[llvm-commits] CVS: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

[Chris Lattner]

On Jun 28, 2007, at 3:09 AM, Duncan Sands wrote:

> Hi,
>
>>> 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.
>
> presumably undef/X should be folded to 0.  [This means that undef/0  
> gets
> folded to 0, not sure if that's OK].

Ah, duncan's right.  I confused myself.

We can fold undef/x -> 0 because there is some value of the undef  
where it is always safe to produce zero for any x.  In particular, if  
the undef was 0, x can be anything (if it is also zero, the result is  
undefined, so zero is fine).

-Chris