Hi Bill,
>> Ralf Koetter's code used a number of constructions like the following:
>>
>> for (y = m2-1-x; y; ) d[y--] = p[y];
>>
>> The clang compiler flagged these with warnings like this:
>>
>> warning: unsequenced modification and access to 'y'
> Yes, that is undefined behaviour.
Yes. Also hard to read and make sense of. :-)
>> Somewhere between gcc 4.6.3 and gcc 4.8.1, the GNU folks did something
>> that changed the ordering of copy and decrement in the original statement.
> Probably more aggressive optimization, compilers are free to reorder
> expressions where the Standard doesn't define a strict sequencing.
FWIW: With the original expressions in the code, gcc 4.8.1 ordered the
operations the "wrong" (read "unintended") way, even with no
optimization flag (or -O0) specified.
-- Joe
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