>>> I think that undef udiv intmax -> 0, no? If not, plz update
>>> instcombine as well.
>>
>> intmax udiv intmax -> 1.
>>
>> It seems like folding undef/X to undef isn't safe either though, with
>> the way it sounds like undef is intended to work. This code:
>>
>> %x = udiv i32 undef, %intmax
>> %y = udiv i32 %x, 2
>>
>> will always set %y to 0. Maybe instcombine can fold the second
>> udiv by looking through its operands, but it can't safely fold the
>> first. The best it could do is try to fold away all of %x's uses so
>> that %x isn't needed anymore.
>
> Ug, excellent point. At this point, I'm inclined to just give up
> folding of udiv undefs. What do you think?
udiv isn't the only one, the way this is going...
%x = mul i32 undef, 2
%y = srem i32 %x, 2
%x = and i32 undef, 0xffff0000
%y = and i32 %x, 0x0000ffff
and so on for a lot of others.
add, sub, and xor, might be different though; the example below is
relevant.
>> Even simple things like undef+X don't seem to be safe to fold.
>>
>> %x = undef;
>> if (%x >= 0)
>> %z = %y / (%x + 1); // don't divide by undef!
>
> Fortunately, this isn't a problem. LLVM has no copy instruction, so
> the code is really this:
>
>> if (undef >= 0)
>> %z = %y / (undef + 1); // don't divide by undef!
>
> There is nothing that specifies the two undefs are the same value.
> Also, in C, if you have an undefined variable, you aren't guaranteed
> to get the same undef value each time you read the variable, so
> transforming C into LLVM is ok :)
In C, an uninitialized variable has an "indeterminate value", which is
potentially a trap representation, which can't even be multiplied by
zero without incurring undefined behavior. I don't know where it
suggests that a variable with indeterminate value might be different
on each read though.
LLVM does so have copy instructions. The syntax is a little odd though,
and the keyword is spelled 'bitcast' ;-).
%x = bitcast i64 undef to i64;
if (%x >= 0)
%z = %y / (%x + 1); // don't divide by undef!
Now what should instcombine do?
Dan
--
Dan Gohman, Cray Inc.
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