Re: [uml-devel] Re: [uml-user] SuSE 9.1 host + 2.4.26 Kernel + GCC 3.3.3 similarities to 2.6.6!?

[Nix <nix@esperi.org.uk>]

On Mon, 14 Jun 2004, BlaisorBlade mused:
> Alle 23:58, domenica 13 giugno 2004, Nix ha scritto:
>> On Sun, 06 Jun 2004, uml@dot1q.net stipulated:
>> > Switch differences between -O1 and -O2 on GCC 3.3.3:
>>
>> Note: -O{n} are *not* the equivalent of groups of -f switches:
>> they turn those switches on, but other things inside the compiler
>> are directly triggered by the optimization level.
> 
> I expected that SuSE 9.1 shipped a buggy gcc, but I didn't found any confirm 
> to this (I remember they did such things in the past). However the fact that 
> the compiler output that dump is anyway a compiler bug...

Oh, yes, certainly.

> Well, ok, more hints, the first possible answer to this one seems to be that 
> SuSE enables -funit-at-a-time:

(unsurprising, given that -funit-at-a-time's primary author is jh@suse ;} )

> http://www.google.com/search?q=%22SuSE+9.1%22+bug+gcc&sourceid=opera&num=25&ie=utf-8&oe=utf-8
> 
> I checked the first hit (read the whole thread):
> 
> http://gcc.gnu.org/ml/gcc-bugs/2004-05/msg01912.html

Yep, -funit-at-a-time will definitely break kernels that aren't
expecting it, both because of elision of unused statics and because some
arches depend on emission of asm's at the top level to emit stuff in the
same order in generated code, and with -funit-at-a-time this just
doesn't happen anymore. (I don't think i386 is such an architecture,
though.)

-funit-at-a-time is enabled by default for C and C++ in GCC 3.4.

> This is the trick:
> "#define ATTRIBUTE_USED
> #if (__GNUC__ > 3) || (__GNUC__ == 3 && __GNUC_MINOR__ >=4)
> #undef ATTRIBUTE_USED
> #define ATTRIBUTE_USED __attribute__((used))
> #endif

The sort of thing you want to stash away in a header file and never lay
eyes on again. :)

> static const int hello ATTRIBUTE_USED  = 56;
> 
> int main()
> {
>         asm(
>         "mov hello, %eax\n"
>         );

Well, in this *specific* case, hello should be marked as clobbered,
which will also stop the compiler eliding it. But this *was* a contrived
testcase...

-- 
`We in no way believe that this Christ was a space alien.'
                    --- A creationist website goes completely bonkers


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