[LLVMdev] ConstantFold 'undef xor undef'

Jianzhou Zhao
Wed, 07 Jul 2010 12:47:49 -0000

On Tue, Jul 6, 2010 at 10:07 PM, Jianzhou Zhao <jianzhou at
seas.upenn.edu> wrote:
> On Tue, Jul 6, 2010 at 8:34 PM, Chris Lattner <clattner at apple.com>
wrote:
>>
>> On Jul 6, 2010, at 3:37 PM, Jianzhou Zhao wrote:
>>
>>> Which semantics is better? I guess both are fine because if we
assume
>>> these two def's are same, then it is 0 as
>>> 'ConstantFoldBinaryInstruction', while if we assume they
are different
>>> then it is equal to undef. But the second case seems to include the
>>> first one. If we let undef xor undef to be undef, later we can use
>>> this undef as 0, but also other values w.r.t contexts. Is there any
>>> reason that ConstantFoldBinaryInstruction uses the first
assumption?
>>
>> The right answer is that undef ^ undef = undef.  Folding it to 0 is a
conservatively correct approximation of undef.  This is done because
(annoyingly) a lot of people write things like this:
>>
>> int x;
>> x = x^x;
>>
>> As a "clever" way of clearing out x, particularly for vectors
which don't have a convenient 0 literal.  This is nonsense, but common
enough to try to not completely break.
>
> Does this also apply to two different variables? say
>   int z x y;
>   z = x ^ y;
> If ConstantFoldBinaryInstruction also folds x ^ y into z, should this
> pass (which uses ConstantFold) also initialize x and y with a same
> initial value? Otherwise at runtime z may not be 0.

I guess my question is what variables can have undef values. In my
understanding, only uninitialized locals and globals can be undefined,
so compilers are free to assign them at runtime, if
ConstantFoldBinaryInstruction ensures that x and y can only be locals
and globals. But function parameters cannot be undefined, which should
be arbitrary symbols, depending on concrete arguments. Is this
correct?

>
>>
>> -Chris
>
>
>
> --
> Jianzhou
>



--
Jianzhou