RFH: compile-time assertions now broken in GCC
 

Hi,

https://bugs.launchpad.net/ubuntu/+source/mksh/+bug/1058035
was just the beginning, as gcc-snapshot in Debian had it (I
wrote that in the bugreport already), but, now, gcc-4.{6,7}
in sid also have it.

I’ve just tracked it down for gcc-4.6 to have been introdu‐
ced between 4.6.3-9 and 4.6.3-10; gcc-4.7 as currently sits
in wheezy (4.7.1-7) doesn’t have the problem, 4.7.2-2 does.

tg@zigo:~/b $ cat t.c
#include <stdint.h>
typedef int32_t mksh_ari_t;
char ari_sign_32_bit_and_wrap[(

(mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 1) >
(mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 2)

) ? 1 : -1];

int main(void) { return (sizeof(ari_sign_32_bit_and_wrap)); }
tg@zigo:~/b $ gcc-4.6 -O -Wextra t.c
t.c:6:62: warning: integer overflow in expression [-Woverflow]
t.c:3:6: error: variably modified ‘ari_sign_32_bit_and_wrap’ at file scope

My question now is, how am I supposed to deal with that? Will
the affected gcc versions transition into wheezy? In that case
I’ll need to apply the hotfix I made for Ubuntu into Debian as
well, so I’d like an answer.

More generally, why does GCC suddenly break such compile-time
check constructs? (mksh guarantees to scripts it runs that all
arithmetic operations are 32-bit and that both signed and un‐
signed operations wrap around, so it *must* know whether the
system does that correctly. And its build script is cross-com‐
pile agnostic, so it doesn’t do run-time checks.)

Thanks in advance,
//mirabilos
--
“It is inappropriate to require that a time represented as
seconds since the Epoch precisely represent the number of
seconds between the referenced time and the Epoch.”
-- IEEE Std 1003.1b-1993 (POSIX) Section B.2.2.2


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RFH: compile-time assertions now broken in GCC
 

On 03.10.2012 15:59, Thorsten Glaser wrote:
> Hi,
>
> https://bugs.launchpad.net/ubuntu/+source/mksh/+bug/1058035
> was just the beginning, as gcc-snapshot in Debian had it (I
> wrote that in the bugreport already), but, now, gcc-4.{6,7}
> in sid also have it.
>
> I’ve just tracked it down for gcc-4.6 to have been introdu‐
> ced between 4.6.3-9 and 4.6.3-10; gcc-4.7 as currently sits
> in wheezy (4.7.1-7) doesn’t have the problem, 4.7.2-2 does.
>
> tg@zigo:~/b $ cat t.c
> #include <stdint.h>
> typedef int32_t mksh_ari_t;
> char ari_sign_32_bit_and_wrap[(
>
> (mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 1) >
> (mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 2)
>
> ) ? 1 : -1];
>
> int main(void) { return (sizeof(ari_sign_32_bit_and_wrap)); }
> tg@zigo:~/b $ gcc-4.6 -O -Wextra t.c
> t.c:6:62: warning: integer overflow in expression [-Woverflow]
> t.c:3:6: error: variably modified ‘ari_sign_32_bit_and_wrap’ at file scope
>
> My question now is, how am I supposed to deal with that? Will
> the affected gcc versions transition into wheezy? In that case
> I’ll need to apply the hotfix I made for Ubuntu into Debian as
> well, so I’d like an answer.
>
> More generally, why does GCC suddenly break such compile-time
> check constructs? (mksh guarantees to scripts it runs that all
> arithmetic operations are 32-bit and that both signed and un‐
> signed operations wrap around, so it *must* know whether the
> system does that correctly. And its build script is cross-com‐
> pile agnostic, so it doesn’t do run-time checks.)

how does gcc-snapshot behave? If you think it's a regression, please could you
forward it upstream?

thanks, Matthias


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RFH: compile-time assertions now broken in GCC
 

Matthias Klose dixit:

>how does gcc-snapshot behave?

It also introduces this failure.

>If you think it's a regression, please could you forward it upstream?

OK. In the meantime, I’ve prepared workarounds upstream (and
made the code a bit less depending on -fwrapv), but I need to
know whether I should upload mksh to sid now with the hotfix
applied to Ubuntu, i.e. whether you plan to have the affected
gcc versions migrate.

bye,
//mirabilos
--
Sometimes they [people] care too much: pretty printers [and syntax highligh-
ting, d.A.] mechanically produce pretty output that accentuates irrelevant
detail in the program, which is as sensible as putting all the prepositions
in English text in bold font. -- Rob Pike in "Notes on Programming in C"


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RFH: compile-time assertions now broken in GCC
 

On 04.10.2012 18:57, Thorsten Glaser wrote:
> Matthias Klose dixit:
>
>> how does gcc-snapshot behave?
>
> It also introduces this failure.
>
>> If you think it's a regression, please could you forward it upstream?
>
> OK. In the meantime, I’ve prepared workarounds upstream (and
> made the code a bit less depending on -fwrapv), but I need to
> know whether I should upload mksh to sid now with the hotfix
> applied to Ubuntu, i.e. whether you plan to have the affected
> gcc versions migrate.

I'll propose 4.7.2-4 for testing, so please update. However I couldn't find the
upstream GCC issue.

Matthias


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RFH: compile-time assertions now broken in GCC
 

Matthias Klose dixit:

>I'll propose 4.7.2-4 for testing, so please update.

OK thanks for the heads-up, will do after I get over
the worst of this cold I caught.

>However I couldn't find the upstream GCC issue.

Didn’t report one yet, had too much dayjob-related
stuff to do this week. Will do.

bye,
//mirabilos
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