[LLVMdev] Handling Masked Vector Operations

Dan Gohman dan433584 at gmail.com
Thu May 2 11:18:01 PDT 2013

On Thu, May 2, 2013 at 10:31 AM, <dag at cray.com> wrote:

> Nadav Rotem <nrotem at apple.com> writes:
> >
> >         For DIV/MOD you can blend the inputs BEFORE the operation. You
> >         can
> >         place ones or zeros depending on the operation.
> >
> >     Quick follow-up on this. What about using "undef" as the input for
> >     false items:
> >
> >     tv1 = select mask, v1, undef
> >     tv2 = select mask, v2, undef
> >     tv3 = div tv1, tv2
> >     v3 = select mask, tv3, undef
> >
> >     I'm always confused about the semantics of undef. Is the above
> >     safe
> >     code? It would simplify things a bit not to have to track which
> >     input
> >     values are safe based on the context of an operation.

The selects with undef do indeed do what you intend here. They return
vectors where the elements corresponding to true bits in the mask have
defined values, and the other elements have undef values. Undef can indeed
exist on a per-element basis, and LLVM is quite careful to implement this

The real problem with this approach is that there's nothing guarding those
undef elements from the div. If the undef elements happen to be 0, you'll
divide by zero, which is undefined behavior. For a divide, it's either
necessary to define the behavior of divide-by-zero (not what LLVM does
today), or mask the divide itself.

> >     -David
> >
> > This is not a correct use of undef. I think that
> > InstCombine/DagCombine will optimize tv1 into 'undef'.

It is not valid to optimize tv1 to undef here. tv3 is the problem.


[0] undef can also exist on a per-bit basis: and(1, undef) is a value where
the least significant bit is undef and the remaining bits are zero.
Amusingly, undef can also exist on a non-bitwise basis: add(mul(urem(undef,
3), 17), 5) is a value which fluctuates among just the values 5, 22, and 39.
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