On 10/12/2015 17:53, Matt Caswell wrote:
> On 10/12/15 11:55, Jayalakshmi bhat wrote:
>> Hi Matt,
>>
>> Thanks for the patch. Unfortunately patch did not work. I continued
>> debugging and found that issue was in constant_time_msb.
>>
>> static inline unsigned int constant_time_msb(unsigned int a) {
>> - *return 0 - (a >> (sizeof(a) * 8 - 1));*
>> + return (((unsigned)((int)(a) >> (sizeof(int) * 8 - 1))));
>> }
> Thanks. Have you analysed why the original version failed? Both versions
> look reasonable to me (ignoring the hardcoded 8 - implying a char is 8
> bits). I'd really like to understand that before replacing it with
> something else which apparently does the same thing. Perhaps you could
> post some sample values for "a" and the return value, before and after
> your change.
Looking at the provided disassembly, it looks like the
1.0.2 version triggers a compiler bug which (at least)
forgets to mask the result down to 8 bits after inlining
in test_is_zero_8(). The missing mask with FF occurs
in multiple functions in the disassembly.
Enjoy
Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S. https://www.wisemo.com
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