmed on FreeBSD-6.1 RELEASE:
>
> .file "bla.c"
> .text
> .p2align 2,,3
> .globl main
> .type main, @function
> main:
> pushl %ebp
> movl %esp, %ebp
> subl $8, %esp
> andl $-16, %esp
> subl $28, %esp
> pushl $0
> call puts
> leave
> ret
> .size main, .-main
> .ident "GCC: (GNU) 3.4.4 [FreeBSD] 20050518"
>
> > It calls "puts(NULL)" with core dump.
> > It means "printf("%s\n", NULL)" is overoptimized.
> > BTW, things like "printf("1%s\n", NULL)" are not overoptimized.
> > Any ideas? Is it right or needs to be fixed?
>
> Given that this is not what the user asked (replacing printf with puts), I
> consider this a bug. GCC made its assumption, and it was incorrect--it's not
> user's fault.
GCC can do whatever it wants here, even printing "foobar42", because the
C standard says that passing a NULL pointer to a %s format will yield
undefined behaviour. It *is* user's fault to have passed NULL to
printf() in the first place.
So, while we could argue that GCC's behaviour here is useless, annoying,
etc, this just can't be called a bug in GCC. As a side note, these
"optimizations" are in place since a *long* time now.
Cheers,
Maxime
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