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Date: Fri, 20 Jun 2014 22:09:17 +0000 (UTC)
From: Clément Vasseur <clement.vasseur@...il.com>
To: musl@...ts.openwall.com
Subject: Re: broken gcc optimization for facilitynames
On 2014-06-19, Rich Felker <dalias@...c.org> wrote:
> On Wed, Jun 18, 2014 at 04:31:58PM +0000, Clément Vasseur wrote:
>> Hello,
>>
>> I might have found a gcc code generation problem with the facilitynames
>> implementation used in musl. See the following reduced test case:
>>
>> $ cat test-facilitynames.c
>> #define SYSLOG_NAMES
>> #include <syslog.h>
>> int main(void)
>> {
>> for (int i = 0; facilitynames[i].c_name; i++)
>> if (facilitynames[i].c_name)
>> return facilitynames[i].c_val;
>> }
>>
>> $ musl-gcc -std=gnu99 -O1 test-facilitynames.c && ./a.out; echo $?
>> 32
>>
>> $ musl-gcc -std=gnu99 -O2 test-facilitynames.c && ./a.out; echo $?
>> 1
>>
>> I see similar results with gcc versions 4.6.1, 4.8.3 and 4.9.0 (with a
>> different return value with -O2).
>
> I can verify that I see this one on gcc 4.7.3. I don't see any UB in
> the code so I'm pretty sure this is a gcc bug.
>
> Note that we should really fix this horrible definition of
> facilitynames (it bloats busybox by quite a bit), but it's a nice
> demonstration of the gcc bug which we should also report and try to
> get fixed..
>
> Can you make a self-contained test case (copy the macros from musl's
> syslog.h into the source file) and file a gcc bug report?
I made some progress towards a standalone test case. Here it is:
struct s1 { int v; };
struct s2 { int v; };
#define a ((struct s2 *)(struct s1 []){{ 42 }})
int main(void)
{
for (int i = 0; a[i].v; i++)
if (a[i].v)
return a[i].v;
}
I also found out which optimization flag causes the broken output, it's
-fstrict-aliasing. I guess the issue here is whether casting `struct s1
[]` to `struct s2 *` violates the strict aliasing rules or not. Indeed,
compiling with -Wstrict-aliasing=1 shows a warning at this location.
Can someone pinpoint the exact location in the ISO C spec where there is
an explanation about the aliasing rules with this kind of pointer
compatibility?
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