[openssl-dev] [openssl.org #3852] bn_gfm2.c: in BN_GF2m_mod_arr() a check is optimized out

[Andy Polyakov via RT]

> I don't think its optimizing it out I agree with you, but your
> suggested change resolved the error so Wim was right about it being
> undefined behaviour, and the tool which uses clang 3.4 was warning us
> about that.
> I added my own debugging statement and ran ectest and it is indeed
> undefined behaviour (though it doesn't matter much as the results not
> used as WIm stated) before your patch:
> 
> printf("DBG: zz=" BN_DEC_FMT1 " d0=%d\n", zz, d0);
> Example Output line (before your patch):
> DBG: zz=1 d0=1 (UB)
> Output line (after your patch):
> DBG: zz=89619874872 d0=7
> 
> 
> On Tue, May 19, 2015 at 4:04 PM, Andy Polyakov via RT <rt at openssl.org> wrote:
>> The commit intention is actually sane. This kind of brings us back to
>> question why was it optimized away? BTW, which compiler version is it
>> and what are your optimization flags? I can't reproduce the problem with
>> clang 3.6 and -O3. I mean I don't see that check for tmp_ulong is
>> optimized away. In either case, as intention is sane, the only possible
>> modification would be
>>
>>         if (d0 && (tmp_ulong = zz >> d1))
>>
>> So that right shift is not attempted if d0 is zero.

Committed as
http://git.openssl.org/gitweb/?p=openssl.git;a=commit;h=86e5d1e32b2d702a5fc777e612cd746e790098ef.