llvm dev - May 2013 - [LLVMdev] Handling Masked Vector Operations

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.

                             -David

> 
>> 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.
> 
>                             -David
This is not a correct use of undef. I think that InstCombine/DagCombine will
optimize tv1 into 'undef'.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20130502/590bb96a/attachment.html>

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.
>     
>     -David
>
> This is not a correct use of undef. I think that
> InstCombine/DagCombine will optimize tv1 into 'undef'. 
But is that really a legal transformation?  Clearly not all elements are
known to be undef.

But thanks for the feedback!  We'll avoid this route for now.

                          -David