Re: mspgcc "undefines" too-many-bits shifts in weird way

On 22/04/13 19:32, Paul Sokolovsky wrote:
> Hello,
>
> On Mon, 22 Apr 2013 17:39:27 +0200
> David Brown <[email protected]> wrote:
>
>> Hi,
>>
>> When I compile the code you gave below (using gcc 4.6.3 from
>> 20120406), I get:
>>
>> warning: left shift count >= width of type [enabled by default]
>>
>> That is even without any sort of warning flags - and you should
>> normally enable lots of warnings.
>>
>> So here you have undefined code, and the compiler tells you of the
>> problem.  You really cannot get better than that by trying to invent
>> your own ideas about how you want to define this undefined behaviour.
>
> Yeah, but can get better by leveraging generally accepted ideas (math
> def of shift) and ideas put by more than just "you" into similar cases
> (other well known gcc ports, like x86) ;-).
>

Defining

	(x << n) = (x * 2^(n % 16)) & 0xffff

is just as good a mathematical definition as your desired

	(x << n) = (x * 2^n) & 0xffff

(for 16-bit ints)


Neither definition is "normal" mathematics.


And note that gcc on the x86 has exactly the same effect - it's just 
that with 32-bit ints, you get it with shifts of 32-bits or more.


> Thanks for all the flame, guys! ;-).
>

You're welcome :-)  It's important to think about these things and get 
the correct.




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